Solid-State NMR Literature Blog

Phys. Chem. Chem. Phys., 2011

2011-02-09T14:40:00.003-05:00

Determination of coordination modes and estimation of the 31P–31P distances in heterogeneous catalyst by solid state double quantum filtered 31P NMR spectroscopy
Si-Yong Zhang, Mei-Tao Wang, Qing-Hua Liu, Bing-Wen Hu, Qun Chen, He-Xing Li and Jean-Paul Amoureux

Phys. Chem. Chem. Phys., 2011, Advance Article
http://dx.doi.org/10.1039/C0CP01191F

To overcome the separation difficulty of the palladium-based homogeneous catalyst, the palladium complex can be anchored on various supports such as silica. However, it is difficult to determine the amounts of the two coordination modes of the Pd nucleus, that is, Pd coordinates with one phosphorus atom and Pd coordinates with two phosphorus atoms. Here a 31P double-quantum filtered (DQ-filtered) method in solid-state NMR is introduced for the palladium-based heterogenous catalyst system. With the DQ-filtered method, we can not only determine the amounts of the two different kinds of palladium coordination modes, we can also estimate the interatomic distance of two 31P nuclei bonded to a palladium nucleus. With the help of this method, we can quickly estimate interatomic distances in our designed system and accurately re-design the palladium system to accommodate either one 31P or two 31P.

A highly ordered mesostructured material containing regularly distributed phenols: preparation and characterization at a molecular level through ultra-fast magic angle spinning proton NMR spectroscopy
Arthur Roussey, David Gajan, Tarun K. Maishal, Anhurada Mukerjee, Laurent Veyre, Anne Lesage, Lyndon Emsley, Christophe Copéret and Chloé Thieuleux

Phys. Chem. Chem. Phys., 2011, Advance Article
http://dx.doi.org/10.1039/C0CP02137G

Highly ordered organic–inorganic mesostructured material containing regularly distributed phenols is synthesized by combining a direct synthesis of the functional material and a protection–deprotection strategy and characterized at a molecular level through ultra-fast magic angle spinning proton NMR spectroscopy.

Influence of particle size on solid solution formation and phase interfaces in Li0.5FePO4 revealed by 31P and 7Li solid state NMR spectroscopy
L. J. M. Davis, I. Heinmaa, B. L. Ellis, L. F. Nazar and G. R. Goward

Phys. Chem. Chem. Phys., 2011, Advance Article
http://dx.doi.org/10.1039/C0CP01922D

Here we report the observation of electron delocalization in nano-dimension xLiFePO4:(1 − x)FePO4 (x = 0.5) using high temperature, static, 31P solid state NMR. The 31P paramagnetic shift in this material shows extreme sensitivity to the oxidation state of the Fe center. At room temperature two distinct 31P resonances arising from FePO4 and LiFePO4 are observed at 5800 ppm and 3800 ppm, respectively. At temperatures near 400 °C these resonances coalesce into a single narrowed peak centered around 3200 ppm caused by the averaging of the electronic environments at the phosphate centers, resulting from the delocalization of the electrons among the iron centers. 7Li MAS NMR spectra of nanometre sized xLiFePO4:(1 − x)FePO4 (x = 0.5) particles at ambient temperature reveal evidence of Li residing at the phase interface between the LiFePO4 and FePO4 domains. Moreover, a new broad resonance is resolved at 65 ppm, and is attributed to Li adjacent to the anti-site Fe defect. This information is considered in light of the 7Li MAS spectrum of LiMnPO4, which despite being iso-structural with LiFePO4 yields a remarkably different 7Li MAS spectrum due to the different electronic states of the paramagnetic centers. For LiMnPO4 the higher 7Li MAS paramagnetic shift (65 ppm) and narrowed isotropic resonance (FWHM ≈ 500 Hz) is attributed to an additional unpaired electron in the t2g orbital as compared to LiFePO4 which has δiso = −11 ppm and a FWHM = 9500 Hz. Only the delithiated phase FePO4 is iso-electronic and iso-structural with LiMnPO4. This similarity is readily observed in the 7Li MAS spectrum of xLiFePO4:(1 − x)FePO4 (x = 0.5) where Li sitting near Fe in the 3+ oxidation state takes on spectral features reminiscent of LiMnPO4. Overall, these spectral features allow for better understanding of the chemical and electrochemical (de)lithiation mechanisms of LiFePO4 and the Li-environments generated upon cycling.

Phase evolution in lithium disilicate glass–ceramics based on non-stoichiometric compositions of a multi-component system: structural studies by 29Si single and double resonance solid state NMR
Christine Bischoff, Hellmut Eckert, Elke Apel, Volker M. Rheinberger and Wolfram Höland

Phys. Chem. Chem. Phys., 2011, Advance Article
http://dx.doi.org/10.1039/C0CP01440K

The crystallization mechanism of a high-strength lithium disilicate glass–ceramic in the SiO2–Li2O–P2O5–Al2O3–K2O–(ZrO2) system, used as restorative dentistry material, has been examined on the basis of quantitative 29Si magic angle spinning (MAS) and 29Si{7Li} rotational echo double resonance (REDOR) NMR spectroscopy. Crystallization occurs in two stages: near 650 °C a significant fraction of the Q(3) units disproportionates into crystalline Li2SiO3 and Q(4) units. Upon further annealing of this glass–ceramic to 850 °C the crystalline Li2SiO3 phase reacts with the Q(4) units of the softened residual glass matrix, resulting in the crystallization of Li2Si2O5. The NMR experiments provide detailed insight into the spatial distribution of the lithium ions suggesting the absence of lithium ion clustering in the residual glassy component of the final glass–ceramic. 31P MAS-NMR spectra indicate that phosphate acts as a lithium ion scavenger, resulting in the predominant formation of orthophosphate (P(0)) and some pyrophosphate (P(1)) groups. Crystallization of Li2SiO3 occurs concomitantly with the formation of a highly disordered Li3PO4 phase as evidenced from strong linebroadening effects in the 31P MAS-NMR spectra. Well-crystallized Li3PO4 is only formed at annealing conditions resulting in the formation of crystalline lithium disilicate. These results argue against an epitaxial nucleation process previously proposed in the literature and rather suggest that the nucleation of both lithium metasilicate and lithium disilicate starts at the phase boundary between the disordered lithium phosphate phase and the glass matrix.

Longer-range distances by spinning-angle-encoding solid-state NMR spectroscopy
Johanna Becker-Baldus, Thomas F. Kemp, Jaan Past, Andres Reinhold, Ago Samoson and Steven P. Brown

Phys. Chem. Chem. Phys., 2011, Advance Article
http://dx.doi.org/10.1039/C0CP02364G

A new spinning-angle-encoding spin-echo solid-state NMR approach is used to accurately determine the dipolar coupling corresponding to a C–C distance over 4 Å in a fully labelled dipeptide. The dipolar coupling dependent spin-echo modulation was recorded off magic angle, switching back to the magic angle for the acquisition of the free-induction decay, so as to obtain optimum sensitivity. The retention of both ideal resolution and long-range distance sensitivity was achieved by redesigning a 600 MHz HX MAS NMR probe to provide fast angle switching during the NMR experiment: for 1.8 mm rotors, angle changes of up to [similar]5° in [similar]10 ms were achieved at 12 kHz MAS. A new experimental design that combines a reference and a dipolar-modulated experiment and a master-curve approach to data interpretation is presented.

2011-01-31T16:50:00.001-05:00

Solid-State NMR and Density Functional Theory Studies of Ionization States of Thiamin

J. Phys. Chem. B, 2011, 115 (4), pp 730–736

Thiamin diphosphate (ThDP) is a key coenzyme in sugar metabolism. The 4′-aminopyrimidine ring of ThDP cycles through several ionization and tautomeric states during enzyme catalysis, but it is not fully understood which states are adopted during the individual steps of the catalytic cycle. Thiamin has been synthesized with labels selectively inserted into the C2 and C6′ positions, as well as into the amino group, creating [C2, C6′-13C2] thiamin and [N4′-15N] thiamin. Magic-angle spinning (MAS) NMR spectroscopy has been employed to record the 13C and 15N chemical shift anisotropy (CSA) tensors for C2, C6′, and N4′ atoms. Our results indicate that the isotropic chemical shifts as well as the principal components of the 13C and 15N CSA tensors are very sensitive to the protonation states in these compounds and, therefore, permit differentiating between the two ionization states, 4-aminopyrimidine and 4-aminopyrimidinium. Using density functional theory (DFT), we have calculated the magnetic shielding anisotropy tensors of C2, C6′, and N4′ and found excellent agreement between the computed and the experimental tensors. Our findings indicate that MAS NMR spectroscopy in conjunction with DFT calculations is a sensitive probe of ionization states in the thiamin cofactor. The results of this study will serve as a guide for characterization of ionization and tautomeric states of thiamin in complexes with thiamin-dependent enzymes.

Siting and Mobility of Deuterium Absorbed in Cosputtered Mg0.65Ti0.35. A MAS 2H NMR Study

J. Phys. Chem. C, 2011, 115 (1), pp 288–297

Nanostructured magnesium titanium alloys are interesting lightweight materials for chemical hydrogen storage. We have therefore investigated the siting and dynamics of deuterium absorbed in a Mg0.65Ti0.35 alloy generated by magnetron cosputtering, and made a comparison to the corresponding features in bulk samples of deuterium-loaded Mg0.65Ti0.35 and Mg0.65Sc0.35 prepared by ball-milling and melt-casting, respectively. Magic-angle spinning 2H NMR of cosputtered Mg0.65Ti0.35D1.1 shows partly resolved signals of deuterium located in nonconductive domains at tetrahedral Mg4 and mixed MgnTi4−n sites (4 ppm) and deuterium at Ti4 sites in conducting TiD2 nanodomains (−29 and −68 ppm). No bulk TiD2 signal at −150 ppm is observed, in contrast to what we find in ball-milled Mg0.65Ti0.35D0.65, which is largely phase separated. The deuterium species with shift values of 4 and −29 ppm undergo complete exchange at a subsecond time scale in one- and two-dimensional exchange NMR and must therefore be close together in the lattice. In contrast, deuterium resonating at −68 ppm does not show deuterium exchange and thus appears to be located at more stable sites. The observed deuterium exchange and the reduced Knight shift compared to bulk TiD2 are explained using a model with TiD2 nanoslabs.

Towards Portable High-Resolution NMR Spectroscopy†

Angew. Chem. Int. Ed. 2011, 50, 354 – 356

Analysis on the go: Portable high-resolution NMR spectroscopy is of great interest for many applications. Recent advances in magnet design, spectrometer stability, and acquisition schemes have placed the realization of low-field spectrometers based on room-temperature permanent magnets and that can deliver chemical shift resolution within reach.

2011-01-31T14:26:00.000-05:00

Probing the local structures and protonic conduction pathways in scandium substituted BaZrO3 by multinuclear solid-state NMR spectroscopy

from RSC - J. Mater. Chem. latest articles by Clare P. Grey

Abstract

A comprehensive multinuclear solid-state NMR study of scandium-substituted BaZrO₃ is reported. Static low field and MQMAS very high field ⁴⁵Sc NMR data revealed the presence of both 5- and 6-coordinated scandium atoms, 5-coordinated scandium arising from Sc nearby an oxygen vacancy. ¹⁷O NMR spectra showed the presence of up to three different chemical oxygen environments assigned to Zr–O–Zr, Zr–O–Sc and Sc–O–Sc. From the ratios of these different oxygen sites, the distribution of the scandium cations was close to random but indicated that the maximum scandium incorporation was lower than expected, consistent with the observation of Sc₂O₃ impurities at substitution levels of 30% Sc for Zr. ¹H and ⁴⁵Sc NMR data on the hydrated materials revealed the presence of scandium next to protonic defects. Finally, variable temperature ¹H NMR showed the presence of at least two different proton environments in between which proton transfer occurs at ambient temperatures (300 K).

2011-01-31T14:24:00.000-05:00

Solid state NMR study on the thermal decomposition pathway of sodium amidoborane NaNH2BH3

from RSC - J. Mater. Chem. latest articles by Yoshitsugu Kojima
Abstract

The thermal decomposition pathway of sodium amidoborane (NaAB; NaNH₂BH₃) has been investigated in detail by using solid state NMR spectroscopy. ²³Na MAS/3QMAS NMR spectra suggested that NaH and an amorphous Na–N–B–H phase started to be formed as decomposition products even at 79 °C, although NaAB was prepared from NaH and NH₃BH₃ by ball milling at room temperature. Based on the quantitative analyses of the ²³Na MAS spectra, we proposed a decomposition reaction to 200 °C to be NaNH₂BH₃ → Na_0.5NBH_0.5 + 0.5NaH + 2.0H₂. The hypothetical phase Na_0.5NBH_0.5 is amorphous, where the basic molecular unit of the original NaAB is polymerized into a [–BN–]_n network structure. It was also found that the diammoniate of diborane (DADB) and polyaminoborane (PAB) were not formed during the decomposition of NaAB, which are both key compounds on the pyrolysis of ammonia borane (AB).

17O Central Transition NMR

2011-01-17T10:31:00.004-05:00

A nice write-up of Gang Wu's solution 17O central transition NMR studies of biomolecules can be found in C&E News

http://pubs.acs.org/cen/news/89/i01/8901notw4.html

The corresponding articles are:

Quadrupole Central Transition 17O NMR Spectroscopy of Biological Macromolecules in Aqueous Solution

Jianfeng Zhu and Gang Wu*

J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/ja1079207

Abstract: We demonstrate a general nuclear magnetic resonance (NMR) spectroscopic approach in obtaining high-resolution 17O (spin-5/2) NMR spectra for biological macromolecules in aqueous solution. This approach, termed quadrupole central transition (QCT) NMR, is based on the multiexponential relaxation properties of half-integer quadrupolar nuclei in molecules undergoing slow isotropic tumbling motion. Under such a circumstance, Redfield’s relaxation theory predicts that the central transition, mI = +1/2 ↔ −1/2, can exhibit relatively long transverse relaxation time constants, thus giving rise to relatively narrow spectral lines. Using three robust protein−ligand complexes of size ranging from 65 to 240 kDa, we have obtained 17O QCT NMR spectra with unprecedented resolution, allowing the chemical environment around the targeted oxygen atoms to be directly probed for the first time. The new QCT approach increases the size limit of molecular systems previously attainable by solution 17O NMR by nearly 3 orders of magnitude (1000-fold). We have also shown that, when both quadrupole and shielding anisotropy interactions are operative, 17O QCT NMR spectra display an analogous transverse relaxation optimized spectroscopy type behavior in that the condition for optimal resolution depends on the applied magnetic field. We conclude that, with the currently available moderate and ultrahigh magnetic fields (14 T and higher), this 17O QCT NMR approach is applicable to a wide variety of biological macromolecules. The new 17O NMR parameters so obtained for biological molecules are complementary to those obtained from 1H, 13C, and 15N NMR studies.

Solid-State 17O NMR Spectroscopy of Large Protein–Ligand Complexes†
Dr. Jianfeng Zhu1, Dr. Eric Ye2, Dr. Victor Terskikh3, Prof. Dr. Gang Wu1
Article first published online: 29 OCT 2010

DOI: 10.1002/anie.201002041

Angewandte Chemie International Edition
Volume 49, Issue 45, pages 8399–8402, November 2, 2010

Keywords:
oxygen-17;protein–ligand interactions;proteins;solid-state NMR spectroscopy;structure refinement

Oxygen, oxygen, everywhere! Poor sensitivity has hindered the development of solid-state 17O NMR spectroscopy as a practical technique for the structural elucidation of protein complexes. However, this has now changed and it has been demonstrated that multinuclear 17O, 27Al, 13C NMR parameters can be used to aid structural refinement for a protein-bound ligand molecule (see picture).

Solid-State NMR

2011-01-10T16:42:00.002-05:00

A practical guide for the setup of a 1H-31P-13C double cross-polarization (DCP) experiment

Publication year: 2010
Source: Solid State Nuclear Magnetic Resonance, In Press, Accepted Manuscript, Available online 13 December 2010
Wlodzimierz, Ciesielski , Hassan, Kassasir , Marek J., Potrzebowski

O-phospho-L-threonine is a convenient sample to setup a ¹H-³¹P-¹³C double cross-polarization (DCP) Hartmann-Hahn match. The ¹H-³¹P-¹³C technique is extremely sensitive to the rate of sample spinning. Both zero-quantum (ZQ) and double-quantum (DQ) cross-polarization operate at an average spinning rate (6–7 kHz). At higher spinning rates (10 kHz), the DQCP mechanism dominates and leads to a reduction of signal intensity, in particular for lower ³¹P rf field strength. The application of two shape pulses during the second cross-polarization greatly improves the signal to noise ratio allowing the recording of better quality spectra. ³¹P-¹³C SPECIFIC-CP (spectrally induced filtering in combination with cross-polarization) experiments can be carried out under ZQCP and DQCP condition if careful attention is paid to the choice of RF field amplitudes and carriers Ω. Application of 1D and 2D ¹H-³¹P-¹³C experiments is demonstrated on model samples; disodium ATP hydrate and O-phospho-L-tyrosine.

Multinuclear NMR study of silica fiberglass modified with zirconia

Publication year: 2010
Source: Solid State Nuclear Magnetic Resonance, In Press, Accepted Manuscript, Available online 29 December 2010
O.B., Lapina , D.F, Khabibulin , V.V., Terskikh

Silica fiberglass textiles are emerging as uniquely suited supports in catalysis which offer unprecedented flexibility in designing advanced catalytic systems for chemical and auto industries. During manufacturing fiberglass materials are often modified with additives of various nature to improve glass properties. Glass network formers, such as zirconia and alumina, are known to provide the glass fibers with higher strength and to slow down undesirable devitrification processes. In this work multinuclear ¹H, ²³Na, ²⁹Si, and ⁹¹Zr NMR spectroscopy was used to characterize the effect of zirconia on the molecular-level fiberglass structure. ²⁹Si NMR results help in understanding why zirconia-modified fiberglass is more stable towards devitrification comparing with pure silica glass. Internal void spaces formed in zirconia-silica glass fibers after acidic leaching correlate with sodium and water distributions in the starting bulk glass as probed by ²³Na and ¹H NMR. These voids spaces are important for stabilization of catalytically active species in the supported catalysts. Potentials of high-field ⁹¹Zr NMR spectroscopy to study zirconia-containing glasses and similarly disordered systems are illustrated.

Kinetics of 1H→13C NMR cross-polarization in polymorphs and solvates of the antipsychotic drug olanzapine

Publication year: 2011
Source: Solid State Nuclear Magnetic Resonance, In Press, Accepted Manuscript, Available online 4 January 2011
Waclaw, Kolodziejski , Joanna, Herold , Marzena, Kuras , Irena, Wawrzycka-Gorczyca , Anna E., Koziol

The ¹H→¹³C NMR cross-polarization (CP) was studied under magic-angle spinning at 7.5 kHz in various crystal forms of the antipsychotic drug olanzapine: two polymorphs (metastable I and stable II) and eight solvates containing organic solvent and water molecules. The CP kinetics followed the non-classical I-I^*-S model, in which CP begins in a spin cluster of proximate abundant spins I^* and rare spins S, then is controlled by spin diffusion of the abundant spins I from bulk to the I^* spins of the spin cluster and finally is governed by spin-lattice relaxation of the abundant spins in the rotating frame. The corresponding CP kinetics parameters were determined and analyzed. It was demonstrated that the, λ and T_df values (the CP time constant, the cluster composition parameter and the ¹H spin-diffusion constant, respectively) were very useful to discriminate the functional groups, especially in the 3D parameter space. In order to conveniently analyze the large amount (1 7 5) of the collected CP parameters, the number of the observed variables was reduced using the principal component (PC) analysis. The 2D plot of PC2 vs. PC1 showed adequate separation of the CH₃, CH₂, CH and C cases (C stands for carbons without adjacent hydrogens). It was demonstrated that those cases were located along the PC1 axis in the order of increasing ¹H-¹³C dipolar couplings: C32. Our study showed the I-I^*-S model at work and established ranges of its parameters for various functional groups.

Inorg. Chem.

2011-01-10T10:47:00.001-05:00

Solvent Effects and Dynamic Averaging of ¹⁹⁵Pt NMR Shielding in Cisplatin Derivatives

Lionel A. Truflandier, Kiplangat Sutter, and Jochen Autschbach*

jochena@buffalo.edu

Inorg. Chem., Article ASAP

DOI: 10.1021/ic102174b

Publication Date (Web): January 4, 2011

The influences of solvent effects and dynamic averaging on the ¹⁹⁵Pt NMR shielding and chemical shifts of cisplatin and three cisplatin derivatives in aqueous solution were computed using explicit and implicit solvation models. Within the density functional theory framework, these simulations were carried out by combining ab initio molecular dynamics (aiMD) simulations for the phase space sampling with all-electron relativistic NMR shielding tensor calculations using the zeroth-order regular approximation. Structural analyses support the presence of a solvent-assisted “inverse” or “anionic” hydration previously observed in similar square-planar transition-metal complexes. Comparisons with computationally less demanding implicit solvent models show that error cancellation is ubiquitous when dealing with liquid-state NMR simulations. After aiMD averaging, the calculated chemical shifts for the four complexes are in good agreement with experiment, with relative deviations between theory and experiment of about 5% on average (1% of the Pt^II chemical shift range).

J. Phys. Chem. A

2011-01-10T10:20:00.002-05:00

Crystal Structure Based Design of Signal Enhancement Schemes for Solid-State NMR of Insensitive Half-Integer Quadrupolar Nuclei

Luke A. O’Dell* and Christopher I. Ratcliffe
luke.odell@nrc-cnrc.gc.ca

J. Phys. Chem. A, Article ASAP

DOI: 10.1021/jp111531e

Publication Date (Web): December 21, 2010

A combination of density functional and optimal control theory has been used to generate amplitude- and phase-modulated excitation pulses tailored specifically for the ³³S nuclei in taurine, based on one of several reported crystal structures. The pulses resulted in significant signal enhancement (stemming from population transfer from the satellite transitions) without the need for any experimental optimization. This allowed an accurate determination of the ³³S NMR interaction parameters at natural abundance and at a moderate magnetic field strength (11.7 T). The ³³S NMR parameters, along with those measured from ¹⁴N using frequency-swept pulses, were then used to assess the accuracy of various proposed crystal structures.

___________________________________________________

Interactions of Volatile Organic Compounds with Syndiotactic Polystyrene Crystalline Nanocavities
Alexandra R. Albunia*, Patrizia Oliva, and Alfonso Grassi
aalbunia@unisa.it

J. Phys. Chem. A, Article ASAP

DOI: 10.1021/jp1090608

Publication Date (Web): December 17, 2010

The interaction of some volatile organic compounds, namely, 1,2-dichloroethane, 1,2-dibromoethane, and 1,1,2,2-tetrachloroethane, included in the δ crystalline phase of syndiotactic polystyrene (sPS) has been studied in terms of conformation, orientation, and dynamical behavior. By combination of X-ray diffraction (XRD), Fourier-transform infrared (FTIR), and solid-state ²H NMR analyses, it has been shown that despite the differences in guest molecular properties (mass, boiling temperature, and volume), stable sPS/guest δ-clathrate cocrystals are formed since the nanoporous δ crystalline form has a flexible structure able to adapt itself to the guest molecule. As a consequence of inclusion, it has been shown that the guest diffusivity is strongly reduced and the dynamical processes are constrained, particularly when these guests are in trans
conformation. This suggests the nanoporous sPS δ form to be an efficient tool for water and air purification through volatile organic compound absorption.

J. Chem. Phys.

2011-01-07T16:39:00.001-05:00

Electrical and ionic conductivity effects on magic-angle spinning nuclear magnetic resonance parameters of CuI

from Journal of Chemical Physics: All Topics

James P. Yesinowski, Harold D. Ladouceur, Andrew P. Purdy, and Joel B. Miller

We investigate experimentally and theoretically the effects of two different types of conductivity, electrical and ionic, upon magic-angle spinning NMR spectra. The experimental demonstration of these effects involves ⁶³Cu, ⁶⁵Cu, and ¹²⁷I variable temperature MAS-NMR experiments on samples of γ-CuI, a Cu⁺-ion conductor at elevated temperatures as well as a wide bandgap semiconductor. We extend previous observations that the chemical shifts depend very strongly upon the square of the spinning-speed as well as the particular sample studied and the magnetic field strength. By using the ²⁰⁷Pb resonance of lead nitrate mixed with the γ-CuI as an internal chemical shift thermometer we show that frictional heating effects of the rotor do not account for the observations. Instead, we find that spinning bulk CuI, a p-type semiconductor due to Cu⁺vacancies in nonstoichiometric samples, in a magnetic field generates induced AC electric currents from the Lorentz force that can resistively heat the sample by over 200 °C. These induced currents oscillate along the rotor spinning axis at the spinning speed. Their associated heating effects are disrupted in samples containing inert filler material, indicating the existence of macroscopic current pathways between micron-sized crystallites. Accurate measurements of the temperature-dependence of the ⁶³Cu and ¹²⁷I chemical shifts in such diluted samples reveal that they are of similar magnitude (ca. 0.27 ppm/K) but opposite sign (being negative for ⁶³Cu), and appear to depend slightly upon the particular sample. This relationship is identical to the corresponding slopes of the chemical shifts versus square of the spinning speed, again consistent with sample heating as the source of the observed large shift changes. Higher drive-gas pressures are required to spin samples that have higher effective electrical conductivities, indicating the presence of a braking effect arising from the induced currents produced by rotating a conductor in a homogeneous magnetic field. We present a theoretical analysis and finite-element simulations that account for the magnitude and rapid time-scale of the resistive heating effects and the quadratic spinning speed dependence of the chemical shift observed experimentally. Known thermophysical properties are used as inputs to the model, the sole adjustable parameter being a scaling of the bulk thermal conductivity of CuI in order to account for the effective thermal conductivity of the rotating powdered sample. In addition to the dramatic consequences of electrical conductivity in the sample,ionic conductivity also influences the spectra. All three nuclei exhibit quadrupolar satellite transitions extending over several hundred kilohertz that reflect defects perturbing the cubic symmetry of the zincblende lattice. Broadening of these satellite transitions with increasing temperature arises from the onset of Cu⁺ ion jumps to sites with different electric field gradients, a process that interferes with the formation of rotational echoes. This broadening has been quantitatively analyzed for the ⁶³Cu and ⁶⁵Cu nuclei using a simple model in the literature to yield an activation barrier of 0.64 eV (61.7 kJ/mole) for the Cu⁺ ion jumping motion responsible for the ionic conductivity that agrees with earlier results based on ⁶³Cu NMR relaxation times of static samples

Communication: Critical dynamics and nuclear relaxation in lipid bilayers

Harden McConnell

Membrane composition fluctuations affect deuterium nuclear magnetic relaxation in lipid bilayers. The time dependence of the fluctuations depends on lipid diffusion. Near a miscibility critical point this diffusion involves an advective hydrodynamic coupling to the aqueous phase. The corresponding diffusion coefficient depends on both the critical length and the fluctuation wavelength. We calculate the effects of these dynamics on transverse deuterium nuclear relaxation in the 0.1^o–10^o range above the critical temperature.

Phys. Rev. Lett.

2011-01-07T16:06:00.000-05:00

Detection of Phase Biaxiality in Liquid Crystals by Use of the Quadrupole Shift in ^{131} Xe NMR Spectra

from Recent Articles in Phys. Rev. Lett. by Jukka P. Jokisaari, Anu M. Kantola, Juhani A. Lounila, and L. Petri Ingman

An experimental method to unambiguously distinguish between uniaxial and biaxial liquid crystal phases is introduced. The method is based on the second order quadrupole shift (SOQS) observable in ¹³¹Xe NMR spectra of xenon dissolved in liquid crystals. It is shown that besides revealing the biaxiality, the ¹³¹Xe SOQS offers a novel method to determine the tilt angle in smectic C phases. As an example, the ¹³¹Xe SOQS in a ferroelectric liquid crystal is reported. It yields up a biaxial phase in between isotropic and smectic C phases.

Phys. Rev. B.

2010-12-17T15:40:00.002-05:00

NMR and NQR study of the tetrahedral frustrated quantum spin system Cu_{2} Te_{2} O_{5} Br_{2} in its paramagnetic phase
Author(s): Arnaud Comment, Hadrien Mayaffre, Vesna Mitrović, Mladen Horvatić, Claude Berthier, Béatrice Grenier, and Patrice Millet
The quantum antiferromagnet Cu₂Te₂O₅Br₂ was investigated by NMR and nuclear quadrupole resonance (NQR). The ¹²⁵Te NMR investigation showed that there is a magnetic transition around 10.5 K at 9 T, in agreement with previous studies. From the divergence of the spin-lattice relaxation rate, we ruled out the possibility that the transition could be governed by a one-dimensional divergence of the spin-spin correlation function. The observed anisotropy of the ¹²⁵Te shift was shown to be due to a spin polarization of the 5s² “E” doublet of the[TeO₃E] tetrahedra, highlighting the importance of tellurium in the exchange paths. In the paramagnetic state, Br NQR and NMR measurements led to the determination of the Br hyperfine coupling and the electric field gradient tensor, and to the spin polarization of Br porbitals. The results demonstrate the crucial role of bromine in the interaction paths between Cu spins.

NMR evidence for the partially gapped state in CeOs_{2} Al_{10}

Author(s): C. S. Lue, S. H. Yang, T. H. Su, and Ben-Li Young

We report the results of a ²⁷Al nuclear magnetic resonance (NMR) study of CeOs₂Al₁₀ at temperatures between 4 and 300 K. This material has been of current interest due to indications of hybridization gap behavior below the transition temperature T_o≃29 K. Five ²⁷Al NMR resonance lines that are associated with five nonequivalent crystallographic aluminum sites have been resolved. For each individual aluminum site, the low-temperature NMR Knight shift goes over a thermally activated response. The temperature-dependent spin-lattice-relaxation rate exhibits a rapid drop below T_o, indicative of the formation of an energy gap in this material. We interpret the Knight shift and the relaxation-rate data in light of the presence of a pseudogap with residual electronic density of states at the Fermi level. Moreover, the magnitude of the pseudogap of 120 K is extracted from NMR results, in agreement with the value obtained from the inelastic neutron-scattering experiment.

Spin order and lattice frustration in optimally doped manganites: A high-temperature NMR study

Author(s): N. Panopoulos, D. Koumoulis, G. Diamantopoulos, M. Belesi, M. Fardis, M. Pissas, and G. Papavassiliou

Understanding the complex glassy phenomena, which accompany polaron formation in optimally doped manganites (ODMs) is a cumbersome issue with many unexplained perspectives. Here, on the basis of ¹³⁹La and ⁵⁵Mn nuclear magnetic resonance (NMR) measurements, performed in the temperature range 80–900 K we show that glass freezing, observed in the paramagnetic (PM) phase of ODM La_0.67Ca_0.33MnO₃, is not a random uncorrelated process but the signature of the formation of a genuine spin-glass state, which forT<T_c consolidates with the ferromagnetic (FM) state into a single thermodynamic phase. Comparison with NMR measurements performed onLa_1−xCa_xMnO₃ systems for 0.0≤x≤0.41 and ODM La_0.70Sr_0.30MnO₃, demonstrates the key role played by the local lattice distortions, which control (i) the stability of the spin-glass phase component and (ii) the kind (first or second order) of the PM-FM phase transition. The experimental results are in agreement with the predictions of the compressible random bond-random field Ising model, where consideration of a strain field induced by lattice distortions is shown to invoke at T_c a discontinuous first-orderlike change in both the FM and the “glassy” Edwards-Anderson order parameters.

Phys. Rev. Lett.

2010-12-16T18:07:00.001-05:00

^{7} Li NMR Investigation of Li-Li Pair Ordering in the Paraelectric Phase of Weakly Substitutionally Disordered K_{1-x} Li_{x} TaO_{3}

by Boštjan Zalar, Andrija Lebar, David C. Ailion, R. O. Kuzian, I. V. Kondakova, and V. V. Laguta

Author(s): Boštjan Zalar, Andrija Lebar, David C. Ailion, R. O. Kuzian, I. V. Kondakova, and V. V. Laguta

Breaking of the average cubic symmetry in Li-doped potassium tantalate was observed with quadrupole-perturbed ⁷Li NMR at temperatures (150–400 K) far above the nominal glass transition temperature (≈50 K for Li concentration x=0.03). The observed spectrum consists of contributions from both isolated Li ions (i.e., with no nearest-neighbor Li) and from Li-Li pairs. The isolated Li ions move among six equivalent off-center sites in a potential having cubic symmetry. These have zero average electric field gradient and, hence, exhibit no quadrupole splitting. In addition, very low intensity, but well resolved, quadrupole satellites having a temperature-dependent splitting were observed. This splitting indicates that the various Li-Li pair configurations are not all equally probable. These are the first direct observations of biased Li ion ordering that persists in the paraelectric phase at temperatures high above the glass phase.

Anisotropic Spin Fluctuations and Superconductivity in “115” Heavy Fermion Compounds: ^{59} Co NMR Study in PuCoGa_{5}

from Recent Articles in Phys. Rev. Lett. by S.-H. Baek, H. Sakai, E. D. Bauer, J. N. Mitchell, J. A. Kennison, F. Ronning, and J. D. Thompson

Author(s): S.-H. Baek, H. Sakai, E. D. Bauer, J. N. Mitchell, J. A. Kennison, F. Ronning, and J. D. Thompson

We report results of ⁵⁹Co nuclear magnetic resonance measurements on a single crystal of superconducting PuCoGa₅ in its normal state. The nuclear spin-lattice relaxation rates and the Knight shifts as a function of temperature reveal an anisotropy of spin fluctuations with finite wave vector q. By comparison with the isostructural members, we conclude that antiferromagnetic XY-type anisotropy of spin fluctuations plays an important role in mediating superconductivity in these heavy fermion materials.

Journal of chemical physics

2010-12-16T17:47:00.001-05:00

High resolution NMR study of T magnetic relaxation dispersion. II. Influence of spin-spin couplings on the longitudinal spin relaxation dispersion in multispin systems
Sergey Korchak, Konstantin Ivanov, Alexandra Yurkovskaya, and Hans-Martin Vieth
Effects of scalar spin-spin interactions on the nuclear magnetic relaxation dispersion (NMRD) of coupled multispin systems were analyzed. Taking spin systems of increasing complexity we demonstrated pronounced influence of the intramolecular spin-spin couplings on the NMRD of protons. First, at low magnetic fields where there is strong coupling of spins the apparent relaxation times of the coupled spins become equal. Second, there are new features, which appear at the positions of the nuclear spin level anticrossings. Finally, in coupled spin systems there can be a coherent contribution to the relaxation kinetics present at low magnetic fields. All these peculiarities caused by spin-spin interactions are superimposed on the features in NMRD, which are conditioned by changes of the motional regime. Neglecting the effects of couplings may lead to misinterpretation of the NMRD curves and significant errors in determining the correlation times of molecular motion. Experimental results presented are in good agreement with theoretical calculations.

Proceedings of National Academy of Sciences, Early Edition

2010-12-06T10:37:00.003-05:00

Strongly bound citrate stabilizes the apatite nanocrystals in bone
Y.-Y. Hu, A. Rawal, and K. Schmidt-Rohr1
+ Author Affiliations

Ames Laboratory and Department of Chemistry, Iowa State University, Ames, IA 50011
Edited by David A. Tirrell, California Institute of Technology, Pasadena, CA, and approved October 12, 2010 (received for review June 27, 2010)

Published online before print December 2, 2010, doi: 10.1073/pnas.1009219107
PNAS December 2, 2010

Abstract: Nanocrystals of apatitic calcium phosphate impart the organic-inorganic nanocomposite in bone with favorable mechanical properties. So far, the factors preventing crystal growth beyond the favorable thickness of ca. 3 nm have not been identified. Here we show that the apatite surfaces are studded with strongly bound citrate molecules, whose signals have been identified unambiguously by multinuclear magnetic resonance (NMR) analysis. NMR reveals that bound citrate accounts for 5.5 wt% of the organic matter in bone and covers apatite at a density of about 1 molecule per (2 nm)2, with its three carboxylate groups at distances of 0.3 to 0.45 nm from the apatite surface. Bound citrate is highly conserved, being found in fish, avian, and mammalian bone, which indicates its critical role in interfering with crystal thickening and stabilizing the apatite nanocrystals in bone.

A nice summary of the article can also be found in C&E News.

http://pubs.acs.org/isubscribe/journals/cen/88/i49/html/8849scic2.html

J. Phys. Chem. B and C, vol. 114, Issues 37-42

2010-12-02T14:29:00.003-05:00

Accurate Determination of Interstrand Distances and Alignment in Amyloid Fibrils by Magic Angle Spinning NMR

Marc A. Caporini†§, Vikram S. Bajaj†, Mikhail Veshtort†, Anthony Fitzpatrick‡, Cait E. MacPhee‡, Michele Vendruscolo‡, Christopher M. Dobson‡, and Robert G. Griffin*†

J. Phys. Chem. B, 2010, 114 (42), pp 13555–13561
DOI: 10.1021/jp106675h
Publication Date (Web): October 6, 2010
Copyright © 2010 American Chemical Society

Abstract: Amyloid fibrils are structurally ordered aggregates of proteins whose formation is associated with many neurodegenerative and other diseases. For that reason, their high-resolution structures are of considerable interest and have been studied using a wide range of techniques, notably electron microscopy, X-ray diffraction, and magic angle spinning (MAS) NMR. Because of the excellent resolution in the spectra, MAS NMR is uniquely capable of delivering site-specific, atomic resolution information about all levels of amyloid structure: (1) the monomer, which packs into several (2) protofilaments that in turn associate to form a (3) fibril. Building upon our high-resolution structure of the monomer of an amyloid-forming peptide from transthyretin (TTR105−115), we introduce single 1-13C labeled amino acids at seven different sites in the peptide and measure intermolecular carbonyl−carbonyl distances with an accuracy of 0.11 A. Our results conclusively establish a parallel, in register, topology for the packing of this peptide into a β-sheet and provide constraints essential for the determination of an atomic resolution structure of the fibril. Furthermore, the approach we employ, based on a combination of a double-quantum filtered variant of the DRAWS recoupling sequence and multispin numerical simulations in SPINEVOLUTION, is general and should be applicable to a wide range of systems.

Solid-State NMR Study of Cysteine on Gold Nanoparticles

Anuji Abraham, Eugene Mihaliuk, Bharath Kumar, Justin Legleiter, and Terry Gullion*
Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States
J. Phys. Chem. C, 2010, 114 (42), pp 18109–18114
DOI: 10.1021/jp107112b
Publication Date (Web): September 30, 2010
Copyright © 2010 American Chemical Society

Abstract: Solid-state NMR spectroscopy is used to characterize the interaction of l-cysteine with gold nanoparticles. The experiments show that there are two types of cysteine in the gold−cysteine complex, with nearly equal populations. We postulate that cysteine forms a two-layer boundary around the gold nanoparticles. The first layer is made of cysteine molecules forming a thiolate bond with the gold surface and having its charged amino and carboxyl groups oriented away from the gold surface. The second layer has its amino and carboxyl groups oriented toward the first layer and its sulfur group oriented away from the gold particles.

Pore Size Distribution Analysis of Mesoporous TiO2 Spheres by 1H Nuclear Magnetic Resonance (NMR) Cryoporometry

Su-Yeol Ryu, Dong Suk Kim, Jae-Deok Jeon, and Seung-Yeop Kwak*

J. Phys. Chem. C, 2010, 114 (41), pp 17440–17445
DOI: 10.1021/jp105496h
Publication Date (Web): September 21, 2010
Copyright © 2010 American Chemical Society

Abstract: Mesoporous TiO2 spheres with various pore sizes were prepared by varying the calcination temperature in the range of 300−700 °C. Increasing calcination temperature was found to increase the crystal size, decrease the surface area, and increase the pore size. The morphologies of mesoporous TiO2 spheres consist of well-defined spherical shapes of monodisperse sizes near 0.8 μm. To determine the pore size distributions (PSDs) of these mesoporous TiO2 spheres, 1H nuclear magnetic resonance (NMR) cryoporometry and Barrett−Joyner−Halenda (BJH) analysis were conducted. NMR cryoporometry is based on the theory of the melting point depression (MPD) of a probe molecule confined within a pore, which is dependent on the pore diameter. MPD was determined by analyzing the variation of the NMR spin−echo intensity with temperature. From the resulting spin−echo intensity versus temperature (I−T) curves, it was found that the maximum MPD of a probe molecule confined within the pores of mesoporous TiO2 decreases with increasing calcination temperature; that is, the pore size increases with increasing calcination temperature. Because mesoporous TiO2 spheres consist of aggregates of nanocrystallite TiO2 and mesopores located at intercrystallites, an increase in the calcination temperature induces an increase in the crystallite size and, thus, in the pore size because the small pores collapse and the large pores increase in size. We also confirmed by BJH analysis that the pore size of mesoporous TiO2 increases with increasing calcination temperature. This trend is in agreement with our 1H NMR cryoporometry results. Overall, these findings indicate that NMR cryoporometry is a very effective method for determining the PSDs of mesoporous TiO2 spheres.

Defect Functionalization of Hexagonal Boron Nitride Nanosheets

Yi Lin*†, Tiffany V. Williams‡, Wei Cao§, Hani E. Elsayed-Ali§, and John W. Connell‡

J. Phys. Chem. C, 2010, 114 (41), pp 17434–17439
DOI: 10.1021/jp105454w
Publication Date (Web): September 21, 2010
Copyright © 2010 American Chemical Society

Abstract:A pristine hexagonal boron nitride (h-BN) powder sample with layered crystalline sheetlike particles of 1−10 μm in lateral sizes and a few hundred nanometers in thicknesses was mechanically treated using a ball-mill to intentionally introduce defect sites. The h-BN was ball-milled for various times and subsequently was functionalized with a long alkyl chain amine via Lewis acid−base interactions between the amino groups and the boron atoms of h-BN to obtain soluble amine-attached nanosheet samples as the products. The functionalized h-BN nanosheet samples were characterized via various microscopic and spectroscopic techniques. The results strongly support a direct correlation between increasing defect site concentrations of the h-BN nanosheet samples and improved reaction efficiency with the amine. This suggests the enhanced reactivity of defect boron atoms in comparison to conjugated ones on an unperturbed h-BN plane. NMR investigations provided the strongest evidence supporting the hypothesis that the amino groups reacted with the h-BN at specific defect sites induced by ball-milling. The mechanistic implications are discussed.

Access to Well-Defined Ruthenium Mononuclear Species Grafted via a Si−Ru Bond on Silane Functionalized Silica†

Fernando Rascn‡, Romain Berthoud‡, Raphal Wischert‡, Wayne Lukens§, and Christophe Copret*‡

J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp1064962
Publication Date (Web): September 20, 2010
Copyright © 2010 American Chemical Society

Abstract: A functionalized silica with T3 silane surface groups (i.e., (≡SiO)3Si−H) was prepared and interacted with Ru(cod)(cot), resulting in the formation of monometallic surface species attached to the surface via a Si−Ru bond, according to EXAFS spectroscopy, infrared spectroscopy, and solid-state NMR.

Time-Resolved and Site-Specific Insights into Migration Pathways of Li+ in α-Li3VF6 by 6Li 2D Exchange MAS NMR

M. Wilkening*†, E. E. Romanova†‡, S. Nakhal§, D. Weber§, M. Lerch§, and P. Heitjans†

J. Phys. Chem. C, 2010, 114 (44), pp 19083–19088
DOI: 10.1021/jp103433h
Publication Date (Web): September 14, 2010
Copyright © 2010 American Chemical Society

Abstract: Two-dimensional (2D) exchange nuclear magnetic resonance (NMR) spectroscopy carried out under magic angle spinning (MAS) conditions is ideally suited to study site-specific Li diffusion parameters of cathode materials required for the target-oriented development of so-called high-energy density 4 V-lithium-ion batteries. In the present study, we took advantage of Li NMR hyperfine shifts to record temperature-variable 1D and mixing-time dependent 2D exchange MAS 6Li NMR spectra on α-Li3VF6 serving as both a potential cathode material as well as an application-oriented model substance with three magnetically inequivalent Li sites. By comparing the NMR results with structural details of the material we were able to obtain detailed insights into the migration pathways and Li exchange rates which are of the order of some hundreds of Li jumps per second at approximately 340 K. Site-specific Li jump rates τ−1 reveal the electrochemically active sites and provide information how to modify the material in order to increase its relatively low Li diffusivity found at room temperature.

Adsorbate Effect on AlO4(OH)2 Centers in the Metal−Organic Framework MIL-53 Investigated by Solid-State NMR Spectroscopy

Christian Lieder, Sabine Opelt, Michael Dyballa, Harald Henning, Elias Klemm, and Michael Hunger*

J. Phys. Chem. C, 2010, 114 (39), pp 16596–16602
DOI: 10.1021/jp105700b
Publication Date (Web): September 10, 2010
Copyright © 2010 American Chemical Society

Abstract:1H and 27Al MAS NMR spectroscopies have been applied for studying the effect of water molecules, nitrogen bases, and o-xylene on the hydroxyl protons of bridging AlOH groups and framework aluminum atoms in the metal−organic framework (MOF) MIL-53. For water molecules adsorbed on the low-temperature form MIL-53lt, two 1H MAS NMR signals were found indicating the formation of different O−H···O hydrogen bonds to neighboring oxygen atoms, such as to carboxylic oxygens. Upon adsorption of the nitrogen bases acetonitrile, ammonia, and pyridine, a linear increase of the quadrupole coupling constant, CQ, of the framework aluminum atoms in dehydrated MIL-53 from CQ = 8.5 MHz (unloaded material) to maximum 10.8 MHz (pyridine-loaded material) as a function of the proton affinity of the adsorbates was observed. Adsorption of o-xylene led to three stepwise changes of the quadrupole coupling constants, CQ, of framework aluminum atoms in dehydrated MIL-53. While the first two stepwise changes of the CQ values (CQ = 8.0 and 8.7 MHz) occur for o-xylene loadings of lower than 4 molecules per unit cell and for all AlO4(OH)2 centers, the third change of the CQ value to 9.4 MHz was observed for o-xylene loadings higher than 4 o-xylene molecules per unit cell and for maximum 50% of the framework aluminum atoms. This third adsorbate-induced change of the CQ value of framework aluminum atoms in MIL-53 is accompanied by a significant decrease of the adsorbate mobility

Impact of Controlling the Site Distribution of Al Atoms on Catalytic Properties in Ferrierite-Type Zeolites†

Yuriy Romn-Leshkov, Manuel Moliner, and Mark E. Davis*
Chemical Engineering, California Institute of Technology, Pasadena, California 91125
J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp106247g
Publication Date (Web): September 9, 2010
Copyright © 2010 American Chemical Society

Abstract: Zeolites with the ferrierite (FER) topology are synthesized using a combination of tetramethylammonium (TMA) cations with differently sized cyclic amines (pyrrolidine (Pyr), hexamethyleneimine (HMI), and 1,4-diazabicyclo[2.2.2]octane (DAB)). Using these organic structure-directing agents (SDAs), low Si/Al ratios and concentrated synthesis mixtures favor the crystallization of FER materials. Increasing the size of the cyclic amine or decreasing the aluminum content leads to the crystallization of other phases or the creation of excessive amounts of connectivity defects. TMA cations play a decisive role in the synthesis of the FER materials, and their presence allows the use of HMI to synthesize FER. Proton MAS NMR is used to quantify the accessibility of pyridine to acid sites in these FER samples, where it is found that the FER+HMI+TMA sample contains only 27% acid sites in the 8-MR channels, whereas FER+Pyr and FER+Pyr+TMA contain 89% and 84%, respectively. The constraint index (CI) test and the carbonylation of dimethyl ether (DME) with carbon monoxide are used as probe reactions to evaluate how changes in the aluminum distribution in these FER samples affect their catalytic behavior. Results show that the use of Pyr as an SDA results in the selective population of acid sites in the 8-MR channels, whereas the use of HMI generates FER zeolites with an increased concentration of acid sites in the 10-MR channels.

J. Phys. Chem. B and C, volume 114, Issues 43 - 45 + November ASAPs

2010-12-01T15:25:00.006-05:00

Quantum Oscillations and Polarization of Nuclear Spins in Photoexcited Triplet States†

Gerd Kothe*‡, Tomoaki Yago‡, Jrg-Ulrich Weidner‡, Gerhard Link‡, Michail Lukaschek‡, and Tien-Sung Lin§

J. Phys. Chem. B, 2010, 114 (45), pp 14755–14762
DOI: 10.1021/jp103508t
Publication Date (Web): July 28, 2010
Copyright © 2010 American Chemical Society

Abstract: The unique physical properties of photoexcited triplet states have been explored in numerous spectroscopic studies employing electron paramagnetic resonance (EPR). So far, however, no quantum interference effects were found in these systems in the presence of a magnetic field. In this study, we report the successful EPR detection of nuclear quantum oscillations in an organic triplet state subject to an external magnetic field. The observed quantum coherences can be rationalized using an analytical theory. Analysis suggests that the nuclear spins are actively involved in the intersystem crossing process. The novel mechanism also acts as a source of oscillatory nuclear spin polarization that gives rise to large signal enhancement in nuclear magnetic resonance (NMR). This opens new perspectives for the analysis of chemically induced dynamic nuclear polarization in mechanistic studies of photoactive proteins.

W/Mo-Oxide Nanomaterials: Structure−Property Relationships and Ammonia-Sensing Studies†

Ying Zhou‡, Kaibo Zheng§, Jan-Dierk Grunwaldt, Thomas Fox‡, Leilei Gu§, Xiaoliang Mo§, Guorong Chen§, and Greta R. Patzke*‡

J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp106439n
Publication Date (Web): November 30, 2010
Copyright © 2010 American Chemical Society

Abstract:W/Mo-oxides of the hexagonal tungsten bronze (HTB) type have been investigated by X-ray absorption spectroscopy to obtain detailed insight into the substitution process of W by Mo that leads to mixed HTB frameworks. Both the morphology of the nanostructured W/Mo-HTBs as well as the oxidation state of Mo are significantly influenced through the incorporation of different alkali cations into the hexagonal channels of this open structure. A variety of complementary analytical methods, including TG, in situ and ex situ XRD, SEM, and solid-state NMR analyses, were applied to determine the thermal stability of the obtained W/Mo-HTB materials with respect to their alkali cation and NH4+ contents. A strong correlation between composition and stability was found with the Rb-W/Mo-HTBs exhibiting the highest structural and morphological resistance among the series (up to 580 °C). The NH3-sensing properties of selected W/Mo-oxides in test atmospheres furthermore point to promising features of the Rb-stabilized hexagonal framework materials

Hydrogen Physisorption in a Cu(II) Metallacycle

Tanja Pietraβ*†, Itza Cruz-Campa‡, Justine Kombarakkaran†, Suman Sirimulla§, Atta M. Arif§, and Juan C. Noveron*‡
J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp104544r
Publication Date (Web): November 19, 2010
Copyright © 2010 American Chemical Society

Abstract: The interaction of molecular hydrogen with a novel microporous dinuclear Cu(II) complex, [bis-μ-di(4-pyridyl)methanol-1,4,7-triazacyclononane copper(II)] triflate (1), and its derivatives formed from oxidation and solvent removal was studied with 2H NMR and density functional theory (DFT). The Cu-complex 1 was characterized with X-ray diffraction methods and consists of a dinuclear macrocycle that forms one-dimensional channels of 9.55 Å in diameter. 2H NMR studies of deuterium gas adsorption by 1 suggest that physisorption condensation of D2 occurs within two distinct microenvironments: in the interior and in-between the microtubular structures. The assignment of NMR resonances to specific adsorption sites is supported by spectral decomposition and analysis of the line widths and integrated signal intensities of the components. The dynamics of the system are probed by spin−lattice relaxation time measurements and spectral hole-burn experiments as a function of temperature and pressure. NMR and DFT calculations suggest that hydrogen uptake is mediated through interactions with the Cu(II) centers via dipole−ion interactions.

Influence of Structure on the Spectroscopic Properties of the Polymorphs of Piroxicam

Wei Liu†, Wei David Wang†, Wei Wang†, Shi Bai*†‡, and Cecil Dybowski‡

J. Phys. Chem. B, ASAP
DOI: 10.1021/jp1084444
Publication Date (Web): November 18, 2010
Copyright © 2010 American Chemical Society

Abstract: The complete 13C NMR chemical-shift tensors for the carbon sites of the two polymorphic forms (PI and PII) and the monohydrate form (PM) of the analgesic drug, piroxicam, are reported. The NMR parameters (isotropic chemical shifts, chemical-shielding anisotropies and asymmetries, and dipolar couplings), X-ray powder diffraction, and density functional calculations of piroxicam are analyzed in terms of hydrogen bonding and structure. The integration of all the data gives an improved model of the local solid-state structures of the polymorphs. In particular, the solid-state NMR spectra demonstrate that the asymmetric unit of the monohydrate, PM, contains two zwitterionic piroxicam molecules.

Heterogeneities in Gelatin Film Formation Using Single-Sided NMR

Sushanta Ghoshal*, Carlos Mattea, Paul Denner, and Siegfried Stapf

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp1068363
Publication Date (Web): November 18, 2010
Copyright © 2010 American Chemical Society

Abstract: Gelatin solutions were prepared in D2O. The drying process of cast solutions was followed with a single-sided nuclear magnetic resonance (NMR) scanner until complete solidification occurred. Spin−spin relaxation times (T2) were measured at different layers with microscopic resolution and were correlated with the drying process during film formation. Additionally, the evaporation of the gelatin solution was observed optically from the reduction of the sample thickness, revealing that at the macroscopic level, the rate of evaporation is not uniform throughout the experiment. A crossover in the spatial evolution of the drying process is observed from the NMR results. At the early stages, the gel appears to be drier in the upper layers near the evaporation front, while this tendency is inverted at the later stages, when drying is faster from the bottom. XRD (X-ray diffraction) data showed that a structural heterogeneity persists in the final film.

Understanding the Properties of the Coagel and Gel Phases: A 2H and 13C NMR Study of Amphiphilic Ascorbic Acid Derivatives

Silvia Borsacchi†, Moira Ambrosi‡, Pierandrea Lo Nostro‡, and Marco Geppi*†

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp107324e
Publication Date (Web): November 15, 2010
Copyright © 2010 American Chemical Society

Abstract: The coagel and gel phases formed by the d and l diastereoisomers of ascorbyl-dodecanoate (ASC12) in deuterated water were studied through solid-state NMR techniques. In particular, the dynamic properties of water and surfactant chains were investigated by 2H and 13C NMR static spectra, respectively. Two fractions of water with very different dynamics were found in the coagel phases, one solidlike and one liquidlike, assigned to water strongly bound to the surfactant polar heads and bulk water, respectively. Only one kind of “intermediate” water was instead detected in the gel phase suggesting that the merging of the two types of water in the interlayers between the surfactant lamellae occurs at the coagel-to-gel transition. Moreover, the surfactant chains, very rigid in the coagel phase, give rise to fast trans−gauche interconformational jumps in the gel phase, where almost isotropic reorientations of the whole aggregates also occur. A different dynamic behavior was found for the two diastereoisomers in particular for what concerns the surfactant molecules in the gel phase and the water molecules in the coagel presumably ascribable to different inter- and intramolecular interactions that involve the polar heads

The “Alkyl” and “Carbenium” Pathways of Methane Activation on Ga-Modified Zeolite BEA: 13C Solid-State NMR and GC-MS Study of Methane Aromatization in the Presence of Higher Alkane

Mikhail V. Luzgin, Anton A. Gabrienko, Vladimir A. Rogov, Alexander V. Toktarev, Valentin N. Parmon, and Alexander G. Stepanov*
J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp1078899
Publication Date (Web): November 11, 2010
Copyright © 2010 American Chemical Society

Abstract:By using 13C solid-state NMR spectroscopy and GC-MS analysis, the activation of methane and coaromatization of methane and propane have been monitored on gallium-modified zeolite BEA at 573−823 K. A noticeable degree involvement of the 13C-label from methane-13C into the aromatic reaction products (benzene, toluene) has been demonstrated. The major intermediate of the methane activation represents gallium-methyl species, which are formed by methane dissociative adsorption on Ga2O3 species of the zeolite. The minor species of methane activation, Ga-methoxy groups, provide the involvement of methane into aromatics by the methylation of aromatic molecules, which are generated exclusively from propane, by the mechanism of electrophilic substitution. Ga-methyl species can serve as methylating nucleophilic agent for the reaction of nucleophilic substitution with participation of aromatic molecules, which contain the electron-withdrawing substitutes.

Slow Exchange Model of Nonrigid Rotational Motion in RNA for Combined Solid-State and Solution NMR Studies

Prashant S. Emani†, Gregory L. Olsen‡, Dorothy C. Echodu‡, Gabriele Varani‡§, and Gary P. Drobny*‡

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp107193z
Publication Date (Web): November 10, 2010
Copyright © 2010 American Chemical Society

Abstract: Functional RNA molecules are conformationally dynamic and sample a multitude of dynamic modes over a wide range of frequencies. Thus, a comprehensive description of RNA dynamics requires the inclusion of a broad range of motions across multiple dynamic rates which must be derived from multiple spectroscopies. Here we describe a slow conformational exchange theoretical approach to combining the description of local motions in RNA that occur in the nanosecond to microsecond window and are detected by solid-state NMR with nonrigid rotational motion of the HIV-1 transactivation response element (TAR) RNA in solution as observed by solution NMR. This theoretical model unifies the experimental results generated by solution and solid-state NMR and provides a comprehensive view of the dynamics of HIV-1 TAR RNA, a well-known paradigm of an RNA where function requires extensive conformational rearrangements. This methodology provides a quantitative atomic level view of the amplitudes and rates of the local and collective displacements of the TAR RNA molecule and provides directly motional parameters for the conformational capture hypothesis of this classical RNA−ligand interaction.

Molecular Dynamics of Amorphous Gentiobiose Studied by Solid-State NMR

Teresa G. Nunes*†, Hermnio P. Diogo†, Susana S. Pinto†, and Joaquim J. Moura Ramos‡

J. Phys. Chem. B, Article ASAP
DOI: 10.1021/jp106371w
Publication Date (Web): November 10, 2010
Copyright © 2010 American Chemical Society

Abstract: A solid-state NMR (SSNMR) study is reported on the effect of temperature on the molecular mobility of amorphous gentiobiose, which is complemented with data obtained from crystalline samples. 13C cross-polarization/magic-angle-spinning (CPMAS) spectra and 1H MAS spectra were obtained for gentiobiose at natural abundance, in the amorphous state, from 293 K up to the glass transformation region (Tg = 359 K). Two well-defined molecular mobility regimes were observed, corresponding to different motional modes. NMR results on molecular dynamics are discussed and compared with those obtained by thermally stimulated depolarization currents (TSDC) and dielectric relaxation spectroscopy (DRS). SSNMR spectra presented evidence for a new polymorphic form of gentiobiose, not yet reported in the literature, which is obtained by slow heating of the amorphous solid up to 364 K inside the NMR zirconia rotor.

73Ge Solid-State NMR of Germanium Oxide Materials: Experimental and Theoretical Studies

Vladimir K. Michaelis and Scott Kroeker*

J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp1071082
Publication Date (Web): November 10, 2010
Copyright © 2010 American Chemical Society

Abstract: A comprehensive series of crystalline germanates has been studied by ultrahigh-field 73Ge NMR and quantum chemical calculations. Despite its low gyromagnetic ratio, low natural abundance and large quadrupole moment, interpretable spectra were obtained in almost all cases, demonstrating that 73Ge is an accessible NMR nucleus. The spectra yield a wide range of quadrupole coupling constants (CQ = 9 to 35 MHz), with calculations indicating a range twice that, which are rationalized principally in terms of the variation in Ge−O bond lengths. The isotropic chemical shifts appear to fall into distinct regions for four-, five-, and six-coordinate Ge, with increasing coordination number corresponding to lower frequencies. Both CASTEP and WIEN2k consistently underestimate the CQs, suggesting that the exchange-correlation functional is poorly optimized for these systems. 73Ge NMR spectra of alkali germanate glasses are broad and featureless, rendering them difficult to interpret in terms of specific structural elements, even with the well understood NMR parameters from the crystalline systems. This study represents the first systematic 73Ge NMR investigation of solids, and shows that valuable structural information can be obtained in favorable cases.

NMR Study of LiBH4 with C60

David T. Shane*†, Robert L. Corey‡, Laura H. Rayhel†, Matthew Wellons§, Joseph A. Teprovich, Jr.§, Ragaiy Zidan§, Son-Jong Hwang, Robert C. Bowman, Jr., and Mark S. Conradi†

J. Phys. Chem. C, 2010, 114 (46), pp 19862–19866
DOI: 10.1021/jp107911u
Publication Date (Web): November 3, 2010
Copyright © 2010 American Chemical Society

Abstract:LiBH4 doped with 1.6 mol % well-dispersed C60 is studied with solid-state nuclear magnetic resonance (NMR). Variable-temperature hydrogen NMR shows large changes between the data upon first heating and after exposure to 300 °C. After heating, a large fraction on the order of 50% of the hydrogen signal appears in a motionally narrowed peak, similar to a previous report of LiBH4 in a porous carbon aerogel nanoscaffold. Magic-angle spinning (MAS) NMR of 13C in a 13C-enriched sample finds the C60 has reacted already in the as-mixed (unheated) material. Dehydriding and rehydriding result in further 13C spectral changes, with nearly all intensity being found in a broad peak corresponding to aromatic carbons. It thus appears that the previously reported improved dehydriding and rehydriding kinetics of this material at least partially result from in situ formation of a carbon framework. The method may offer a new route to dispersal of hydrides in carbon support structures.

Investigation of Si Atom Migration in the Framework of MSE-Type Zeolite YNU-2

Takuji Ikeda*†, Satoshi Inagaki‡, Taka-aki Hanaoka†, and Yoshihiro Kubota‡

J. Phys. Chem. C, 2010, 114 (46), pp 19641–19648
DOI: 10.1021/jp1079586
Publication Date (Web): November 2, 2010
Copyright © 2010 American Chemical Society

Abstract: The change in distribution of Si atom defects in the framework of zeolite YNU-2 by steam treatment was investigated using powder X-ray diffraction and solid-state NMR spectroscopy. The precursor of zeolite YNU-2 (abbreviated to YNU-2P) with a three-dimensional pore system has a large number of Si atom defects (more than 10% of all T sites in a unit cell) around the supercage. These defect sites were confirmed by observation of a Q3 ((−SiO)3Si−OH) resonance peak by 29Si magic angle spinning NMR spectroscopy. We have shown that steam treatment of YNU-2P at 523 K for 24 h significantly decreases the relative intensity ratio of the observed Q3 resonance peak for the Q4((−SiO)4Si) peak. The Rietveld analysis of steam-treated YNU-2P (YNU-2PST) shows a marked increase in the site occupancies of the defective Si sites. Furthermore, the Si atom defects in YNU-2PST almost disappeared after calcination, yielding siliceous zeolite YNU-2. These results indicate that the defective framework structure was almost restored by steam treatment. Experimental results suggest that Si atom migration in the framework of YNU-2P takes place during steam treatment. The migrated Si atom fragment fills defect sites and is connected with adjacent silanol groups. In addition, the quantity of the half amount of structure-directing agent molecules was removed from the micropores in YNU-2PST by steam treatment.

Ryan M. Ravenelle†, Florian Schüβler‡, Andrew D’Amico†, Nadiya Danilina§, Jeroen A. van Bokhoven§, Johannes A. Lercher‡, Christopher W. Jones†, and Carsten Sievers*†

J. Phys. Chem. C, 2010, 114 (46), pp 19582–19595
DOI: 10.1021/jp104639e
Publication Date (Web): November 2, 2010
Copyright © 2010 American Chemical Society

Abstract: Zeolites Y and ZSM-5 with varying Si/Al ratios are treated in liquid water at 150 and 200 °C under autogenic pressure to assess their hydrothermal stability. The changes in the structure are characterized by atomic absorption spectroscopy, X-ray diffraction, scanning electron microscopy, argon physisorption, 27Al and 29Si MAS NMR spectroscopy, temperature-programmed desorption of ammonia, and pyridine adsorption followed by IR spectroscopy. During treatment in hot water, zeolite Y with a Si/Al ratio of 14 or higher is transformed into an amorphous material, and the rate of this degradation increases with increasing Si/Al ratio. In contrast, ZSM-5 is not modified under the same conditions. The main degradation mechanism is suggested to be hydrolysis of the siloxane bonds (Si−O−Si) as opposed to dealumination, which dominates under steaming conditions. In the resulting amorphous phase, Al remains tetrahedrally coordinated, but the micropore volume and concentration of accessible acid sites is reduced dramatically. The results demonstrate that potential structural changes of zeolites have to be considered when these materials are used as catalysts for aqueous phase conversion of biomass.

Structure and Characterization of KSc(BH4)4

Radovan ern*†, Dorthe B. Ravnsbæk‡, Godwin Severa§, Yaroslav Filinchuk, Vincenza D’ Anna, Hans Hagemann, Drthe Haase#, Jørgen Skibsted‡, Craig M. Jensen*§, and Torben R. Jensen*‡

J. Phys. Chem. C, 2010, 114 (45), pp 19540–19549
DOI: 10.1021/jp106280v
Publication Date (Web): October 25, 2010
Copyright © 2010 American Chemical Society

Abstract: A new potassium scandium borohydride, KSc(BH4)4, is presented and characterized by a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. The title compound, KSc(BH4)4, forms at ambient conditions in ball milled mixtures of potassium borohydride and ScCl3 together with a new ternary chloride K3ScCl6, which is also structurally characterized. This indicates that the formation of KSc(BH4)4 differs from a simple metathesis reaction, and the highest scandium borohydride yield (31 mol %) can be obtained with a reactant ratio KBH4:ScCl3 of 2:1. KSc(BH4)4 crystallizes in the orthorhombic crystal system, a = 11.856(5), b = 7.800(3), c = 10.126(6) Å, V = 936.4(8) Å3 at RT, with the space group symmetry Pnma. KSc(BH4)4 has a BaSO4 type structure where the BH4 tetrahedra take the oxygen positions. Regarding the packing of cations, K+, and complex anions, [Sc(BH4)4]−, the structure of KSc(BH4)4 can be seen as a distorted variant of orthorhombic neptunium, Np, metal. Thermal expansion of KSc(BH4)4 in the temperature range RT to 405 K is anisotropic, and the lattice parameter b shows strong nonlinearity upon approaching the melting temperature. The vibrational and NMR spectra are consistent with the structural model, and previous investigations of the related compounds ASc(BH4)4 with A = Li, Na. KSc(BH4)4 is stable from RT up to 405 K, where the compound melts and then releases hydrogen in two rapid steps approximately at 460−500 K and 510−590 K. The hydrogen release involves the formation of KBH4, which reacts with K3ScCl6 and forms a solid solution, K(BH4)1−xClx. The ternary potassium scandium chloride K3ScCl6 observed in all samples has a monoclinic structure at room temperature, P21/a, a = 12.729(3), b = 7.367(2), c = 12.825(3) Å, β = 109.22(2)°, V = 1135.6(4) Å3, which is isostructural to K3MoCl6. The monoclinic polymorph transforms to cubic at 635 K, a = 10.694 Å (based on diffraction data measured at 769 K), which is isostructural to the high temperature phase of K3YCl6.

Phase Behavior and 13C NMR Spectroscopic Analysis of the Mixed Methane + Ethane + Propane Hydrates in Mesoporous Silica Gels

Seungmin Lee, Inuk Cha, and Yongwon Seo*

J. Phys. Chem. B, 2010, 114 (46), pp 15079–15084
DOI: 10.1021/jp108037m
Publication Date (Web): October 21, 2010
Copyright © 2010 American Chemical Society

Abstract: In this study, the phase behavior and quantitative determination of hydrate composition and cage occupancy for the mixed CH4 + C2H6 + C3H8 hydrates were closely investigated through the experimental measurement of three-phase hydrate (H)−water-rich liquid (LW)−vapor (V) equilibria and 13C NMR spectra. To examine the effect of pore size and salinity, we measured hydrate phase equilibria for the quaternary CH4 (90%) + C2H6 (7%) + C3H8 (3%) + water mixtures in silica gel pores of nominal diameters of 6.0, 15.0, and 30.0 nm and for the quinary CH4 (90%) + C2H6 (7%) + C3H8 (3%) + NaCl + water mixtures of two different NaCl concentrations (3 and 10 wt %) in silica gel pores of a nominal 30.0 nm diameter. The value of hydrate−water interfacial tension for the CH4 (90%) + C2H6 (7%) + C3H8 (3%) hydrate was found to be 47 ± 4 mJ/m2 from the relation of the dissociation temperature depression with the pore size of silica gels at a given pressure. At a specified temperature, three-phase H−LW−V equilibrium curves of pore hydrates were shifted to higher pressure regions depending on pore sizes and NaCl concentrations. From the cage-dependent 13C NMR chemical shifts of enclathrated guest molecules, the mixed CH4 (90%) + C2H6 (7%) + C3H8 (3%) gas hydrate was confirmed to be structure II. The cage occupancies of each guest molecule and the hydration number of the mixed gas hydrates were also estimated from the 13C NMR spectra.

Biomimetic Apatite Mineralization Mechanisms of Mesoporous Bioactive Glasses as Probed by Multinuclear 31P, 29Si, 23Na and 13C Solid-State NMR

Philips N. Gunawidjaja†, Andy Y. H. Lo†, Isabel Izquierdo-Barba‡§, Ana Garca‡§, Daniel Arcos‡§, Baltzar Stevensson†, Jekabs Grins, Mara Vallet-Reg‡§, and Mattias Edn*†

J. Phys. Chem. C, 2010, 114 (45), pp 19345–19356
DOI: 10.1021/jp105408c
Publication Date (Web): October 21, 2010
Copyright © 2010 American Chemical Society

Abstract: An array of magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy experiments is applied to explore the surface reactions of a mesoporous bioactive glass (MBG) of composition Ca0.10Si0.85P0.04O1.90 when subjected to a simulated body fluid (SBF) for variable intervals. Powder X-ray diffraction and 31P NMR techniques are employed to quantitatively monitor the formation of an initially amorphous calcium phosphate surface layer and its subsequent crystallization into hydroxycarbonate apatite (HCA). Prior to the onset of HCA formation, 1H → 29Si cross-polarization (CP) NMR evidence dissolution of calcium ions; a slightly increased connectivity of the speciation of silicate ions is observed at the MBG surface over 1 week of SBF exposure. The incorporation of carbonate and sodium ions into the bioactive orthophosphate surface layer is explored by 1H → 13C CPMAS and 23Na NMR, respectively. We discuss similarities and distinctions in composition−bioactivity relationships established for traditional melt-prepared bioglasses compared to MBGs. The high bioactivity of phosphorus-bearing MBGs is rationalized to stem from an acceleration of their surface reactions due to presence of amorphous calcium orthophosphate clusters of the MBG pore wall.

Analysis of the 7Li NMR signals in the Monoclinic Li3Fe2(PO4)3 and Li3V2(PO4)3 Phases
A. Castets, D. Carlier*, K. Trad, C. Delmas, and M. Mntrier

J. Phys. Chem. C, 2010, 114 (44), pp 19141–19150
DOI: 10.1021/jp106871z
Publication Date (Web): October 21, 2010
Copyright © 2010 American Chemical Society

Abstract: The monoclinic Li3Fe2(PO4)3 and Li3V2(PO4)3 phosphates are materials for positive electrodes in Li-ion batteries. They also have interesting structures to test and improve the understanding of Li NMR signals in paramagnetic compounds. The position of such signals is governed by the transfer of electron spin density from the transition metal ion to the Li nucleus. These mechanisms are based on delocalization and polarization effects which induce positive and negative Fermi contact shifts, respectively. We have characterized Li3Fe2(PO4)3 by Li NMR. To understand the signals observed, we have analyzed the electron spin density transfer mechanisms (i) by considering the different Li environments, (ii) by using DFT calculations. We compare our analysis to the one very recently reported by Davis et al. These analyses have been extended to Li3V2(PO4)3 studied by NMR by Cahill et al.

Mechanically, Magnetically, and “Rotationally Aligned” Membrane Proteins in Phospholipid Bilayers Give Equivalent Angular Constraints for NMR Structure Determination

Sang Ho Park, Bibhuti B. Das, Anna A. De Angelis, Mario Scrima, and Stanley J. Opella*

J. Phys. Chem. B, 2010, 114 (44), pp 13995–14003
DOI: 10.1021/jp106043w
Publication Date (Web): October 20, 2010
Copyright © 2010 American Chemical Society

Abstract: The native environment for membrane proteins is the highly asymmetric phospholipid bilayer, and this has a large effect on both their structure and dynamics. Reproducing this environment in samples suitable for spectroscopic and diffraction experiments is a key issue, and flexibility in sample preparation is essential to accommodate the diverse size, shape, and other physical properties of membrane proteins. In most cases, to ensure that the biological activities are maintained, this means reconstituting the proteins in fully hydrated planar phospholipid bilayers. The asymmetric character of protein-containing bilayers means that it is possible to prepare either oriented or unoriented (powder) samples. Here we demonstrate the equivalence of mechanical, magnetic, and what we refer to as “rotational alignment” of membrane proteins in phospholipid bilayer samples for solid-state NMR spectroscopy. The trans-membrane domain of virus protein “u” (Vpu) from human immunodeficiency virus (HIV-1) and the full-length membrane-bound form of fd bacteriophage coat protein in phospholipid bilayers are used as examples. The equivalence of structural constraints from oriented and unoriented (powder) samples of membrane proteins is based on two concepts: (1) their alignment is defined by the direction of the bilayer normal relative to the magnetic field and (2) they undergo rapid rotational diffusion about the same bilayer normal in liquid crystalline membranes. The measurement of angular constraints relative to a common external axis system defined by the bilayer normal for all sites in the protein is an essential element of oriented sample (OS) solid-state NMR.

Controlled Interactions between Anhydrous Keggin-Type Heteropolyacids and Silica Support: Preparation and Characterization of Well-Defined Silica-Supported Polyoxometalate Species

Eva Grinenval†, Xavier Rozanska§, Anne Baudouin†, Elise Berrier‡, Franoise Delbecq§, Philippe Sautet§, Jean-Marie Basset†, and Frdric Lefebvre*†

J. Phys. Chem. C, 2010, 114 (44), pp 19024–19034
DOI: 10.1021/jp107317s
Publication Date (Web): October 20, 2010
Copyright © 2010 American Chemical Society

Abstract:Anhydrous Keggin-type phosphorus heteropolyacids were deposited on partially dehydroxylated silica by using the surface organometallic chemistry (SOMC) strategy. The resulting solids were characterized by a combination of physicochemical methods including IR, Raman, 1D and 2D 1H, and 31P MAS NMR, electron microscopy experiments and density functional theory (DFT) calculations. It is shown that the main surface species is [≡Si(OH...H+)]2[H+]1[PM12O403−] where the polyoxometalate is linked to the support by proton interaction with two silanols. Two other minor species (10% each) are formed by coordination of the polyoxometalate to the surface via the interaction between all three protons with three silanol groups or via three covalent bonds formed by dehydroxylation of the above species. Comparison of the reactivity of these solids and of compounds prepared by a classical way shows that the samples prepared by the SOMC approach contain ca. 7 times more acid sites

Thermal Spreading As an Alternative for the Wet Impregnation Method: Advantages and Downsides in the Preparation of MoO3/SiO2−Al2O3 Metathesis Catalysts

Damien P. Debecker*†, Mariana Stoyanova‡, Uwe Rodemerck‡, Pierre Eloy†, Alexandre Lonard§, Bao-Lian Su§, and Eric M. Gaigneaux*†

J. Phys. Chem. C, 2010, 114 (43), pp 18664–18673
DOI: 10.1021/jp1074994
Publication Date (Web): October 14, 2010
Copyright © 2010 American Chemical Society

Abstract:Silica−alumina-supported MoO3 catalysts are classically prepared via impregnation of the support with a molybdenum salt solution, usually ammonium heptamolybdate, and subsequent drying and calcination (three steps). The downsides of such a route for the synthesis of heterogeneous metathesis catalysts are linked to the limited control on the nature of the MoOx stabilized at the surface, to the uneven distribution of the deposit in the pores of the support, and to the build up of inactive species that find their origin in the wet step of the preparation. In opposition, the direct thermal spreading of molybdenum oxide onto the support is a straightforward (one step) method involving no wet stage. It allows the conversion of bulk MoO3 crystals to amorphous molybdate species dispersed at the surface of the silica−alumina support. This contribution shows that the catalysts obtained via both methods exhibit similar performances in the self-metathesis of propene to butene and ethene. However, based on XRD, XPS, Raman spectroscopy, ICP-AES, N2 physisorption, TEM, and MAS-NMR spectroscopy, it is shown that the origin of active and inactive species in the two systems is different. Whereas the activity of wet-made catalysts is limited by the formation of bulky MoO3 crystals and of aluminum molybdate, the performances of dry-made catalysts are limited by the incomplete spreading of MoO3 nanocrystallites.

On the Performance of Spin Diffusion NMR Techniques in Oriented Solids: Prospects for Resonance Assignments and Distance Measurements from Separated Local Field Experiments

Nathaniel J. Traaseth†, T. Gopinath†, and Gianluigi Veglia*†‡

J. Phys. Chem. B, 2010, 114 (43), pp 13872–13880
DOI: 10.1021/jp105718r
Publication Date (Web): October 11, 2010
Copyright © 2010 American Chemical Society

Abstract: NMR spin diffusion experiments have the potential to provide both resonance assignment and internuclear distances for protein structure determination in oriented solid-state NMR. In this paper, we compared the efficiencies of three spin diffusion experiments: proton-driven spin diffusion (PDSD), cross-relaxation-driven spin diffusion (CRDSD), and proton-mediated proton transfer (PMPT). As model systems for oriented proteins, we used single crystals of N-acetyl-L-15N-leucine (NAL) and N-acetyl-L-15N-valyl-L-15N-leucine (NAVL) to probe long and short distances, respectively. We demonstrate that, for short 15N/15N distances such as those found in NAVL (3.3 Å), the PDSD mechanism gives the most intense cross-peaks, while, for longer distances (>6.5 Å), the CRDSD and PMPT experiments are more efficient. The PDSD was highly inefficient for transferring magnetization across distances greater than 6.5 Å (NAL crystal sample), due to small 15N/15N dipolar couplings (<4.5>

Bryan's Blog Update, Part II

2010-11-10T13:02:00.023-05:00

Hope you're ready for more SSNMR literature!

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Chem. Commun., 2010, 46, 4514-4516
DOI: 10.1039/B924936B , Communication

On-line monitoring of a microwave-assisted chemical reaction by nanolitre NMR-spectroscopy

M. Victoria Gomez, Hein H. J. Verputten, Angel Díaz-Ortíz, Andres Moreno, Antonio de la Hoz and Aldrik H. Velders

Abstract

We report the use of a nanolitre nuclear magnetic resonance (NMR) spectroscopy microfluidic chip hyphenated to a continuous-flow microlitre-microwave irradiation set-up, for on-line monitoring and rapid optimization of reaction conditions.

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Chem. Commun., 2010, 46, 4532-4534

DOI: 10.1039/C0CC00113A , Communication

Rational synthesis, enrichment, and ¹³C NMR spectra of endohedral C₆₀ and C₇₀ encapsulating a helium atom

Yuta Morinaka, Fumiyuki Tanabe, Michihisa Murata, Yasujiro Murata and Koichi Komatsu

Abstract

Endohedral fullerenes encapsulating a helium atom, i.e., He@C₆₀ and He@C₇₀, at occupation levels of 30% were prepared by rational chemical synthesis. The existence of weak interactions between the inner helium and the outer fullerene cages was demonstrated by experimental and computational investigations.

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Chem. Commun., 2010, 46, 4982-4984
DOI: 10.1039/C0CC01007C

Direct observation of a transient polymorph during crystallization

Colan E. Hughes and Kenneth D. M. Harris

Abstract

Application of a technique developed for in situ solid-state ¹³C NMR studies of crystallization processes reveals direct evidence that crystallization of glycine from a methanol/water solution involves the initial transient formation of the β polymorph, which then undergoes a solution-mediated polymorphic transformation to yield the more stable α polymorph.

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Chem. Commun., 2010, 46, 5879-5881
DOI: 10.1039/C0CC01271H

Probing heterocycle conformation with residual dipolar couplings

Chakicherla Gayathri, M. Carmen de la Fuente, Burkhard Luy, Roberto R. Gil and Armando Navarro-Vázquez

Abstract

Residual dipolar couplings (RDCs) obtained in a stretched polydimethylsiloxanegel are applied to determine the 7-membered ring conformation in a 2-phenyl-3-benzazepine derivative, and to simultaneously assign all methyleneproton pairs using only ¹D_CH RDCs and DFT molecular modelling data.

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Chem. Commun., 2010, 46, 6714-6716
DOI: 10.1039/C0CC00829J

Solid-state NMR evidence for elastin-like β-turn structure in spider dragline silk

Janelle E. Jenkins, Melinda S. Creager, Emily B. Butler, Randolph V. Lewis, Jeffery L. Yarger and Gregory P. Holland

Abstract

Two-dimensional homo- and heteronuclear solid-state MAS NMR experiments on ¹³C/¹⁵N-proline labeled Argiope aurantia dragline silk provide evidence for an elastin-like β-turn structure for the repetitive Gly-Pro-Gly-X-X motif prevalent in major ampullate spidroin 2 (MaSp2).

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Chem. Commun., 2010, 46, 6774-6776
DOI: 10.1039/C0CC01902J

Ultra-wideline ¹⁴N NMR spectroscopy as a probe of molecular dynamics

Luke A. O'Dell and Christopher I. Ratcliffe

Abstract

We show that ultra-wideline solid-state ¹⁴N NMR can be used as a quantitative probe of molecular dynamics. Jump rates for the molecular flipping mechanism in crystalline urea are determined at various temperatures and are shown to be in good agreement with other NMR techniques.

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Chem. Commun., 2010, 46, 7533-7535
DOI: 10.1039/C0CC01846E

Two-dimensional heteronuclear saturation transfer difference NMR reveals detailed integrin αvβ6 protein–peptide interactions

Jane L. Wagstaff, Sabari Vallath, John F. Marshall, Richard A. Williamson and Mark J. Howard

Abstract

We report the first example of peptide–protein heteronuclear two-dimensional (2D) saturation transfer difference nuclear magnetic resonance (STD NMR). This method, resulting in dramatically reduced overlap, was applied to the interaction of the integrin αvβ6 with a known peptide ligand and highlights novel contact points between the substrate and target protein.

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Chem. Commun., 2010, 46, 8192-8194
DOI: 10.1039/C0CC01953D

Proton hyperpolarisation preserved in long-lived states

Puneet Ahuja, Riddhiman Sarkar, Sami Jannin, Paul R. Vasos and Geoffrey Bodenhausen

Abstract

The polarisation of abundant protons, rather than dilute nuclei with low gyromagnetic ratios, can be enhanced in less than 10 min using dissolution DNP and converted into a long-lived state delocalised over an ensemble of three coupled protons. The process is more straightforward than the hyperpolarisation of heteronuclei followed by magnetisation transfer to protons.

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Chem. Commun., 2010, 46, 8273-8275
DOI: 10.1039/C0CC02730H

Artifact-free measurement of residual dipolar couplings in DMSO by the use of cross-linked perdeuterated poly(acrylonitrile) as alignment medium

Grit Kummerlöwe, Marc Behl, Andreas Lendlein and Burkhard Luy

Abstract

The effect of cation–π interactions on some NMR data of ethylenic complexes has been investigated by B97-1, PBE1KCIS and MPWKCIS1K methods using 6-311++G(3df,3pd) basis set. Unlike 3JH–H(cis), the chemical shift of ethylenic hydrogen (δ^H) and increases by cation–π interaction. The changes of NMR data have been considered with regard to geometry and direct electronic effects. Also, the distance dependence of NMR parameters has been studied in these complexes.
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Chemical Physics Letters
Volume 494, Issues 1-3, 9 July 2010, Pages 104-110
Improved resolution in dipolar NMR spectra using constant time evolution PISEMA experiment
Pages 104-110
T. Gopinath, Gianluigi Veglia
doi:10.1016/j.cplett.2010.05.078

Abstract

The atomic structure of small molecules and polypeptides can be attained from anisotropic NMR parameters such as dipolar couplings (DC) and chemical shifts (CS). Separated local field experiments resolve DC and CS correlations into two dimensions. However, crowded NMR spectra represent a significant obstacle for the complete resolution of these anisotropic parameters. Using the polarization inversion spin exchange at the magic angle (PISEMA) experiment as a foundation, we designed new pulse schemes that use a constant time evolution in the dipolar (indirect) dimension to measure DC and CS correlations at high resolution. We demonstrated this approach on a 4-pentyl-4′-cyanobiphenyl (5CB) liquid crystal sample, achieving a resolution enhancement ranging from 30% to 60% for the resonances in the dipolar dimension. These new experiments open the possibility of obtaining significant resolution enhancement for multidimensional NMR experiments carried out on oriented liquid crystalline samples as well as oriented membrane proteins.

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Chemical Physics Letters
Volume 494, Issues 4-6, 19 July 2010, Pages 326-330

Broadband heteronuclear dipolar recoupling without ¹H decoupling in solid-state NMR using simple cross-polarization methods
Pages 326-330
Morten Bjerring, Anders Bodholt Nielsen, Zdenek Tosner, Niels Chr. Nielsen

doi:10.1016/j.cplett.2010.06.018

Abstract

Heteronuclear dipolar recoupling experiments without ¹H decoupling based on simple cross polarization are introduced for applications in biological solid-state NMR. It is shown that standard or adiabatic variants of the cross-polarization experiment with irradiation on the low-γ (e.g., ¹³C,¹⁵N) spins even at modest spinning frequencies enable efficient band-selective or broadband dipolar recoupling without the need for intense ¹H decoupling. This facilitates experiments on expensive isotope-labelled protein samples for which sample heating by intense ¹H decoupling may lead to sample detoriation. The principle is demonstrated numerically and experimentally on uniformly ¹³C,¹⁵N-labelled samples of GB1 and fibrils of hIAPP [20], [21], [22], [23], [24], [25], [26], [27], [28] and [29] from the human islet amyloid labelled on the FGAIL part.

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Chemical Physics Letters
Volume 494, Issues 4-6, 19 July 2010, Pages 331-336

doi:10.1016/j.cplett.2010.06.019

Optimal control NMR differentiation between fast and slow sodium
Pages 331-336
Jae-Seung Lee, Ravinder R. Regatte, Alexej Jerschow

Abstract

Sodium ions in tissues and organs may experience motion on a variety of timescales, leading to NMR relaxation effects with quadrupolar coupling as the primary mechanism. The various effects that this fluctuating interaction has on spin dynamics can be exploited for distinguishing slow sodium ions from fast ones. Techniques such as triple-quantum filtering have been used for this purpose in the past. In this work we present optimal pulses which significantly improve the selectivity towards slow-tumbling sodium. These pulses can also be modified for robustness against magnetic field inhomogeneities, and could hence also become useful as MRI contrast methods.

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Chemical Physics Letters
Volume 495, Issues 4-6, 10 August 2010, Pages 287-291

doi:10.1016/j.cplett.2010.06.064

NMR relaxometry: Spin lattice relaxation times in the laboratory frame versus spin lattice relaxation times in the rotating frame
Pages 287-291
Emilie Steiner, Mehdi Yemloul, Laouès Guendouz, Sébastien Leclerc, Anthony Robert, Daniel Canet

Abstract

Relaxometry dispersion curves display the spin lattice relaxation rate as a function of the measurement frequency. However, as far as proton NMR is considered, dispersion curves usually start around 5 kHz and thus miss the very low frequency region. This gap can be filled by the measurement of the spin–lattice relaxation rate in the rotating frame. The issue of connecting both relaxation rates is considered for two relaxation mechanisms: (i) randomly varying magnetic fields, (ii) dipolar interaction within a system of two equivalent spins. Appropriate data processing is presented and the random field mechanism turns out to be adequate.

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Chemical Physics Letters
Volume 496, Issues 1-3, 20 August 2010, Pages 148-151

doi:10.1016/j.cplett.2010.07.063

Metal-alkyl species are formed on interaction of small alkanes with gallium oxide: Evidence from solid-state NMR
Pages 148-151
Anton A. Gabrienko, Sergei S. Arzumanov, Alexander V. Toktarev, Alexander G. Stepanov

Abstract

¹³C CP MAS NMR analysis of the products of the interaction of methane, ethane and propane with α-Ga₂O₃ or Ga-modified zeolite BEA at 523–623 K shows that dissociative adsorption of C₁–C₃ alkanes on the surface of gallium oxide or Ga-modified zeolite BEA results to the formation of Ga-methyl, Ga-ethyl and Ga-propyl species. This observation allows one to conclude that Ga-alkyls, rather than earlier suggested alkoxy species, could be the intermediates in small alkane dehydrogenation and aromatization on these catalysts.
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Chemical Physics Letters
Volume 496, Issues 1-3, 20 August 2010, Pages 162-166
doi:10.1016/j.cplett.2010.07.016
A magic-angle turning NMR experiment for separating spinning sidebands of half-integer quadrupolar nuclei
Pages 162-166
Ivan Hung, Zhehong Gan

Abstract

A two-dimensional magic-angle turning NMR experiment is described for separating spinning sidebands of half-integer quadrupolar nuclei. The experiment is an alternative to quadrupolar phase-adjusted sideband separation (QPASS) [Chem. Phys. Lett. 272 (1997) 295] that yields infinite speed spectra in one dimension and spinning sideband manifolds in the other dimension. A shear transformation is introduced for processing of quadrupolar magic-angle turning (QMAT) data with time evolution of only one rotor period. The QMAT experiment has the advantages of averaging the first-order modulation of the pulse sequence efficiencies giving smaller residual spinning sidebands, linearly incrementing pulse timings for convenient practical implementation, and easily adjustable pulse spacings suitable for higher spinning frequencies.

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The ‘molecular’ value of the nuclear quadrupole moment of the ⁷⁵As nucleus is determined combining the experimental data for quadrupole coupling constant and the calculated electric field gradient in the AsP molecule. The accurate calculations have been carried out at the CCSD(T) level. The relativistic effects were accounted for using the infinite-order two-component method in the scalar approximation. The new recommended value of the nuclear quadrupole moment of ⁷⁵As obtained in this work is 311(2) mb and is more precise than the previous ‘muonic’ value.
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Chemical Physics Letters
Volume 498, Issues 1-3, 30 September 2010, Pages 42-44
doi:10.1016/j.cplett.2010.08.054
Determination of chemical shift of gas-phase hydrogen molecules by ¹H nuclear magnetic resonance
Pages 42-44
Hirotada Fujiwara, Junichiro Yamabe, Shin Nishimura

Abstract

The precise and detailed chemical shift of gas-phase hydrogen molecules was successfully determined by ¹H nuclear magnetic resonance (NMR), avoiding the intervention of neighboring molecules such as in hydrogen occluding materials (free hydrogen). The measurement was conducted with double walled NMR sample tube taking into consideration the change of hydrogen pressure. The inner tube was filled with standard substance (DHO in D₂O at 4.8 ppm). The chemical shift of free hydrogen molecules was determined to be 7.40 ± 0.01 ppm at 0.18 MPa, 25 °C, which is different from previously reported chemical shifts of hydrogen gas with intervention of neighboring molecules.

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Chemical Physics Letters
Volume 498, Issues 1-3, 30 September 2010, Pages 214-220

doi:10.1016/j.cplett.2010.08.038

Homonuclear dipolar decoupling with very large scaling factors for high-resolution ultrafast magic angle spinning ¹H solid-state NMR spectroscopy
Pages 214-220
Elodie Salager, Jean-Nicolas Dumez, Robin S. Stein, Stefan Steuernagel, Anne Lesage, Bénédicte Elena-Herrmann, Lyndon Emsley

Abstract

We present a new phase modulated radio-frequency pulse sequence for homonuclear dipolar decoupling in proton solid-state NMR spectroscopy, eDUMBO-PLUS-1, with a chemical shift scaling factor of 0.73. This sequence was determined by screening random sequences, and experimentally optimizing the best candidates directly on ¹H NMR spectra with 60 kHz magic angle spinning. It yields efficient decoupling with linewidths as little as 150 Hz for 1.3 mm MAS probes on different spectrometers. Experiments and calculations support the hypothesis of a radio-frequency and MAS joint averaging regime, in which the large scaling factor contributes significantly to the overall performance of the decoupling sequence.

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Chemical Physics Letters
Volume 498, Issues 4-6, 8 October 2010, Pages 270-276

doi:10.1016/j.cplett.2010.08.077

Distinguishing hydrogen bonding networks in α-d-galactose using NMR experiments and first principles calculations
Pages 270-276
Mikhail Kibalchenko, Daniel Lee, Limin Shao, Mike C. Payne, Jeremy J. Titman, Jonathan R. Yates

Abstract

First principles calculations and solid-state NMR experiments are used to distinguish between possible hydrogen bonding networks in α-d-galactose. In contrast to ¹³C, the ¹H chemical shift parameters show differences which are sufficient to allow the correct network to be identified by comparison with experiments which make use of modern homonuclear decoupling schemes. In addition, clear linear correlations are established between both ¹H chemical shift and chemical shift anisotropy, and hydrogen bond length.

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Chemical Physics Letters
Volume 500, Issues 1-3, 10 November 2010, Pages 54-58

doi:10.1016/j.cplett.2010.09.061

Assignment of the He@C₈₄ isomers in experimental NMR spectra using density functional calculations
Pages 54-58
Petr Štěpánek, Petr Bouř, Michal Straka

Abstract

The ³He chemical shifts were calculated for He_n@C₈₄ (n = 1, 2) fullerenes to obtain characteristic NMR patterns for distinguishing their isomers in a mixture. The density functional methods were calibrated on experimental data. Accuracy within 1 ppm could be reached without further fitting of individual shifts. Such precision allows for a semi-quantitative assignment of ³He NMR spectra. Additional criteria in the identification are discussed, such as the relative energies of the isomers, positions of the satellite di-helium peaks, and the differential ³He shifts in the fullerenes reduced to anions. Endohedral ³He shifts are predicted for so far experimentally unknown He@C₈₄ and isomers.

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Journal updates

2010-10-20T11:27:00.002-04:00

J. Am. Chem. Soc., 2010, 132 (40), pp 13984–13987

High Resolution Measurement of Methyl 13Cm−13C and 1Hm−13Cm Residual Dipolar Couplings in Large Proteins
Chenyun Guo, Raquel Godoy-Ruiz, and Vitali Tugarinov*

Abstract
NMR methodology is developed for high-resolution, accurate measurements of methyl 1Hm−13Cm (1DCH) and 13Cm−13C (1DCC) residual dipolar couplings (RDCs) in ILV-methyl-protonated high-molecular-weight proteins. Both types of RDCs are measured in a three-dimensional (3D) mode that allows dispersion of correlations to the third (13Cβ/γ) dimension, alleviating the problem of overlap of methyl resonances in highly complex and methyl-abundant protein structures. The methodology is applied to selectively ILV-protonated 82-kDa monomeric enzyme malate synthase G (MSG) that contains 273 ILV methyl groups with substantial overlap of methyl resonances in 2D methyl 1H−13C correlation maps. A good agreement is observed between the measured RDCs of both types and those calculated from the crystallographic coordinates of MSG for the residues with low-amplitude internal dynamics. Although the measurement of 1DCH RDCs from the acquisition dimension of NMR spectra imposes certain limitations on the accuracy of obtained 1DCH values, 1DCH couplings can be approximately corrected for cross-correlated relaxation effects. The ratios of 1DCH and 1DCC
couplings (1DCH/1DCC) are independent of methyl axis dynamics and the details of residual alignment [Ottiger, M.; Bax, A. J. Am. Chem. Soc. 1999, 121, 4690.].
The 1DCH/1DCC ratios obtained in MSG can therefore validate the employed correction scheme.

J. Am. Chem. Soc., 2010, 132 (40), pp 14015–14017

The Structure of Formaldehyde-Inhibited Xanthine Oxidase Determined by 35 GHz 2H ENDOR Spectroscopy
Muralidharan Shanmugam†, Bo Zhang‡, Rebecca L. McNaughton†, R. Adam Kinney†, Russ Hille*‡, and Brian M. Hoffman*†

Abstract
The formaldehyde-inhibited Mo(V) state of xanthine oxidase (I) has been studied for four decades, yet it has not proven possible to distinguish unequivocally among the several structures proposed for this form. The uniquely large isotropic hyperfine coupling for 13C from CH2O led to the intriguing suggestion of a direct Mo−C bond for the active site of I. This suggestion was supported by the recent crystal structures of glycol- and glycerol-inhibited forms of aldehyde oxidoreductase, a member of the xanthine oxidase family. 1H and 2H ENDOR spectra of I(C1,2H2O) in H2O/D2O buffer now have unambiguously revealed that the active-site structure of I contains a CH2O adduct of Mo(V) in the form of a four-membered ring with S and O linking the C to Mo and have ruled out a direct Mo−C bond. Density functional theory computations are consistent with this conclusion. We interpret the large 13C coupling as resulting from a “transannular hyperfine interaction”.

J. Phys. Chem. A, 2010, 114 (24), pp 6622–6629
Solid-State NMR Spectra and Long, Intra-Dimer Bonding in the π-[TTF]22+ (TTF = Tetrathiafulvalene) Dication
Merrill D. Halling, Joshua D. Bell, Ronald J. Pugmire, David M. Grant* and Joel S. Miller*

Abstract
The 13C chemical-shift tensor principal values for TTF and π-[TTF]22+ (TTF = tetrathiafulvalene) dimer dications have been measured in order to better understand the electronic structure and long intradimer bonding of these TTF-based dimer structures. The structure of π-[TTF]22+ is abnormal due to its two C−C and four S−S ca. 3.4 Å intradimer separations, which is less than the sum of the sulfur van der Waals radii, and has a singlet 1A1g electronic ground state. This study of TTF and [TTF]22+ was conducted to determine how the NMR chemical-shift tensor principal values change as a function of electronic structure. This study also establishes a better understanding of the interactions that lead to spin-pairing of the monomeric radical units. The density functional theory (DFT) calculated nuclear shielding tensors are correlated with the experimentally determined principal chemical-shift values. The embedded ion method (EIM) was used to investigate the electrostatic lattice potential in [TTF]22+. These theoretical methods provide information on the tensor magnitudes and orientations of their tensor principal values with respect to the molecular frame. The experimental chemical-shift principal values agree with the calculated quantum mechanical chemical-shielding principal values, within typical errors commonly seen for this class of molecular system. Relatively weak Wiberg bond orders between the two [TTF]+ components of the dimer dication correlate with the long bonds linking the two [TTF]+ monomers and substantiate the claim that there is weak multicenter bonding present.

Inorg. Chem., 2010, 49 (12), pp 5522–5529
Incorporation of Phosphorus Guest Ions in the Calcium Silicate Phases of Portland Cement from 31P MAS NMR SpectroscopySøren L. Poulsen, Hans J. Jakobsen and Jørgen Skibsted*

Abstract
Portland cements may contain small quantities of phosphorus (typically below 0.5 wt % P2O5), originating from either the raw materials or alternative sources of fuel used to heat the cement kilns. This work reports the first 31P MAS NMR study of anhydrous and hydrated Portland cements that focuses on the phase and site preferences of the (PO4)3− guest ions in the main clinker phases and hydration products. The observed 31P chemical shifts (10 to −2 ppm), the 31P chemical shift anisotropy, and the resemblance of the lineshapes in the 31P and 29Si MAS NMR spectra strongly suggest that (PO4)3− units are incorporated in the calcium silicate
phases, alite (Ca3SiO5) and belite (Ca2SiO4), by substitution for (SiO4)4−tetrahedra. This assignment is further supported by a determination of the spin−lattice relaxation times for 31P in alite and belite, which exhibit the same ratio as observed for the corresponding 29Si relaxation times. From simulations of the intensities, observed in inversion−recovery spectra for a white Portland cement, it is deduced that 1.3% and 2.1% of the Si sites in alite and belite, respectively, are replaced by phosphorus. Charge balance may potentially be achieved to some extent by a coupled substitution mechanism where Ca2+ is replaced by Fe3+ ions, which may account for the interaction of the 31P spins with paramagnetic Fe3+ ions as observed for the ordinary Portland cements. A minor fraction of phosphorus may also be present in the separate phase Ca3(PO4)2, as indicated by the observation of a narrow resonance at δ(31P) = 3.0 ppm for two of the studied cements. 31P{1H} CP/MAS NMR spectra following the hydration of a white Portland cement show that the resonances from the hydrous phosphate species fall in the same spectral range as observed for (PO4)3− incorporated in alite. This similarity and the absence of a large 31P chemical shift ansitropy indicate that the hydrous (PO4)3− species are incorporated in the interlayers of the calcium−silicate−hydrate (C−S−H) phase, the principal phase formed upon hydration of alite and belite.

Inorg. Chem., 2010, 49 (12), pp 5573–5583
Basic Coordination Chemistry Relevant to DNA Adducts Formed by the Cisplatin Anticancer Drug. NMR Studies on Compounds with Sterically Crowded Chiral LigandsJamil S. Saad*†§, Michele Benedetti‡, Giovanni Natile‡ and Luigi G. Marzilli*†

Abstract
Me4DABPtG2 adducts with the bulky C2-symmetric chiral diamine, Me4DAB (N,N,N′,N′-tetramethyl-2,3-diaminobutane with R,R and S,S configurations at the chelate ring C atom, G = guanine derivative), exhibit slow conformer interchange and are amenable to characterization by NMR methods. The investigation of the cis-PtA2G2 adducts formed by clinically widely used anticancer drugs [A2 = diaminocyclohexane, (NH3)2] is impeded by the rapid conformer interchange permitted by the low A2 bulk near the inner coordination sphere. Me4DABPtG2 adducts exist as a mixture of exclusively head-to-tail (HT) conformers. No head-to-head (HH) conformer was observed. The Me4DAB chirality significantly influences which HT chirality is favored (ΔHT for S,S and ΛHT for R,R). For simple G ligands, the ratio of favored HT conformer to less favored HT conformer is 2:1. For guanosine monophosphate (GMP) ligands, the phosphate group cis G N1H hydrogen bonding favors the ΛHT and the ΔHT conformers for 5′-GMP and 3′-GMP adducts, respectively. For both HT conformers of cis-PtA2G2 adducts, the G nucleobase plane normally cants with respect to the coordination plane in the same direction, left or right, for a given A2 chirality. In contrast, the results for Me4DABPtG2 adducts provide the first examples of a change in the canting direction between the two HT conformers; this unusual behavior is attributed to the fact that canting always gives long G O6 to N−Me distances and that these Me4DAB ligands have bulk both above and below the coordination plane. These results and ongoing preliminary studies of Me4DABPt adducts with G residues linked by a phosphodiester backbone, which normally favors HH conformers, all indicate that a high percentage of HT conformer is present. Collectively, these findings advance fundamental concepts in Pt-DNA chemistry and may eventually help define the role of the carrier-ligand steric effects on anticancer activity.

Journal of magnetic resonance

2010-10-07T10:57:00.002-04:00

On the choice of heteronuclear dipolar decoupling scheme in solid-state NMR

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 9 September 2010
Subhradip, Paul , N.D., Kurur , P.K., Madhu

We present here a comparison of different heteronuclear dipolar decoupling sequences at the moderate magic-angle spinning (MAS) frequency (ν_r) of 30 kHz. The radio-frequency (RF) amplitude (ν₁) ranges from the low power (ν₁ < ν_r) to the high power regime (ν₁ > 2ν_r) and includes the rotary resonance conditions (ν₁ = nν_r) wheren = 1, 2. For decoupling at the rotary resonance condition, we recently introduced a modification of TPPM, namely high-phase TPPM, whose properties will be discussed here. Finally, based on earlier published and current experimental results we suggest the optimal sequence for heteronuclear dipolar decoupling at any RF amplitude and MAS frequencies up to 35 kHz.

Z-spectroscopy with Alternating Phase Irradiation

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 15 September 2010
Johanna, Närväinen , Penny L., Hubbard , Risto A., Kauppinen , Gareth A., Morris

Magnetization transfer (MT) MRI and Z-spectroscopy are tools to study both water–macromolecule interactions and pH-sensitive exchange dynamics between water and the protons of mobile chemical groups within these macromolecules. Both rely on saturation of frequencies offset from water and observation of the on-resonance water signal. In this work, an RF saturation method called Z-spectroscopy with Alternating-Phase Irradiation (ZAPI) is introduced. Based on the T₂-selectivity of the irradiation pulse, ZAPI can be used to separate the different contributions to a Z-spectrum, as well as to study the T₂ distribution of the macromolecules contributing to the MT signal. ZAPI can be run at resonance for water and with low power, thus minimizing problems with specific absorption rate (SAR) limits in clinical applications. In this paper, physical and practical aspects of ZAPI are discussed and the sequence is applied in vitro to sample systems and in vivo to rat head to demonstrate the method.

Application of Optimal Control to CPMG Refocusing Pulse Design

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 15 September 2010
Troy W., Borneman , Martin D., Hürlimann , David G., Cory

We apply optimal control theory (OCT) to the design of refocusing pulses suitable for the CPMG sequence that are robust over a wide range of B₀ and B₁ offsets. We also introduce a model, based on recent progress in the analysis of unitary dynamics in the field of quantum information processing (QIP), that describes the multiple refocusing dynamics of the CPMG sequence as a dephasing Pauli channel. This model provides a compact characterization of the consequences and severity of residual pulse errors. We illustrate the methods by considering a specific example of designing and analyzing broadband OCT refocusing pulses of length 10 t₁₈₀that are constrained by the maximum instantaneous pulse power. We show that with this refocusing pulse, the CPMG sequence can refocus over 98% of magnetization for resonance offsets up to 3.2 times the maximum RF amplitude, even in the presence of ±10% RF inhomogeneity.

A simple one-dimensional method of chemical shift anisotropy determination under MAS conditions

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 18 September 2010
Piotr, Bernatowicz

A method of determination of chemical shift anisotropy (CSA) tensor principal components under MAS condition is presented. It is a simple, one-dimensional, and robust alternative to the commonly exploited, but more complicated 2D-PASS. The required CSA components are delivered by simultaneous numerical analysis of a few regular MAS spectra acquired under different spinning rates.

On the measurement of 15N-{1H} nuclear Overhauser effects. 2. Effects of the saturation scheme and water signal suppression

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 24 September 2010
Fabien, Ferrage , Amy, Reichel , Shibani, Battacharya , David, Cowburn , Ranajeet, Ghose

Measurement of steady-state ¹⁵N-{¹H} nuclear Overhauser effects forms a cornerstone of most methods to determine protein backbone dynamics from spin-relaxation data, since it is the most reliable probe of very fast motions on the ps-ns timescale. We have, in two previous publications (J. Magn. Reson. 192 (2008), 302-313; J. Am. Chem. Soc. 131 (2009), 6048-6049) reevaluated spin-dynamics during steady-state (or “saturated”) and reference experiments, both of which are required to determine the NOE ratio. Here we assess the performance of several windowed and windowless sequences to achieve effective saturation of protons in steady-state experiments. We also evaluate the influence of the residual water signal due to radiation damping on the NOE ratio. We suggest a recipe that allows one to determine steady-state ¹⁵N-{¹H} NOE’s without artifacts and with the highest possible accuracy.

Hydration Water Dynamics in Biopolymers from NMR Relaxation in the Rotating Frame

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 24 September 2010
Barbara, Blicharska , Hartwig, Peemoeller , Magdalena, Witek

Assuming dipole-dipole interaction as the dominant relaxation mechanism of protons of water molecules adsorbed onto macromolecule (biopolymer) surfaces we have been able to model the dependences of relaxation rates on temperature and frequency. For adsorbed water molecules the correlation times are of the order of 10^-5 s, for which the dispersion region of spin-lattice relaxation rates in the rotating frame R₁ρ = 1/T₁ρ appears over a range of easily accessible B₁ values. Measurements of T₁ρ at constant temperature and different B₁ values then give the “dispersion profiles” for biopolymers. Fitting a theoretical relaxation model to these profiles allows for the estimation of correlation times. This way of obtaining the correlation time is easier and faster than approaches involving measurements of the temperature dependence of R₁ = 1/T₁. The T₁ρ dispersion approach, as a tool for molecular dynamics study, has been demonstrated for several hydrated biopolymer systems including crystalline cellulose, starch of different origins (potato, corn, oat, wheat), paper (modern, old) and lyophilized proteins (albumin, lysozyme).

IPAP- HSQMBC: Measurement of Long-Range Heteronuclear Coupling Constants from Spin-State Selective Multiplets

Publication year: 2010
Source: Journal of Magnetic Resonance, In Press, Accepted Manuscript, Available online 29 September 2010
Sergi, Gil , Juan Félix, Espinosa , Teodor, Parella

A new NMR approach is proposed for the measurement of long-range heteronuclear coupling constants (ⁿJ_XH, n>1) in natural abundance molecules. Two complementary in-phase (IP) and anti-phase (AP) data are separately recorded from a modified HSQMBC experiment and then added/subtracted to provide spin-state-selective α/β-HSQMBC spectra. The magnitude of ⁿJ_XHcan be directly determined by simple analysis of the relative displacement between α- and β-cross-peaks. The robustness of this IPAP-HSQMBC experiment is evaluated experimentally and by simulation using a variety of different conditions. Important aspects such as signal intensity dependence and presence of unwanted cross-talk effects are discussed and examples on the measurement of small proton-carbon (ⁿJ_CH) and proton-nitrogen (ⁿJ_NH) coupling constants are provided.

Characterization of a 3D MEMS fabricated micro solenoid at 9.4T

M. Mohmmadzadeh^a^,, N. Baxan^a, V. Badilita^b, K. Kratt^b, H. Weber^a, J.G. Korvink^c^,^d, U. Wallrabe^b^,^d, J. Hennig^a and D. von Elverfeldt^a

We present for the first time a complete characterization of a micro-solenoid for high resolution MR imaging of mass- and volume-limited samples based on three-dimensional B₀, B₁ per unit current (B_1unit) and SNR maps. The micro-solenoids are fabricated using a fully micro-electromechanical systems (MEMS) compatible process in conjunction with an automatic wire-bonder. We present 15 μm isotropic resolution 3D B₀ maps performed using the phase difference method. The resulting B₀ variation in the range of [-0.07 ppm-0.157 ppm] around the coil center, compares favorably with the 0.5 ppm limit accepted for MR microscopy. 3D B_1unit maps of 40 μm isotropic voxel size were acquired according to the extended multi flip angle (ExMFA) method. The results demonstrate that the characterized microcoil provides a high and uniform sensitivity distribution around its center (B_1unit = 3.4 mT/A ± 3.86%) which is in agreement with the corresponding 1D theoretical data computed along the coil axis. The 3D SNR maps reveal a rather uniform signal distribution around the coil center with a mean value of 53.69 ± 19%, in good agreement with the analytical 1D data along coil axis in the axial slice. Finally, we prove the microcoil capabilities for MR microscopy by imaging Eremosphaera Viridis cells with 18 μm isotropic resolution.

Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization

2010-09-20T17:28:00.003-04:00

A nice write-up in C&E News about DNP SSNMR of molecules bound to silica surfaces.

http://pubs.acs.org/cen/news/88/i38/8838notw6.html

The corresponding JACS communication can be found at:

http://pubs.acs.org/doi/abs/10.1021%2Fja104771z

Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization

Anne Lesage†, Moreno Lelli†, David Gajan‡, Marc A. Caporini§, Veronika Vitzthum§, Pascal Miville§, Johan Alauzun, Arthur Roussey‡, Chlo Thieuleux‡, Ahmad Medhi, Geoffrey Bodenhausen§, Christophe Copret‡, and Lyndon Emsley*†

J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/ja104771z
Publication Date (Web): September 10, 2010
Copyright © 2010 American Chemical Society

Abstract: It is shown that surface NMR spectra can be greatly enhanced using dynamic nuclear polarization. Polarization is transferred from the protons of the solvent to the rare nuclei (here carbon-13 at natural isotopic abundance) at the surface, yielding at least a 50-fold signal enhancement for surface species covalently incorporated into a silica framework

J. Chem. Phys.

2010-09-20T11:34:00.002-04:00

Internal symmetry of basic elements in symmetry-based recoupling sequences under magic-angle spinning

from Journal of Chemical Physics: All Topics

Fang-Chieh Chou, Hsin-Kuan Lee, and Jerry C. C. Chan

In solid-state NMR, many powerful pulse sequences under the condition of magic-angle spinning can be analyzed on the basis of the C- and R-sequences developed by Levitt and co-workers. It has been speculated for some years that the basic elements commonly used in symmetry-based recoupling pulse sequences have certain kind of internal symmetries. We show by a detailed analysis that a set of internal selection rules does exist for many basic elements. These internal selection rules may allow a more versatile design of CNnν or RNnν sequences when n is an integer or half-integer multiple of N. As an illustration, we have derived the symmetry arguments to rationalize the observation that the C-REDOR pulse sequence can suppress homonuclear dipole-dipole interaction, leading to the design of new windowed basic elements usable for heteronuclear dipolar recoupling with active suppression of homonuclear dipole-dipole interaction. Numerical simulations and experiments measured for [U–¹³C,¹⁵N]-L-alanine have been used to validate our approach. On a more general note, the symmetry rules discussed in this work can also be applied for the design of supercycles.

Puckering free energy of pyranoses: A NMR and metadynamics-umbrella sampling investigation

from Journal of Chemical Physics: All Topics

1 person liked this

E. Autieri, M. Sega, F. Pederiva, and G. Guella

We present the results of a combined metadynamics-umbrella sampling investigation of the puckered conformers of pyranoses described using the GROMOS 45a4 force field. The free energy landscape of Cremer–Pople puckering coordinates has been calculated for the whole series of α and β aldohexoses, showing that the current force field parameters fail in reproducing proper puckering free energy differences between chair conformers. We suggest a modification to the GROMOS 45a4 parameter set which improves considerably the agreement of simulation results with theoretical and experimental estimates of puckering free energies. We also report on the experimental measurement of altrose conformer populations by means of NMR spectroscopy, which show good agreement with the predictions of current theoretical models.

J. Phys. Chem. B and C, vol. 114, Issues 36

2010-09-14T15:28:00.001-04:00

Combined Solid-State NMR and Theoretical Calculation Studies of Brønsted Acid Properties in Anhydrous 12-Molybdophosphoric Acid

Ningdong Feng†, Anmin Zheng*†, Shing-Jong Huang‡, Hailu Zhang§, Ningya Yu‡, Chih-Yi Yang‡, Shang-Bin Liu*‡, and Feng Deng*†

J. Phys. Chem. C, 2010, 114 (36), pp 15464–15472
DOI: 10.1021/jp105683y
Publication Date (Web): August 20, 2010
Copyright © 2010 American Chemical Society

Abstract: The strength and distribution of Brønsted acidic protons in anhydrous phosphomolybdic acid (H3PMo12O40, HPMo) have been studied by solid-state magic-angle-spinning (MAS) NMR, using trimethylphosphine oxide (TMPO) as the probe molecule in conjunction with density functional theory (DFT) calculations. Brønsted acid sties with strengths exceeding the threshold of superacidity (Zheng, A. et al. J. Phys. Chem. B 2008, 112, 4496) were observed for HPMo. In addition, the locations and adsorption structures of Brønsted protons on various oxygen sites in HPMo were also identified. The preferred location of the acidic proton was found to follow the trend: corner-sharing (Ob) > edge-sharing (Oc) terminal (Od) sites. Moreover, a tendency of hybridization among Brønsted protons residing at Ob and Oc sites of HPMo was inferred by experimental as well as theoretical 31P chemical shifts of the adsorbed TMPO.

J. Phys. Chem. B and C, v114, Issues 32 - 35

2010-09-07T12:19:00.004-04:00

Structure and Disorder in Amorphous Alumina Thin Films: Insights from High-Resolution Solid-State NMR

Sung Keun Lee*†, Sun Young Park†, Yoo Soo Yi† and Jaehyun Moon‡

J. Phys. Chem. C, 2010, 114 (32), pp 13890–13894
Publication Date (Web): July 28, 2010

Abstract:Revealing the extent of disorder in amorphous oxides is one of the remaining puzzles in physical chemistry, glass sciences, and geochemistry. Here, we report the 27Al NMR results for amorphous Al2O3 thin films obtained from two different deposition methods (i.e., physical vapor-deposition and atomic layer-deposition), revealing two distinct amorphous states defined by a fraction of five-coordinated Al ([5]Al). The fractions of [4]Al and [5]Al are dominant (92−95%) in both films. While the overall similarity between these two states suggests a narrow stability of available amorphous states, the fraction of [5]Al in atomic layer-deposited thin films is apparently larger and thus more disordered than that in physical vapor-deposited films. Such results require that varying extents of disorder exist in the amorphous oxides prepared under different processing conditions. As the [5]Al site (<1%) in crystalline Al2O3 is known to control its catalytic ability over [4]Al and [6]Al, the significant fractions (40%) of [5]Al in our amorphous thin films suggest that amorphous Al2O3 may be potentially useful as a new class of catalysts.

X-ray Diffraction, FT-IR, and 13C CP/MAS NMR Structural Studies of Solvated and Desolvated C-Methylcalix[4]resorcinarene

Rafal Kuzmicz†, Violetta Kowalska†, Sławomir Domagała‡, Marcin Stachowicz‡, Krzysztof Woniak‡ and Waclaw Kolodziejski*†

J. Phys. Chem. B, 2010, 114 (32), pp 10311–10320
DOI: 10.1021/jp1015565
Publication Date (Web): July 26, 2010

Abstract: Solid C-methylcalix[4]resorcinarene solvated by acetonitrile and water (CAL-Me) and then modified by slow solvent evaporation (CAL-Me*) was studied using single-crystal and powder X-ray diffraction, FT-IR, and 13C CP/MAS NMR. The CAL-Me solvate crystallizes in the monoclinic P21/n space group with three CH3CN and two H2O molecules in the asymmetric part of the unit cell. The CAL-Me molecules adopt a typical crown conformation with all of the hydroxyl groups of the aryl rings oriented up and all of the methyl groups disposed down (the rccc isomeric form). The crystalline network is formed by resorcinarene, CH3CN, and H2O molecules and assembled by intermolecular hydrogen bonds and weak C−H···A or C−H···π interactions. The desolvated CAL-Me* loses its crystalline character and becomes partly amorphous. It is devoid of CH3CN and deficient in water. However, the resorcinarene molecules still remain in the crown conformation supported by intramolecular hydrogen bonds, while intermolecular hydrogen bonds are considerably disintegrated. The work directs general attention to the problem of stability and polymorphism of resorcinarene solvates. It shows that the joint use of diffractometric and spectroscopic methods is advantageous in the structural studies of complex crystalline macromolecular systems. On the other hand, the solid-state IR and NMR spectroscopic analyses applied in tandem have been found highly beneficial to elucidate the disordered structure of poorly crystalline, desolvated resorcinarene

Conformational Changes at Mesophase Transitions in a Ferroelectric Liquid Crystal by Comparative DFT Computational and 13C NMR Study

Alberto Marini* and Valentina Domenici

J. Phys. Chem. B, 2010, 114 (32), pp 10391–10400
DOI: 10.1021/jp105095m
Publication Date (Web): July 26, 2010

Abstract: In this work, we report a detailed investigation on both the conformational and the orientational ordering properties of a ferroelectric liquid crystal mesogen, namely, M10/**, through the combination of high resolution solid state 13C NMR and density functional theory (DFT) computational methods. The trends of the observed 13C chemical shift in the blue, cholesteric, and ferroelectric SmC* phases of M10/** were analyzed in terms of conformational changes occurring in the flexible parts of the molecule. In particular, we focused on the aliphatic alpha methylenoxy carbons because of their high sensitivity to mesophase environment, as evidenced by experimental 13C chemical shift anisotropy (CSA). DFT computation of the chemical shift tensors as a function of geometrical parameters, such as dihedral angles, put in evidence significant changes in the average conformation at the mesophase transitions. The conformations predicted by DFT have been validated by comparing the calculated 13C chemical shifts with those experimentally observed for the alkoxylic carbons, whose relative orientation plays a key role in establishing the overall conformation of the molecule in each liquid crystalline phase. Furthermore, the orientational order parameters of the relevant flexible fragments were calculated and found to be in good agreement with those characterizing similar systems, thus validating our approach.

Glass-to-Vitroceramic Transition in the Yttrium Aluminoborate System: Structural Studies by Solid-State NMR

Heinz Deters†‡, Andrea S. S. de Camargo†§, Cristiane N. Santos§ and Hellmut Eckert*†

J. Phys. Chem. C, 2010, 114 (34), pp 14618–14626
Publication Date (Web): August 6, 2010

Abstract: The crystallization of laser glasses in the system (B2O3)0.6{(Al2O3)0.4−y(Y2O3)y} (0.1 ≤ y ≤ 0.25) doped with different levels of ytterbium oxide has been investigated by X-ray powder diffraction, differential thermal analysis, and various solid-state NMR techniques. The homogeneous glasses undergo major phase segregation processes resulting in crystalline YBO3, crystalline YAl3(BO3)4, and residual glassy B2O3 as the major products. This process can be analyzed in a quantitative fashion by solid-state 11B, 27Al, and 89Y NMR spectroscopies as well as 11B{27Al} rotational echo double resonance (REDOR) experiments. The Yb dopants end up in both of the crystalline components, producing increased line widths of the corresponding 11B, 27Al, and 89Y NMR resonances that depend linearly on the Yb/Y substitution ratio. A preliminary analysis of the composition dependence suggests that the Yb3+ dopant is not perfectly equipartitioned between both crystalline phases, suggesting a moderate preference of Yb to substitute in the crystalline YBO3 component

Chemical Degradation of Nafion Membranes under Mimic Fuel Cell Conditions as Investigated by Solid-State NMR Spectroscopy

Lida Ghassemzadeh†‡, Klaus-Dieter Kreuer†, Joachim Maier† and Klaus Mller*§

J. Phys. Chem. C, 2010, 114 (34), pp 14635–14645
Publication Date (Web): August 5, 2010

Abstract: A new ex situ method has been developed to mimic the degradation of the polymer membranes in polymer electrolyte membrane fuel cells (PEMFCs), caused by the cross-leakage of H2 and O2. In this ex situ setup, it is possible to expose membranes to flows of different gases with a controlled temperature and humidity. H+-form Nafion films with and without an electrode layer (Pt) have been treated in the presence of different gases in order to simulate the anode and cathode side of a PEMFC. The changes of the chemical structure occurring during the degradation tests were primarily examined by solid-state 19F NMR spectroscopy. For completion, liquid-state NMR studies and ion-exchange capacity measurements were performed. The molecular mobility changes of the ionomer membrane upon degradation were examined for the first time by variable-temperature 19F NMR line-shape, T1 and T1ρ relaxation experiments. It was found that degradation occurs only when both H2 and O2 are present (condition of gas cross-leakage) and when the membrane is coated with a Pt catalyst. The chemical degradation rate is found to be highest for H2-rich mixtures of H2 and O2, which corresponds to the anode under OCV conditions. It is further shown that side-chain disintegration is very important for chemical degradation, although backbone decomposition also takes place. The temperature-dependent line-width and spectral anisotropy alterations were explained by the reduction of static disorder in the Nafion membrane. From the relaxation data, there is evidence for structural annealing, which is independent of the chemical degradation. Chemical degradation is considered to reduce the chain flexibility, as expressed by smaller motional amplitudes, most probably due to chain cross-linking.

Activation of Ammonia Borane Hybridized with Alkaline−Metal Hydrides: A Low-Temperature and High-Purity Hydrogen Generation Material

Yu Zhang, Keiji Shimoda, Takayuki Ichikawa* and Yoshitsugu Kojima
J. Phys. Chem. C, 2010, 114 (34), pp 14662–14664
Publication Date (Web): August 5, 2010

Abstract: Recently, alkali−metal amidoborane complexes have been highlighted as materials that satisfy many of the criteria required to make hydrogen-storage media. In this paper, ammonia borane was successfully activated by the existence of hybrid alkaline−metal hydrides. The desorption results showed that this activation strategy can significantly decrease the dehydrogenation temperature and, furthermore, can successfully suppress ammonia gas release and volume expansion. These results will be helpful for the design of future hydrogen-storage media.

Solid-State 2H NMR and MD Simulations of Positional Isomers of a Monounsaturated Phospholipid Membrane: Structural Implications of Double Bond Location

Stephen R. Wassall*†, M. Alan McCabe†, Cynthia D. Wassall†, Richard O. Adlof‡ and Scott E. Feller§

J. Phys. Chem. B, 2010, 114 (35), pp 11474–11483
DOI: 10.1021/jp105068g
Publication Date (Web): August 13, 2010
Copyright © 2010 American Chemical Society

Abstract: The impact that the position of double bonds has upon the properties of membranes is investigated using solid-state 2H NMR and MD simulations to compare positional isomers of 1-palmitoyl-2-octadecenoylphosphatidylcholine (16:0-18:1PC) bilayers that are otherwise identical apart from the location of a single cis double bond at the Δ6, Δ9, Δ12, or Δ15 position in the 18:1 sn-2 chain. Moment analysis of 2H NMR spectra recorded for isomers perdeuterated in the 16:0 sn-1 chain reveals that average order parameters CD change by more than 35% and that the temperature for chain melting Tm varies by 40 °C. At equal temperature, the CD values exhibit a minimum, as do Tm values, when the double bond is in the middle of the 18:1 sn-2 chain and increase as it is shifted toward each end. Order parameter profiles generated from depaked (“dePaked”) spectra for the 16:0 sn-1 chain all possess the same shape with a characteristic “plateau” region of slowly decreasing order in the upper portion before progressively decreasing more in the lower portion. The NMR results are interpreted on the basis of MD simulation results obtained on each of the four systems. The simulations support the idea that the order parameter changes reflect differences in molecular surface areas, and furthermore that the molecular areas are a function of the strength of the acyl chain attractions.

A Solid-State 17O NMR Study of l-Tyrosine in Different Ionization States: Implications for Probing Tyrosine Side Chains in Proteins

Jianfeng Zhu, Justin Y. C. Lau and Gang Wu*
J. Phys. Chem. B, 2010, 114 (35), pp 11681–11688
DOI: 10.1021/jp1055123
Publication Date (Web): August 16, 2010
Copyright © 2010 American Chemical Society

Abstract: We report experimental characterization of 17O quadrupole coupling (QC) and chemical shift (CS) tensors for the phenolic oxygen in three l-tyrosine (l-Tyr) compounds: l-Tyr, l-Tyr·HCl, and Na2(l-Tyr). This is the first time that these fundamental 17O NMR tensors are completely determined for phenolic oxygens in different ionization states. We find that, while the 17O QC tensor changes very little upon phenol ionization, the 17O CS tensor displays a remarkable sensitivity. In particular, the isotropic 17O chemical shift increases by approximately 60 ppm upon phenol ionization, which is 6 times larger than the corresponding change in the isotropic 13C chemical shift for the Cζ nucleus of the same phenol group. By examining the CS tensor orientation in the molecular frame of reference, we discover a “cross-over” effect between δ11 and δ22 components for both 17O and 13C CS tensors. We demonstrate that the knowledge of such “cross-over” effects is crucial for understanding the relationship between the observed CS tensor components and chemical bonding. Our results suggest that solid-state 17O NMR can potentially be used to probe the ionization state of tyrosine side chains in proteins.

J. Chem. Phys.

2010-09-07T11:32:00.002-04:00

Self-diffusion of poly(propylene glycol) in nanoporous glasses studied by pulsed field gradient NMR: A study of molecular dynamics and surface interactions
A. Schonhals, F. Rittig, and J. Karger
Pulsed field gradient NMR is applied to investigate the self-diffusion of poly(proypylene glycol) in nanoporous glasses (nominal pore sizes of 2.5–7.5 nm). In general, the diffusion is slowed down by the confinement compared to the bulk. For native pore surfaces covered by hydroxyl groups the spin echo attenuation Ψ displays a bimodal behavior versus q²t (q-norm of a generalized scattering vector). This was explained assuming spatial regions of different diffusivities in a two-phase model. The slow component is assigned to segments forming a surface layer close to the pore walls in which the segments have a lower mobility than those located in the center of the pores. By variation of observation time it was concluded that time constant for the dynamic exchange of segments between these two regions is around 100 ms at room temperature. For silanized pores, the bimodal behavior in the spin echo attenuation Ψ shows a stretched exponential decay versus q²t. The estimated diffusion coefficients decrease strongly with decreasing pore size. The temperature dependence of the diffusion coefficient can be approximated by an Arrhenius law where the activation energy increases with decreasing pore size. The observed pore size dependence for the diffusion of poly(propylene glycol) in silanized nanoporous glasses can be discussed assuming interaction and confining size effects.

Dynamical effects in ab initio NMR calculations: Classical force fields fitted to quantum forces

Mark Robinson and Peter D. Haynes
NMR chemical shifts for an L-alanine molecular crystal are calculated using ab initio plane wave density functional theory. Dynamical effects including anharmonicity may be included by averaging chemical shifts over an ensemble of structural configurations generated using molecular dynamics (MD). The time scales required mean that ab initio MD is prohibitively expensive. Yet the sensitivity of chemical shifts to structural details requires that the methodologies for performing MD and calculating NMR shifts be consistent. This work resolves these previously competing requirements by fitting classical force fields to reproduce ab initio forces. This methodology is first validated by reproducing the averaged chemical shifts found using ab initio molecular dynamics. Study of a supercell of L-alanine demonstrates that finite size effects can be significant when accounting for dynamics.

s

Inorganic Chemistry

2010-08-16T12:46:00.002-04:00

Separation of Geometric Isomers of a Dicopper Complex by Using a 19F-Labeled Ligand: Dynamics, Structures, and DFT Calculations
Stphanie Durot, Laila H. Hossain, Sylvain Hamman, Hlne Jamet, Maylis Orio, Isabelle Gautier-Luneau, Dominique Luneau, Christian Philouze, Jean-Louis Pierre and Catherine Belle
DOI: 10.1021/ic1006567
Introducing a fluorine group on two pyridines of the HLCH3 ligand (2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol) allows the separation of two geometric isomers after complexation by two copper(II) ions. Methods for isolating the isomers (1meso and 1rac) as a μ-phenoxo,μ-hydroxo dicopper(II) complex as a crystalline product have been developed. Both isomers (1meso and 1rac) have been characterized by X-ray crystallography and 19F NMR. The isomerism is determined by the disposition of the fluorine atoms with respect to the plane containing the Cu2O2 core. Density functional theory calculations using different functionals were performed to provide additional support for the existence of these two forms. Dissolution of 1meso in acetone or acetonitrile causes its spontaneous isomerization into the 1rac form at room temperature. Combined experimental studies (UV−vis, 19F NMR) and theoretical calculations support this process. Paramagnetic 19F NMR appears as a unique and powerful probe for distinguishing the two isomers and supplying direct evidence of this isomerization process in solution.
_____________________________________________________
A Pd6 Molecular Cage via Multicomponent Self-Assembly Incorporating Both Neutral and Anionic Linkers
Arun Kumar Bar, Golam Mostafa and Partha Sarathi Mukherjee
DOI: 10.1021/ic101139s
A Pd6 molecular cage [{(tmen)Pd}6(bpy)3(tma)2](NO3)6 [1; where tmen = N,N,N′,N′-tetramethylethylene diamine, bpy = 4,4′-bipyridyl, and H3tma = trimesic acid] was prepared via the template-free three-component self-assembly of a cis-blocked palladium(II) acceptor in combination with a tricarboxylate and a dipyridyl donor. Complex 1 represents the first example of a 3D palladium(II) cage of defined shape incorporating anionic and neutral linkers. Guest-induced exclusive formation of this cage was also monitored by an NMR study.

J. Chem. Phys.

2010-08-13T12:00:00.002-04:00

First principles nuclear magnetic resonance signatures of graphene oxide

Ning Lu, Ying Huang , Hai-bei Li Zhenyu Li, and Jinlong Yang
Nuclear magnetic resonance (NMR) has been widely used in graphene oxide (GO) structure studies. However, the detailed relationship between its spectroscopic features and the GO structural configuration remains elusive. Based on first principles ¹³C chemical shift calculations using the gauge including projector augmented waves method, we provide a reliable spectrum-structure connection. The ¹³C chemical shift in GO is found to be very sensitive to the atomic environment, even for the same type of oxidation groups. Factors determining the chemical shifts of epoxy and hydroxy groups have been discussed. GO structures previously reported in the literature have been checked from the NMR point of view. The energetically favorable hydroxy chain structure is not expected to be widely existed in real GO samples according to our NMR simulations. The epoxy pair species we proposed previously is also supported by chemical shift calculations.

Determination of the antisymmetric part of the chemical shift anisotropy tensor via spin relaxation in nuclear magnetic resonance

Raphael Paquin, Philippe Pelupessy, Luminita Duma, Christel Gervais, and Geoffrey Bodenhausen
Relaxation processes induced by the antisymmetric part of the chemical shift anisotropy tensor (henceforth called anti-CSA) are usually neglected in NMR relaxation studies. It is shown here that anti-CSA components contribute to longitudinal relaxation rates of the indole ¹⁵N nucleus in tryptophan in solution at different magnetic fields and temperatures. To determine the parameters of several models for rotational diffusion and internal dynamics, we measured the longitudinal relaxation rates R₁ = 1/T₁ of ¹⁵N, the ¹⁵N–¹H dipole-dipole (DD) cross-relaxation rates (Overhauser effects), and the cross-correlated CSA/DD relaxation rates involving the second-rank symmetric part of the CSA tensor of ¹⁵N at four magnetic fields B₀ = 9.4, 14.1, 18.8, and 22.3 T (400, 600, 800, and 950 MHz for protons) over a temperature range of 270<T<310>. A good agreement between experimental and theoretical rates can only be obtained if the CSA tensor is assumed to comprise first-rank antisymmetric (anti-CSA) components. The magnitude of the hitherto neglected antisymmetric components is of the order of 10% of the CSA.

Noninvasive bipolar double-pulsed-field-gradient NMR reveals signatures for pore size and shape in polydisperse, randomly oriented, inhomogeneous porous media

Noam Shemesh, Evren Ozarslan, Tal Adiri, Peter J. Basser, and Yoram Cohen
Noninvasive characterization of pore size and shape in opaque porous media is a formidable challenge. NMR diffusion-diffraction patterns were found to be exceptionally useful for obtaining such morphological features, but only when pores are monodisperse and coherently placed. When locally anisotropic pores are randomly oriented, conventional diffusion NMR methods fail. Here, we present a simple, direct, and general approach to obtain both compartment size and shape even in such settings and even when pores are characterized by internal field gradients. Using controlled porous media, we show that the bipolar-double-pulsed-field-gradient (bp-d-PFG) methodology yields diffusion-diffraction patterns from which pore size can be directly obtained. Moreover, we show that pore shape, which cannot be obtained by conventional methods, can be directly inferred from the modulation of the signal in angular bp-d-PFG experiments. This new methodology significantly broadens the types of porous media that can be studied using noninvasive diffusion-diffraction NMR.

Analyzing molecular static linear response properties with perturbed localized orbitals

Jochen Autschbach and Harry F. King
Perturbed localized molecular orbitals (LMOs), correct to first order in an applied static perturbation and consistent with a chosen localization functional, are calculated using analytic derivative techniques. The formalism is outlined for a general static perturbation and variational localization functionals. Iterative and (formally) single-step approaches are compared. The implementation employs an iterative sequence of 2×2 orbital rotations. The procedure is verified by calculations of molecular electric-field perturbations. Boys LMO contributions to the electronic static polarizability and the electric-field perturbation of the 〈r²〉 expectation value are calculated and analyzed for ethene, ethyne, and fluoroethene (H₂CCHF). For ethene, a comparison is made with results from a Pipek–Mezey localization. The calculations show that a chemically intuitive decomposition of the calculated properties is possible with the help of the LMO contributions and that the polarizability contributions in similar molecules are approximately transferable.

Trading sensitivity for information: CarrPurcellMeiboomGill acquisition in solid-state NMR

Krishna K. Dey, Jason T. Ash, Nicole M. Trease, and Philip J. Grandinetti
The Carr–Purcell–Meiboom–Gill (CPMG) experiment has gained popularity in solid-state NMR as a method for enhancing sensitivity for anisotropically broadened spectra of both spin 1/2 and half integer quadrupolar nuclei. Most commonly, the train of CPMG echoes is Fourier transformed directly, which causes the NMR powder pattern to break up into a series of sidebands, sometimes called “spikelets.” Larger sensitivity enhancements are observed as the delay between the π pulses is shortened. As the duration between the π pulses is shortened, however, the echoes become truncated and information about the nuclear spin interactions is lost. We explored the relationship between enhanced sensitivity and loss of information as a function of the product Ω 2τ, where Ω is the span of the anisotropic lineshape and 2τ is the π pulse spacing. For a lineshape dominated by the nuclear shielding anisotropy, we found that the minimum uncertainty in the tensor values is obtained using Ω 2τ values in the range Ω 2τ ≈ 12−1+6 and Ω 2τ ≈ 9−3+3 for η_s = 0 and η_s = 1, respectively. For an anisotropic second-order quadrupolar central transition lineshape under magic-angle spinning (MAS), the optimum range of Ω 2τ ≈ 9−2+3 was found. Additionally, we show how the Two-dimensional One Pulse (TOP) like processing approach can be used to eliminate the cumbersome sideband pattern lineshape and recover a more familiar lineshape that is easily analyzed with conventional lineshape simulation algorithms

Optimized basis sets for the calculation of indirect nuclear spin-spin coupling constants involving the atoms B, Al, Si, P, and Cl

Patricio F. Provasi and Stephan P. A. Sauer
The aug-cc-pVTZ-J series of basis sets for indirect nuclear spin-spin coupling constants has been extended to the atoms B, Al, Si, P, and Cl. The basis sets were obtained according to the scheme previously described by Provasi et al. [J. Chem. Phys. 115, 1324 (2001)] . First, the completely uncontracted correlation consistent aug-cc-pVTZ basis sets were extended with four tight s and three tight d functions. Second, the s and p basis functions were contracted with the molecular orbital coefficients of self-consistent-field calculations performed with the uncontracted basis sets on the simplest hydrides of each atom. As a first illustration, we have calculated the one-bond indirect spin-spin coupling constants in BH4−, BF, AlH, AlF, SiH₄, SiF₄, PH₃, PF₃, H₂S, SF₆, HCl, and ClF at the level of density functional theory using the Becke three parameter Lee–Yang–Parr and the second order polarization propagator approximation with coupled cluster singles and doubles amplitudes.

Recoupling of native homonuclear dipolar couplings in magic-angle-spinning solid-state NMR by the double-oscillating field technique

Lasse Arnt Straaso and Niels Chr. Nielsen
A new solid-state NMR method, the double-oscillating field technique (DUO), that under magic-angle-spinning conditions produces an effective Hamiltonian proportional to the native high-field homonuclear dipole-dipole coupling operator is presented. The method exploits one part of the radio frequency (rf) field to recouple the dipolar coupling interaction with a relatively high scaling factor and to eliminate offset effects over a reasonable bandwidth while in the recoupling frame, the other part gives rise to a sufficiently large longitudinal component of the residual rf field that averages nonsecular terms and in addition ensures stability toward rf inhomogeneity and rf miscalibration. The capability of the DUO experiment to mediate transfer of polarization is described theoretically and compared numerically and experimentally with finite pulse rf driven recoupling and experimentally with dipolar-assisted rotational resonance. Two-dimensional recoupling experiments were performed on antiparallel amyloid fibrils of the decapeptide SNNFGAILSS with the FGAIL fragment uniformly labeled with ¹³C and ¹⁵N.

2010-08-11T15:13:00.000-04:00

J. Am. Chem. Soc., 2010, 132 (28), pp 9561–9563

Rapid Acquisition of Multidimensional Solid-State NMR Spectra of Proteins Facilitated by Covalently Bound Paramagnetic Tags
Philippe S. Nadaud, Jonathan J. Helmus, Ishita Sengupta and Christopher P. Jaroniec

We describe a condensed data collection approach that facilitates rapid acquisition of multidimensional magic-angle spinning solid-state nuclear magnetic resonance (SSNMR) spectra of proteins by combining rapid sample spinning, optimized low-power radio frequency pulse schemes and covalently attached paramagnetic tags to enhance protein 1H spin−lattice relaxation. Using EDTA-Cu2+-modified K28C and N8C mutants of the B1 immunoglobulin binding domain of protein G as models, we demonstrate that high resolution and sensitivity 2D and 3D SSNMR chemical shift correlation spectra can be recorded in as little as several minutes and several hours, respectively, for samples containing 0.1−0.2 μmol of 13C,15N- or 2H,13C,15N-labeled protein. This mode of data acquisition is naturally suited toward the structural SSNMR studies of paramagnetic proteins, for which the typical 1H longitudinal relaxation time constants are inherently a factor of at least 3−4 lower relative to their diamagnetic counterparts. To illustrate this, we demonstrate the rapid site-specific determination of backbone amide 15N longitudinal paramagnetic relaxation enhancements using a pseudo-3D SSNMR experiment based on 15N−13C correlation spectroscopy, and we show that such measurements yield valuable long-range 15N−Cu2+ distance restraints which report on the three-dimensional protein fold.

J. Am. Chem. Soc., 2010, 132 (29), pp 9952–9953
Validation of a Lanthanide Tag for the Analysis of Protein Dynamics by Paramagnetic NMR Spectroscopy
Mathias A. S. Hass, Peter H. J. Keizers, Anneloes Blok, Yoshitaka Hiruma and Marcellus Ubbink

Paramagnetic lanthanide tags potentially can enhance the effects of microsecond to millisecond dynamics in proteins on NMR signals and provide structural information on lowly populated states encoded in the pseudocontact shifts. We have investigated the microsecond to millisecond mobility of a two-point attached lanthanide tag, CLaNP-5, using paramagnetic 1H CPMG relaxation dispersion methods. CLaNP-5 loaded with Lu3+, Yb3+, or Tm3+ was attached to three sites on the surface of two proteins, pseudoazurin and cytochrome c. The paramagnetic center causes large relaxation dispersion effects for two attachment sites, suggesting that local dynamics of the protein at the attachment site causes mobility of the paramagnetic center. At one site the relaxation dispersions are small and limited to the immediate environment of the tag. It is concluded that paramagnetic relaxation dispersion could represent a sensitive method to probe protein dynamics. However, the selection of a rigid attachment site is of critical importance.

J. Am. Chem. Soc., 2010, 132 (29), pp 9956–9957
Solid-State 13C NMR Assignment of Carbon Resonances on Metallic and Semiconducting Single-Walled Carbon Nanotubes
Chaiwat Engtrakul*†, Mark F. Davis†, Kevin Mistry†, Brian A. Larsen†, Anne C. Dillon†, Michael J. Heben‡ and Jeffrey L. Blackburn*†

Solid-state 13C NMR spectroscopy was used to investigate the chemical shift of nanotube carbons on m- and s-SWNTs (metallic and semiconducting single-walled nanotubes) for samples with widely varying s-SWNT content, including samples highly enriched with nearly 100% m- and s-SWNTs. High-resolution 13C NMR was found to be a sensitive probe for m- and s-SWNTs in mixed SWNT samples with diameters of 1.3 nm. The two highly enriched m- and s-SWNT samples clearly exhibited features for m- and s-SNWT 13C nuclei (123 and 122 ppm, respectively) and were successfully fit with a single Gaussian, while five mixed samples required two Gaussians for a satisfactory fit.

J. Am. Chem. Soc., 2010, 132 (29), pp 9979–9981
Probing Slow Protein Dynamics by Adiabatic R1ρ and R2ρ NMR Experiments

Silvia Mangia, Nathaniel J. Traaseth, Gianluigi Veglia, Michael Garwood‡ and Shalom Michaeli

Slow μs/ms dynamics involved in protein folding, binding, catalysis, and allostery are currently detected using NMR dispersion experiments such as CPMG (Carr−Purcell−Meiboom−Gill) or spin-lock R1ρ. In these methods, protein dynamics are obtained by analyzing relaxation dispersion curves obtained from either changing the time spacing between 180° pulses or by changing the effective spin-locking field strength. In this Communication, we introduce a new method to induce a dispersion of relaxation rates. Our approach relies on altering the shape of the adiabatic full passage pulse and is conceptually different from existing approaches. By changing the nature of the adiabatic radiofrequency irradiation, we are able to obtain rotating frame R1ρ and R2ρ dispersion curves that are sensitive to slow μs/ms protein dynamics (demonstrated with ubiquitin). The strengths of this method are to (a) extend the dynamic range of the relaxation dispersion analysis, (b) avoid the need for multiple magnetic field strengths to extract dynamic parameters, (c) measure accurate relaxation rates that are independent of frequency offset, and (d) reduce the stress to NMR hardware (e.g., cryoprobes).

J Phys Chem A

2010-07-26T17:05:00.000-04:00

Triple-Decker Sandwiches and Related Compounds of the First-Row Transition Metals Containing Cyclopentadienyl and Benzene Rings

Haibo Liu, Qian-shu Li, Yaoming Xie, R. Bruce King and Henry F. Schaefer

J. Phys. Chem. A, Article ASAP
DOI: 10.1021/jp104689r
Publication Date (Web): July 21, 2010

The triple-decker sandwich compound trans-Cp2V2(η6:η6-μ-C6H6) has been synthesized, as well as “slipped” sandwich compounds of the type trans-Cp2Co2(η4:η4-μ-arene) and the cis-Cp2Fe2(η4:η4-μ-C6R6) derivatives with an Fe−Fe bond (Cp = η5-cyclopentadienyl). Theoretical studies show that the symmetrical triple-decker sandwich structures trans-Cp2M2(η6:η6-μ-C6H6) are the global minima for M = Ti, V, and Mn but lie 10 kcal/mol above the global minimum for M = Cr. The nonbonding M···M distances and spin states in these triple decker sandwich compounds can be related to the occupancies of the frontier bonding molecular orbitals. The global minimum for the chromium derivative is a singlet spin state cis-Cp2Cr2(η4:η4-μ-C6H6) structure with a very short CrCr distance of 2.06 Å, suggesting a formal quadruple bond. A triplet state cis-Cp2Cr2(η4:η4-μ-C6H6) structure with a predicted Cr≡Cr distance of 2.26 Å lies only 3 kcal/mol above this global minimum. For the later transition metals the global minima are predicted to be cis-Cp2M2(η6:η6-μ-C6H6) structures with a metal−metal bond, rather than triple decker sandwiches. These include singlet cis-Cp2Fe2(η4:η4-μ-C6H6) with a predicted Fe═Fe double bond distance of 2.43 Å, singlet cis-Cp2Co2(η3:η3-μ-C6H6) with a predicted Co—Co single bond distance of 2.59 Å, and triplet cis-Cp2Ni2(η3:η3-μ-C6H6) with a predicted Ni—Ni distance of 2.71 Å.

J. Phys. Chem. C, vol.114, Issue 29

2010-07-26T00:04:00.002-04:00

Molecular Modeling, Multinuclear NMR, and Diffraction Studies in the Templated Synthesis and Characterization of the Aluminophosphate Molecular Sieve STA-2

Maria Castro†, Valerie R. Seymour†, Diego Carnevale†, John M. Griffin†, Sharon E. Ashbrook*†, Paul A. Wright*†, David C. Apperley‡, Julia E. Parker§, Stephen P. Thompson§, Antoine Fecant and Nicolas Bats

J. Phys. Chem. C, 2010, 114 (29), pp 12698–12710
DOI: 10.1021/jp104120y
Copyright © 2010 American Chemical Society

Abstract: Molecular modeling has been used to assist in the design of a new structure directing agent (SDA) for the synthesis of the AlPO4 form of STA-2, bis-diazabicyclooctane-butane (BDAB). This is incorporated as a divalent cation within the large cages of STA-2, as determined via a combination of solid-state 13C and 15N MAS NMR, supported by 14N and 1H-15N HMQC solution NMR and density functional calculations. As-prepared AlPO4 STA-2 containing cationic SDA molecules achieves neutrality by the inclusion of hydroxide ions bridging between 5-fold coordinated framework Al atoms. Synchrotron X-ray powder diffraction data of the dehydrated as-prepared form indicates triclinic symmetry (Al12P12O48(OH)2·BDAB, P1, a = 12.3821(2) Å, b = 12.3795(2) Å, c = 12.3797(3) Å, α = 63.3585(8)°, β = 63.4830(7)°, γ = 63.4218(7)°) with the distortion from rhombohedral R symmetry resulting from the partial order of hydroxide ions in bridging Al−OH−Al sites within cancrinite cages. Upon calcination in oxygen, the organic SDA is removed, leaving AlPO4 STA-2 with a pore volume of 0.22 cm3 g−1 (R, Al36P36O144, a = 12.9270(2) Å, c = 30.7976(4) Å). Dehydrated calcined AlPO4 STA-2 has two crystallographically distinct P and Al sites: 31P MAS NMR resolves the two distinct P sites, and although 27Al MAS NMR only partially resolves the two Al sites, they are separated by MQMAS. Furthermore, 2D 27Al → 31P MQ-J-HETCOR correlation spectroscopy confirms that each framework Al is linked to the two different P sites via Al−O−P connections in a 3:1 ratio (and vice versa for P linked to different Al). The 27Al and 31P resonances are assigned to the crystallographic Al and P sites by calculation of the NMR parameters using the CASTEP DFT program for an energy-minimized AlPO4(SAT) framework.

Propane Aromatization on Zn-Modified Zeolite BEA Studied by Solid-State NMR in Situ

Anton A. Gabrienko†, Sergei S. Arzumanov†, Dieter Freude‡ and Alexander G. Stepanov*†
J. Phys. Chem. C, 2010, 114 (29), pp 12681–12688
DOI: 10.1021/jp103580f
Copyright © 2010 American Chemical Society

Abstract:The conversion of propane (propane-1-13C and propane-2-13C) on Zn/H-BEA zeolite at 520−620 K has been studied by 1H and 13C (CP) MAS NMR. Propene adsorption complex with zinc sites (π-complex) and σ-allylzinc species as intermediates have been identified in the course of propane conversion to aromatics. The mechanism leading to the formation of methane and ethane, which are constituents of an undesirable route in propane conversion, has been examined by kinetic modeling of the expected reaction network based on in situ 1H MAS NMR kinetic measurements of the reaction performance. The pathways for propane aromatization and hydrogenolysis have been proposed. Hydrogenolysis of propane has been concluded to occur with the involvement of both Brønsted acid sites and Zn sites.

13C Chemical Shift of Adsorbed Acetone for Measuring the Acid Strength of Solid Acids: A Theoretical Calculation Study

Hanjun Fang†‡, Anmin Zheng*†, Yueying Chu†‡ and Feng Deng*†
J. Phys. Chem. C, 2010, 114 (29), pp 12711–12718
DOI: 10.1021/jp1044749
Copyright © 2010 American Chemical Society

Abstract: Adsorption of basic probe molecules is one of the widely used methods to characterize the acid strength of solid acids. In this contribution, the adsorptions of acetone on various Brønsted and Lewis acid sites (from weak acid to superacid) are theoretically studied, in order to elucidate the quantitative relationships between 13C chemical shifts of acetone and intrinsic acid strength of solid acids. The Brønsted acid sites are represented by a series of 8T zeolite models with varying terminal Si−H bond lengths, and the different extents of acidic proton transfer from these acid sites to acetone are revealed explicitly. We found that three adsorption conformations (hydrogen-bonded, proton-shared, and ion-pair) exist for acetone, and concurrently, a correlation of three-broken lines is obtained for the 13C chemical shift of acetone versus the deprotonation energy (DPE). The correlation can be used as a scale for quantitatively measuring the Brønsted acid strength of solid acids. A threshold of 245 ppm is determined for superacidity, in good agreement with the experimental value (244 ppm). The Lewis acid sites are modeled by tricoordinate framework aluminum species and various extra-framework aluminum cations or neutral species such as Al3+, AlO+, AlOH2+, Al(OH)2+, Al(OH)3, and AlOOH. We found that acetone is coordinately adsorbed on the aluminum atoms of Lewis acid sites and that the 13C chemical shift of acetone is almost linear to the lowest unoccupied molecular orbital (LUMO) energy of the acid sites.

J. Phys. Chem C. v. 114, issue 28

2010-07-20T21:09:00.004-04:00

45Sc Spectroscopy of Solids: Interpretation of Quadrupole Interaction Parameters and Chemical Shifts

Mara D. Alba*†, Pablo Chain†, Pierre Florian‡ and Dominique Massiot‡

J. Phys. Chem. C, 2010, 114 (28), pp 12125–12132
Publication Date (Web): June 28, 2010

Abstract: The aims of the present study is to describe for the first time the 45Sc MAS NMR spectra of X2-Sc2SiO5 and C-Sc2Si2O7, to combine the spectroscopic information with the structures published from diffraction data, and to propose a rational interpretation of the chemical shifts and quadrupolar parameters. For that purposed, we have correlated the experimental quadrupole coupling parameters of 45Sc determined for a number of scandium compounds to those found by a simple electrostatic calculation and we have found that the isotropic chemical shift of the 45Sc is linearly correlated to the shift parameter, calculated by bond-valence theory. We also show that a simple point charge calculation can approximate the electric field gradient to a sufficiently good approximation that it provides a valuable mean to assign the NMR spectra.

Nuclear Magnetic Resonance Study of Reorientational Motion in α-Mg(BH4)2
Alexander V. Skripov*†, Alexei V. Soloninin†, Olga A. Babanova†, Hans Hagemann‡ and Yaroslav Filinchuk§
J. Phys. Chem. C, 2010, 114 (28), pp 12370–12374
Copyright © 2010 American Chemical Society

Abstract: To study the reorientational motion of BH4 groups in the low-temperature (α) phase of Mg(BH4)2, we have performed nuclear magnetic resonance (NMR) measurements of the 1H and 11B spin−lattice relaxation rates in this compound over wide ranges of temperature (82−443 K) and resonance frequency (14−90 MHz for 1H and 14−28 MHz for 11B). It is found that the thermally activated reorientational motion in α-Mg(BH4)2 is characterized by a coexistence of at least three jump processes with strongly differing activation energies. Taking into account the anisotropy of the local environment of BH4 groups in α-Mg(BH4)2, these jump processes can be attributed to different types of reorientation. The nearly linear coordination of BH4 groups by two Mg atoms suggests that the fastest jump process corresponds to the rotation around the 2-fold axis connecting B and two Mg atoms, whereas the slowest process is associated with the rotation around two other 2-fold axes perpendicular to the Mg−B−Mg line.

Journal of Magnetic Resonance ASAP

2010-07-15T14:43:00.002-04:00

A bit of shameless self-promotion:

Journal of Magnetic Resonance
Article in Press
doi:10.1016/j.jmr.2010.05.018

The application of frequency swept pulses for the acquisition of nuclear quadrupole resonance spectra
Aaron J. Rossinia, Hiyam Hamaeda and Robert W. Schurko, a,

Abstract:The acquisition of nuclear quadrupole resonance (NQR) spectra with wideband uniform rate and smooth truncation (WURST) pulses is investigated. 75As and 35Cl NQR spectra acquired with the WURST echo sequence are compared to those acquired with standard Hahn-echo sequences and echo sequences which employ composite refocusing pulses. The utility of WURST pulses for locating NQR resonances of unknown frequency is investigated by monitoring the integrated intensity and signal to noise of 35Cl and 75As NQR spectra acquired with transmitter offsets of several hundreds kilohertz from the resonance frequencies. The WURST echo sequence is demonstrated to possess superior excitation bandwidths in comparison to the pulse sequences which employ conventional monochromatic rectangular pulses. The superior excitation bandwidths of the WURST pulses allows for differences in the characteristic impedance of the receiving and excitation circuits of the spectrometer to be detected. Impedance mismatches have previously been reported by Marion and Desvaux [D.J.Y. Marion, H. Desvaux, J. Magn. Reson. (2008) 193(1) 153–157] and Muller et al. [M. Nausner, J. Schlagnitweit, V. Smrecki, X. Yang, A. Jerschow, N. Muller, J. Magn. Reson. (2009) 198(1) 73–79]. In this regard, WURST pulse sequences may afford an efficient new method for experimentally detecting impedance mismatches between receiving and excitation circuits, allowing for the optimization of solids and solution NMR and NQR spectrometer systems. The use of the Carr–Purcell Meiboom–Gill (CPMG) pulse sequence for signal enhancement of NQR spectra acquired with WURST pulses and conventional pulses is also investigated. Finally, the utility of WURST pulses for the acquisition of wideline NQR spectra is demonstrated by acquiring part of the 63/65Cu NQR spectrum of CuCN.

Magn. Reson. Chem. - July 2010

2010-07-12T12:05:00.001-04:00

New perspectives in the PAW/GIPAW approach: JP-O-Si coupling constants, antisymmetric parts of shift tensors and NQR predictions

from Magnetic Resonance in Chemistry by Christian Bonhomme, Christel Gervais, Cristina Coelho, Frédérique Pourpoint, Thierry Azaïs, Laure Bonhomme-Coury, Florence Babonneau, Guy Jacob, Maude Ferrari, Daniel Canet, Jonathan R. Yates, Chris J. Pickard, Siân A. Joyce, Francesco Mauri, Dominique Massiot

In 2001, Pickard and Mauri implemented the gauge including projected augmented wave (GIPAW) protocol for first-principles calculations of NMR parameters using periodic boundary conditions (chemical shift anisotropy and electric field gradient tensors). In this paper, three potentially interesting perspectives in connection with PAW/GIPAW in solid-state NMR and pure nuclear quadrupole resonance (NQR) are presented: (i) the calculation of J coupling tensors in inorganic solids; (ii) the calculation of the antisymmetric part of chemical shift tensors and (iii) the prediction of 14N and 35Cl pure NQR resonances including dynamics. We believe that these topics should open new insights in the combination of GIPAW, NMR/NQR crystallography, temperature effects and dynamics. Points (i), (ii) and (iii) will be illustrated by selected examples: (i) chemical shift tensors and heteronuclear 2JP[bond]O[bond]Si coupling constants in the case of silicophosphates and calcium phosphates [Si5O(PO4)6, SiP2O7 polymorphs and [alpha]-Ca(PO3)2]; (ii) antisymmetric chemical shift tensors in cyclopropene derivatives, C3X4 (X = H, Cl, F) and (iii) 14N and 35Cl NQR predictions in the case of RDX (C3H6N6O6), [beta]-HMX (C4H8N8O8), [alpha]-NTO (C2H2N4O3) and AlOPCl6. RDX, [beta]-HMX and [alpha]-NTO are explosive compounds.

Received: 31 March 2010; Revised: 17 May 2010; Accepted: 20 May 2010
Digital Object Identifier (DOI)
10.1002/mrc.2635

Computation and NMR crystallography of terbutaline sulfate
from Magnetic Resonance in Chemistry by Robin K. Harris, Paul Hodgkinson, Vadim Zorin, Jean-Nicolas Dumez, Bénédicte Elena-Herrmann, Lyndon Emsley, Elodie Salager, Robin S. Stein

This article addresses, by means of computation and advanced experiments, one of the key challenges of NMR crystallography, namely the assignment of individual resonances to specific sites in a crystal structure. Moreover, it shows how NMR can be used for crystal structure validation. The case examined is form B of terbutaline sulfate. CPMAS 13C and fast MAS 1H spectra have been recorded and the peaks assigned as far as possible. Comparison of 13C chemical shifts computed using the CASTEP program (incorporating the Gauge Including Projector Augmented Wave principle) with those obtained experimentally enable the accuracy of the two distinct single-crystal evaluations of the structure to be compared and an error in one of these is located. The computations have substantially aided in the assignments of both 13C and 1H resonances, as has a series of two-dimensional (2D) spectra (HETCOR, DQ-CRAMPS and proton-proton spin diffusion). The 2D spectra have enabled many of the proton chemical shifts to be pinpointed. The relationships of the NMR shifts to the specific nuclear sites in the crystal structure have therefore been established for most 13C peaks and for some 1H signals. Emphasis is placed on the effects of hydrogen bonding on the proton chemical shifts.

Received: 26 March 2010; Revised: 20 May 2010; Accepted: 24 May 2010
Digital Object Identifier (DOI)
10.1002/mrc.2636

Prediction of NMR J-coupling in solids with the planewave pseudopotential approach
from Magnetic Resonance in Chemistry by Jonathan R. Yates
We review the calculation of NMR J-coupling in solid materials using the planewave pseudopotential formalism of Density Functional Theory. The methodology is briefly summarised and an account of recent applications is given. We discuss various aspects of the calculations which should be taken into account when comparing results with solid-state NMR experiments including anisotropy and orientation of the J tensors, the reduced coupling constant, and the relation between J and crystal structure. Copyright © 2010 John Wiley & Sons, Ltd.

Received: 20 April 2010; Revised: 2 June 2010; Accepted: 4 June 2010
Digital Object Identifier (DOI)
10.1002/mrc.2646

Comparing quantum-chemical calculation methods for structural investigation of zeolite crystal structures by solid-state NMR spectroscopy
from Magnetic Resonance in Chemistry by Darren H. Brouwer, Igor L. Moudrakovski, Richard J. Darton, Russell E. Morris

Combining quantum-chemical calculations and ultrahigh-field NMR measurements of 29Si chemical shielding (CS) tensors has provided a powerful approach for probing the fine details of zeolite crystal structures. In previous work, the quantum-chemical calculations have been performed on 'molecular fragments' extracted from the zeolite crystal structure using Hartree-Fock methods (as implemented in Gaussian). Using recently acquired ultrahigh-field 29Si NMR data for the pure silica zeolite ITQ-4, we report the results of calculations using recently developed quantum-chemical calculation methods for periodic crystalline solids (as implemented in CAmbridge Serial Total Energy Package (CASTEP) and compare these calculations to those calculated with Gaussian. Furthermore, in the context of NMR crystallography of zeolites, we report the completion of the NMR crystallography of the zeolite ITQ-4, which was previously solved from NMR data. We compare three options for the 'refinement' of zeolite crystal structures from 'NMR-solved' structures: (i) a simple target-distance based geometry optimization, (ii) refinement of atomic coordinates in which the differences between experimental and calculated 29Si CS tensors are minimized, and (iii) refinement of atomic coordinates to minimize the total energy of the lattice using CASTEP quantum-chemical calculations. All three refinement approaches give structures that are in remarkably good agreement with the single-crystal X-ray diffraction structure of ITQ-4.

Received: 31 March 2010; Revised: 27 May 2010; Accepted: 2 June 2010
Digital Object Identifier (DOI)
10.1002/mrc.2642

Prog. in NMR Spectrosc. - July 2010 - Floquet theory

2010-07-12T11:54:00.003-04:00

Progress in Nuclear Magnetic Resonance Spectroscopy
doi:10.1016/j.pnmrs.2010.06.002
Floquet Theory in Solid-State Nuclear Magnetic Resonance
Michal Leskes, P.K. Madhu, Shimon Vega

Received 12 April 2010; accepted 11 June 2010. Available online 30 June 2010.

Keywords: Solid-state NMR; Floquet Theory; van Vleck transformation; Average Hamiltonian Theory

Journal Update

2010-06-25T17:10:00.000-04:00

J. Am. Chem. Soc., 2010, 132 (16), pp 5538–5539

Breaking the T1 Constraint for Quantitative Measurement in Magic Angle Spinning Solid-State NMR Spectroscopy
Guangjin Hou, Shangwu Ding, Limin Zhang and Feng Deng

Quantitative solid-state NMR experimental schemes that break the conventional T1 constraint are described. The combination of broad-band homonuclear recoupling techniques and the conventional single pulse or cross-polarization (CP) schemes (referred as QUSP or QUCP) render the long T1 of low-? spins no longer a constraint for obtaining quantitative NMR spectra. During the mixing time when dipolar recoupling occurs, the nonuniformly CP enhanced or recovered spin magnetization is redistributed under the reintroduced homonuclear dipole-dipole interactions so that uniformly enhanced or recovered magnetization is achieved when the system reaches the quasi-equilibrium state. It is shown that quantitative NMR spectra can be obtained for the recycle delays substantially shorter than the conventionally required 5T1. In addition, the high efficiency gain can be achieved in QUSP and QUCP experiments with a relatively short recycle delay.

J. Am. Chem. Soc., 2010, 132 (16), pp 5546–5547

Detection of Transient Interchain Interactions in the Intrinsically Disordered Protein a-Synuclein by NMR Paramagnetic Relaxation Enhancement
Kuen-Phon Wu and Jean Baum

NMR paramagnetic relaxation enhancement experiments were applied to the intrinsically disordered protein a-synuclein, the primary protein in Parkinson's disease, to directly characterize transient intermolecular complexes at neutral and low pH. At neutral pH, we observed weak N- to C-terminal interchain contacts driven by electrostatic interactions, while at low pH, the C- to C-terminal interchain interactions are significantly stronger and driven by hydrophobic contacts. Characterization of these first interchain interactions will provide fundamental insight into the mechanism of amyloid formation.

J. Am. Chem. Soc., 2010, 132 (16), pp 5556–5557

Fibrillar vs Crystalline Full-Length ß-2-Microglobulin Studied by High-Resolution Solid-State NMR Spectroscopy
Emeline Barbet-Massin†, Stefano Ricagno‡§, Józef R. Lewandowski†, Sofia Giorgetti§, Vittorio Bellotti‡§, Martino Bolognesi, Lyndon Emsley† and Guido Pintacuda*†

Elucidating the fine structure of amyloid fibrils as well as understanding their processes of nucleation and growth remains a difficult yet essential challenge, directly linked to our current poor insight into protein misfolding and aggregation diseases. Here we consider ß-2-microglobulin (ß2m), the MHC-1 light chain component responsible for dialysis-related amyloidosis, which can give rise to amyloid fibrils in vitro under various experimental conditions, including low and neutral pH. We have used solid-state NMR to probe the structural features of fibrils formed by full-length ß2m (99 residues) at pH 2.5 and pH 7.4. A close comparison of 2D 13C-13C and 15N-13C correlation experiments performed on ß2m, in both the crystalline and fibrillar states, suggests that, in spite of structural changes affecting the protein loops linking the protein ß-strands, the protein chain retains a substantial share of its native secondary structure in the fibril assembly. Moreover, variations in the chemical shifts of the key Pro32 residue suggest the involvement of a cis-trans isomerization in the process of ß2m fibril formation. Lastly, the analogy of the spectra recorded on ß2m fibrils grown at different pH values hints at a conserved architecture of the amyloid species thus obtained.

J. Am. Chem. Soc., 2010, 132 (16), pp 5558–5559

Ultrafast MAS Solid-State NMR Permits Extensive 13C and 1H Detection in Paramagnetic Metalloproteins
Ivano Bertini, Lyndon Emsley§, Moreno Lelli, Claudio Luchinat, Jiafei Mao and Guido Pintacuda

We show here that by combining tailored approaches based on ultrafast (60 kHz) MAS on the CoII-replaced catalytic domain of matrix metalloproteinase 12 (CoMMP-12) we can observe and assign, in a highly paramagnetic protein in the solid state, 13C and even 1H resonances from the residues coordinating the metal center. In addition, by exploiting the enhanced relaxation caused by the paramagnetic center, and the low power irradiation enabled by the fast MAS, this can be achieved in remarkably short times and at very high field (21.2 T), with only less than 1 mg of sample. Furthermore, using the known crystal structure of the compound, we are able to distinguish and measure pseudocontact (PCS) contributions to the shifts up to the coordinating ligands and to unveil structural information.

J. Am. Chem. Soc., 2010, 132 (16), pp 5672–5676

NMR-Based Structural Modeling of Graphite Oxide Using Multidimensional 13C Solid-State NMR and ab Initio Chemical Shift Calculations
Leah B. Casabianca†, Medhat A. Shaibat†, Weiwei W. Cai‡, Sungjin Park‡, Richard Piner‡, Rodney S. Ruoff‡ and Yoshitaka Ishii†

Chemically modified graphenes and other graphite-based materials have attracted growing interest for their unique potential as lightweight electronic and structural nanomaterials. It is an important challenge to construct structural models of noncrystalline graphite-based materials on the basis of NMR or other spectroscopic data. To address this challenge, a solid-state NMR (SSNMR)-based structural modeling approach is presented on graphite oxide (GO), which is a prominent precursor and interesting benchmark system of modified graphene. An experimental 2D 13C double-quantum/single-quantum correlation SSNMR spectrum of 13C-labeled GO was compared with spectra simulated for different structural models using ab initio geometry optimization and chemical shift calculations. The results show that the spectral features of the GO sample are best reproduced by a geometry-optimized structural model that is based on the Lerf-Klinowski model (Lerf, A. et al. Phys. Chem. B 1998, 102, 4477); this model is composed of interconnected sp2, 1,2-epoxide, and COH carbons. This study also convincingly excludes the possibility of other previously proposed models, including the highly oxidized structures involving 1,3-epoxide carbons (Szabo, I. et al. Chem. Mater. 2006, 18, 2740). 13C chemical shift anisotropy (CSA) patterns measured by a 2D 13C CSA/isotropic shift correlation SSNMR were well reproduced by the chemical shift tensor obtained by the ab initio calculation for the former model. The approach presented here is likely to be applicable to other chemically modified graphenes and graphite-based systems.

J. Am. Chem. Soc., 2010, 132 (16), pp 5779–5788

Proton-Evolved Local-Field Solid-State NMR Studies of Cytochrome b5 Embedded in Bicelles, Revealing both Structural and Dynamical Information
Ronald Soong†, Pieter E. S. Smith†, Jiadi Xu†, Kazutoshi Yamamoto†, Sang-Choul Im‡, Lucy Waskell‡ and Ayyalusamy Ramamoorthy*†

Structural biology of membrane proteins has rapidly evolved into a new frontier of science. Although solving the structure of a membrane protein with atomic-level resolution is still a major challenge, separated local field (SLF) NMR spectroscopy has become an invaluable tool in obtaining structural images of membrane proteins under physiological conditions. Recent studies have demonstrated the use of rotating-frame SLF techniques to accurately measure strong heteronuclear dipolar couplings between directly bonded nuclei. However, in these experiments, all weak dipolar couplings are suppressed. On the other hand, weak heteronuclear dipolar couplings can be measured using laboratory-frame SLF experiments, but only at the expense of spectral resolution for strongly dipolar coupled spins. In the present study, we implemented two-dimensional proton-evolved local-field (2D PELF) pulse sequences using either composite zero cross-polarization (COMPOZER-CP) or windowless isotropic mixing (WIM) for magnetization transfer. These PELF sequences can be used for the measurement of a broad range of heteronuclear dipolar couplings, allowing for a complete mapping of protein dynamics in a lipid bilayer environment. Experimental results from magnetically aligned bicelles containing uniformly 15N-labeled cytochrome b5 are presented and theoretical analyses of the new PELF sequences are reported. Our results suggest that the PELF-based experimental approaches will have a profound impact on solid-state NMR spectroscopy of membrane proteins and other membrane-associated molecules in magnetically aligned bicelles.

J. Am. Chem. Soc., 2010, 132 (16), pp 5803–5811

Changes in Transmembrane Helix Alignment by Arginine Residues Revealed by Solid-State NMR Experiments and Coarse-Grained MD Simulations
Vitaly V. Vostrikov‡†, Benjamin A. Hall§†, Denise V. Greathouse‡, Roger E. Koeppe, II*‡ and Mark S. P. Sansom*§

Independent experimental and computational approaches show agreement concerning arginine/membrane interactions when a single arginine is introduced at selected positions within the membrane-spanning region of acetyl-GGALW5LALALAL12AL14ALALW19LAGA-ethanolamide, designated GWALP23. Peptide sequence isomers having Arg in position 12 or position 14 display markedly different behaviors, as deduced by both solid-state NMR experiments and coarse-grained molecular dynamics (CG-MD) simulations. With respect to the membrane normal of DOPC or DPPC lipid bilayer membranes, GWALP23-R14 shows one major state whose apparent average tilt is 10° greater than that of GWALP23. The presence of R14 furthermore induces bilayer thinning and peptide displacement to "lift" the charged guanidinium toward the bilayer surface. By contrast, GWALP23-R12 exhibits multiple states that are in slow exchange on the NMR time scale, with CG-MD simulations indicating two distinct positions with different screw rotation angles in the membrane, along with an increased tendency to exit the lipid bilayer.

Hiyam's Journal Update

2010-06-23T14:55:00.001-04:00

J. Am. Chem. Soc., 2010, 132 (21), pp 7321–7337

Molecular Silicate and Aluminate Species in Anhydrous and Hydrated Cements
Aditya Rawal, Benjamin J. Smith†, George L. Athens, Christopher L. Edwards, Lawrence Roberts, Vijay Gupta and Bradley F. Chmelka

The compositions and molecular structures of anhydrous and hydrated cements are established by using advanced solid-state nuclear magnetic resonance (NMR) spectroscopy methods to distinguish among different molecular species and changes that occur as a result of cement hydration and setting. One- and two-dimensional (2D) solid-state 29Si and 27Al magic-angle spinning NMR methodologies, including T1-relaxation-time- and chemical-shift-anisotropy-filtered measurements and the use of very high magnetic fields (19 T), allow resonances from different silicate and aluminate moieties to be resolved and assigned in complicated spectra. Single-pulse 29Si and 27Al NMR spectra are correlated with X-ray fluorescence results to quantify the different crystalline and disordered silicate and aluminate species in anhydrous and hydrated cements. 2D 29Si{1H} and 27Al{1H}heteronuclear correlation NMR spectra of hydrated cements establish interactions between water and hydroxyl moieties with distinct 27Al and 29Si species. The use of a 29Si T1-filter allows anhydrous and hydrated silicate species associated with iron-containing components in the cements to be distinguished, showing that they segregate from calcium silicate and aluminate components during hydration. The different compositions of white Portland and gray oilwell cements are shown to have distinct molecular characteristics that are correlated with their hydration behaviors.

A Resonance Assignment Method for Oriented-Sample Solid-State NMR of Proteins
Robert W. Knox†, George J. Lu‡, Stanley J. Opella‡ and Alexander A. Nevzorov

J. Am. Chem. Soc., 2010, 132 (24), pp 8255–8257

A general sequential assignment strategy for uniformly 15N-labeled uniaxially aligned membrane proteins is proposed. Mismatched Hartmann−Hahn magnetization transfer is employed to establish proton-mediated correlations among the neighboring 15N backbone spins. Magnetically aligned Pf1 phage coat protein was used to illustrate the method. Exchanged and nonexchanged separated local field spectra were acquired and overlaid to distinguish the cross-peaks from the main peaks. Most of the original assignments from the literature were confirmed without selectively labeled samples. This method is applicable to proteins with arbitrary topology and will find use in assigning solid-state NMR spectra of oriented membrane proteins for their subsequent structure determination.

The Polar Phase of NaNbO3: A Combined Study by Powder Diffraction, Solid-State NMR, and First-Principles Calculations
Karen E. Johnston†, Chiu C. Tang‡, Julia E. Parker‡, Kevin S. Knight§, Philip Lightfoot*† and Sharon E. Ashbrook*†

J. Am. Chem. Soc., 2010, 132 (25), pp 8732–8746

A polar phase of NaNbO3 has been successfully synthesized using sol-gel techniques. Detailed characterization of this phase has been undertaken using high-resolution powder diffraction (X-ray and neutron) and 23Na multiple-quantum (MQ) MAS NMR, supported by second harmonic generation measurements and density functional theory calculations. Samples of NaNbO3 were also synthesized using conventional solid-state methods and were observed to routinely comprise of a mixture of two different polymorphs of NaNbO3, namely, the well-known orthorhombic phase (space group Pbcm) and the current polar phase, the relative quantities of which vary considerably depending upon precise reaction conditions. Our studies show that each of these two polymorphs of NaNbO3 contains two crystallographically distinct Na sites. This is consistent with assignment of the polar phase to the orthorhombic space group P21ma, although peak broadenings in the diffraction data suggest a subtle monoclinic distortion. Using carefully monitored molten salt techniques, it was possible to eradicate the polar polymorph and synthesize the pure Pbcm phase.

NMR Methods for Characterizing the Pore Structures and Hydrogen Storage Properties of Microporous Carbons
Robert J. Anderson†, Thomas P. McNicholas‡, Alfred Kleinhammes*†, Anmiao Wang‡, Jie Liu‡ and Yue Wu†

J. Am. Chem. Soc., 2010, 132 (25), pp 8618–8626

1H NMR spectroscopy is used to investigate a series of microporous activated carbons derived from a poly(ether ether ketone) (PEEK) precursor with varying amounts of burnoff (BO). In particular, properties relevant to hydrogen storage are evaluated such as pore structure, average pore size, uptake, and binding energy. High-pressure NMR with in situ H2 loading is employed with H2 pressure ranging from 100 Pa to 10 MPa. An N2-cooled cryostat allows for NMR isotherm measurements at both room temperature (290 K) and 100 K. Two distinct 1H NMR peaks appear in the spectra which represent the gaseous H2 in intergranular pores and the H2 residing in micropores. The chemical shift of the micropore peak is observed to evolve with changing pressure, the magnitude of this effect being correlated to the amount of BO and therefore the structure. This is attributed to the different pressure dependence of the amount of adsorbed and non-adsorbed molecules within micropores, which experience significantly different chemical shifts due to the strong distance dependence of the ring current effect. In pores with a critical diameter of 1.2 nm or less, no pressure dependence is observed because they are not wide enough to host
non-adsorbed molecules; this is the case for samples with less than 35% BO. The largest estimated pore size that can contribute to the micropore peak is estimated to be around 2.4 nm. The total H2 uptake associated with pores of this size or smaller is evaluated via a calibration of the isotherms, with the highest amount being observed at 59% BO. Two binding energies are present in the micropores, with the lower, more dominant one being on the order of 5 kJ mol−1 and the higher one ranging from 7 to 9 kJ mol−1.

Characterization of RNA Invasion by 19F NMR Spectroscopy
Anu Kiviniemi and Pasi Virta*

J. Am. Chem. Soc., 2010, 132 (25), pp 8560–8562

19F NMR spectroscopy offers an efficient tool for monitoring RNA invasion. The invasion of 2′-O-methyl oligoribonucleotides into a 19F-labeled HIV-1 TAR RNA model and the temperature-dependent behavior of the complexes obtained have been examined.

J. Phys. Chem. A

2010-06-21T10:19:00.000-04:00

Solid-State NMR Spectra and Long, Intra-Dimer Bonding in the π-[TTF]22+ (TTF = Tetrathiafulvalene) Dication

Merrill D. Halling, Joshua D. Bell, Ronald J. Pugmire, David M. Grant* and Joel S. Miller

J. Phys. Chem. A, 2010, 114 (24), pp 6622–6629
DOI: 10.1021/jp910509f
Publication Date (Web): June 2, 2010

The 13C chemical-shift tensor principal values for TTF and π-[TTF]22+ (TTF = tetrathiafulvalene) dimer dications have been measured in order to better understand the electronic structure and long intradimer bonding of these TTF-based dimer structures. The structure of π-[TTF]22+ is abnormal due to its two C−C and four S−S ca. 3.4 Å intradimer separations, which is less than the sum of the sulfur van der Waals radii, and has a singlet 1A1g electronic ground state. This study of TTF and [TTF]22+ was conducted to determine how the NMR chemical-shift tensor principal values change as a function of electronic structure. This study also establishes a better understanding of the interactions that lead to spin-pairing of the monomeric radical units. The density functional theory (DFT) calculated nuclear shielding tensors are correlated with the experimentally determined principal chemical-shift values. The embedded ion method (EIM) was used to investigate the electrostatic lattice potential in [TTF]22+. These theoretical methods provide information on the tensor magnitudes and orientations of their tensor principal values with respect to the molecular frame. The experimental chemical-shift principal values agree with the calculated quantum mechanical chemical-shielding principal values, within typical errors commonly seen for this class of molecular system. Relatively weak Wiberg bond orders between the two [TTF]+ components of the dimer dication correlate with the long bonds linking the two [TTF]+ monomers and substantiate the claim that there is weak multicenter bonding present.
_________________________________________

Interpretation of Indirect Nuclear Spin−Spin Couplings in Isomers of Adenine: Novel Approach to Analyze Coupling Electron Deformation Density Using Localized Molecular Orbitals

Radek Marek*, Aneka Kstkov, Kateina Malikov, Jaromr Touek, Jaromr Marek, Michal Hocek, Olga L. Malkina* and Vladimir G. Malkin

J. Phys. Chem. A, 2010, 114 (24), pp 6689–6700
DOI: 10.1021/jp102186r
Publication Date (Web): June 2, 2010

Adenine, an essential building block of nucleic acids present in all living systems, can occur in several tautomeric forms. The phenomenon of tautomerism can be investigated by several experimental methods, including nuclear magnetic resonance. In this study, long-range 1H−13C and 1H−15N coupling constants for N-alkyl derivatives related to four tautomers of adenine are investigated in DMSO and DMF solutions. To investigate the structural dependence of the coupling constants and to understand how polarization propagates in the system, Fermi contact (FC) terms were calculated for the individual isomers and analyzed by using density functional theory (DFT), and the coupling pathways were visualized using real-space functions. The coupling electron deformation densities (CDD) of several 1H−X (X = 13C, 15N) pairs are evaluated and compared. In order to analyze the CDD in more detail, a new approach to break down the CDD into contributions from Boys or Pipek−Mezey localized molecular orbitals (LMOs) has been developed. A similar approach has been applied to split the value of the FC contribution to the J coupling into the LMO contributions. On the basis of chemical concepts, the contributions of σ-bonds, π-electrons, and lone pairs of electrons are discussed. The lone pair of electrons at the nitrogen atom contributes significantly to the 1H−C═15N coupling, whereas the 1H−C═N−13C coupling is affected in a somewhat different way. Surprisingly, the contribution of the intervening C═N bond to the FC term for 1H−C═15N coupling originates exclusively in σ-electrons, with a vanishingly small contribution calculated for the π-electrons of this fragment. This behavior is rationalized by introducing the concept of “hard and soft J elements” derived from the polarizability of the individual components.
________________________________________

NMR Spectroscopic Parameters of Molecular Systems with Strong Hydrogen Bonds

Natalia Zarycz and Gustavo A. Aucar*

J. Phys. Chem. A, Article ASAP
DOI: 10.1021/jp1019334
Publication Date (Web): June 16, 2010

A series of closed H-bonded molecules that have (or not) delocalized bonds were studied. The dependence of both NMR spectroscopic parameters σ and J-couplings, and also the energy stability of such molecules with H-bond strength, were analyzed. The selected basic geometrical structure was that of malonaldehyde. From its full optimized geometry, the corresponding geometry of 3-OH propanal was obtained, fixing either the d(O−O) distance or a more extended local geometry and then optimizing the other part of the whole structure. Nitromalonaldehyde and nitromalonamide were also studied because they should have stronger H-bonds and their basic structure is also malonaldehyde. The last one also has electronic effects that may be varied by rotating the amino groups. By doing this it is possible to show that the effects on acidity of donors are more important than the equivalent effects on the basicity of acceptors. It is also shown that J-couplings that involve atoms close to the H-bond have important noncontact contributions that must be included in order to reproduce total J values. Noncontact contributions are more important than the Fermi contact (FC) one for J(O−O) in malonaldehyde. In nitromalonamide all three terms, FC, paramagnetic spin-orbital, and spin-dipolar are of the same order of magnitude when both amino groups are rotated. This does not happen for its planar configuration. Nuclear magnetic shielding of the hydrogen belonging to the H-bond is quite sensitive to it. The magnetic behavior of such hydrogen atom is modified when it is part of a closed H-bonded molecule. Then a relationship between the H-bond strength with the paramagnetic contributions of the shieldings of both atoms, C and O of the donor substructure, was obtained. We have found a cubic correlation between σp (C) of the C−O donor bond with σ (H) of the H-bonded hydrogen. It is observed that both the noncontact J-coupling contributions and shieldings on atoms belonging to the donor substructure, give a clear evidence about the presence of the resonance phenomenon in the model compounds that have been studied, malonaldehyde, nitromalonaldehyde, and nitromalonamide.
_________________________________

Spin-State-Corrected Gaussian-Type Orbital Basis Sets

Marcel Swart*, Mireia Gell, Josep M. Luis and Miquel Sola

J. Phys. Chem. A, Article ASAP
DOI: 10.1021/jp102712z
Publication Date (Web): June 16, 2010

Recently, we reported that the basis set has a profound influence on the computed values for spin-state splittings [ J. Phys. Chem. A 2008, 112, 6384]. In particular, small Gaussian-type orbital (GTO) basis sets were shown to be unreliable for the prediction of them. Here, we report simple modifications of the small Pople-type Gaussian-type orbital basis sets (3-21G, 3-21G*, 6-31G, 6-31G*), which correct their faulty behavior for the spin-state energies. We have investigated the basis sets for a set of 13 first-row transition-metal complexes for which reliable reference data have been obtained at the OPBE/TZ2P(STO) level. For several systems, we have used single and double spin-contamination corrections to avoid ambiguity of the results because of spin contamination, that is, the energies and geometries were obtained for the pure spin states. The spin ground states as predicted by the spin-state-corrected GTO basis sets (s6-31G, s6-31G*) are in complete agreement with the reference Slater-type orbital (STO) data, while those of the original basis sets and a recent modification by Baker and Pulay (m6-31G*) are not for all cases. The spin-state-corrected GTO basis sets also improve upon the original and modified basis sets for the accuracy of geometry optimization, while the accuracy of the vibrational frequencies is as good or better. At a limited additional cost, one therefore obtains very reliable results for these important spin-state energies.

Journal of Materials Chemistry

2010-06-15T12:20:00.001-04:00

Probing the local structures and protonic conduction pathways in scandium substituted BaZrO₃ by multinuclear solid-state NMR spectroscopy

Lucienne Buannic ^a, Frédéric Blanc ^a, Ivan Hung ^b, Zhehong Gan ^b and Clare P. Grey *^ac

A comprehensive multinuclear solid-state NMR study of scandium-substituted BaZrO₃ is reported. Static low field and MQMAS very high field ⁴⁵Sc NMR data revealed the presence of both 5- and 6-coordinated scandium atoms, 5-coordinated scandium arising from Sc nearby an oxygen vacancy. ¹⁷O NMR spectra showed the presence of up to three different chemical oxygen environments assigned to Zr–O–Zr, Zr–O–Sc and Sc–O–Sc. From the ratios of these different oxygen sites, the distribution of the scandium cations was close to random but indicated that the maximum scandium incorporation was lower than expected, consistent with the observation of Sc₂O₃ impurities at substitution levels of 30% Sc for Zr. ¹H and ⁴⁵Sc NMR data on the hydrated materials revealed the presence of scandium next to protonic defects. Finally, variable temperature ¹H NMR showed the presence of at least two different proton environments in between which proton transfer occurs at ambient temperatures (300 K).

http://www.rsc.org/delivery/_ArticleLinking/ArticleLinking.cfm?JournalCode=JM&Year=2010&ManuscriptID=c0jm00155d&Iss=Advance_Article

DOI: 10.1039/c0jm00155d

Journal of Physical Chemistry B and C, v114, Issues 23

2010-06-10T14:48:00.002-04:00

Crystalline Aluminum Hydroxide Fluorides AlFx(OH)3−x·H2O: Structural Insights from 1H and 2H Solid State NMR and Vibrational Spectroscopy

G. Scholz*, S. Brehme, R. Knig, D. Heidemann and E. Kemnitz*
J. Phys. Chem. C, 2010, 114 (23), pp 10535–10543
DOI: 10.1021/jp1023857

AbstractFor the first time, 1H/2H MAS NMR signals of crystalline hydroxide fluorides AlFx(OH)3−x·H2O, as well as of the dehydrated samples, both with pyrochlore structure, were resolved, identified, and assigned in direct correlation with vibrational bands of respective FT IR spectra. The use of magnetically diluted samples in combination with 1H spin−echo experiments, 2H MAS, and 19F−2H CP and 1H−2H CP MAS NMR experiments gave information on different 2H (1H) sites in relation to present structural motifs known from the crystal structure.

Angewandte Chemie International Edition

2010-06-10T11:10:00.001-04:00

Angewandte Chemie International Edition
Early View (Articles online in advance of print)
Published Online: 8 Jun 2010

High-Resolution Studies of Uniformly 13C,15H-Labeled RNA by Solid-State NMR Spectroscopy

Alexey V. Cherepanov, Dr., Clemens Glaubitz, Prof. Dr., Harald Schwalbe, Prof. Dr.

Keywords: conformational analysis • freeze-quenching • NMR spectroscopy • RNA • solid-state structures

Abstract: Chilling out: Solid-state 13C NMR correlation spectroscopy was used to assign the signals of a uniformly labeled RNA hairpin infrozen aqueous solution. Conformational analysis shows that solutions of biologically relevant RNAs can freeze withoutsignificant changes in RNA structure and without critical loss of resolution and sensitivity in NMR experiments.

10.1002/anie.200906885

Angewandte Chemie International Edition
Early View (Articles online in advance of print)
Published Online: 8 Jun 2010

The Elusive Enamine Intermediate in Proline-Catalyzed Aldol Reactions: NMR Detection, Formation Pathway, and Stabilization Trends

Markus B. Schmid, Kirsten Zeitler, Dr., Ruth M. Gschwind, Prof. Dr.

Keywords: aldol reaction • enamines • NMR spectroscopy • organocatalysis • proline catalysis

Abstract: The missing link: The elusive enamine intermediate of nucleophilic proline catalysis was detected and stereochemicallycharacterized by NMR analysis of the aldehyde self-aldolization reaction in dipolar aprotic solvents. NMR exchange spectroscopy(EXSY) was used to observe direct enamine formation from oxazolidinones. Additionally, the stabilization of the intermediate bythe appropriate choice of solvent and substitution pattern on the aldehyde is presented.

10.1002/anie.200906629

Phys. Chem. Chem. Phys. 2010 Vol. 12 Issue 22

2010-06-08T16:17:00.002-04:00

An entire issue dedicated to dynamic nuclear polarization NMR (cross polarization from an added paramagnetic agent to the nucleus of interest). Requires additional hardware but it is an interesting concept. The following is a good introductory journal:

Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results

Melanie Rosay, Leo Tometich, Shane Pawsey, Reto Bader, Robert Schauwecker, Monica Blank, Philipp M. Borchard, Stephen R. Cauffman, Kevin L. Felch, Ralph T. Weber, Richard J. Temkin, Robert G. Griffin and Werner E. Maas

________________________________________

Dynamic Nuclear Polarization (DNP) experiments transfer polarization from electron spins to nuclear spins with microwave irradiation of the electron spins for enhanced sensitivity in nuclear magnetic resonance (NMR) spectroscopy. Design and testing of a spectrometer for magic angle spinning (MAS) DNP experiments at 263 GHz microwave frequency, 400 MHz 1H frequency is described. Microwaves are generated by a novel continuous-wave gyrotron, transmitted to the NMR probe via a transmission line, and irradiated on a 3.2 mm rotor for MAS DNP experiments. DNP signal enhancements of up to 80 have been measured at 95 K on urea and proline in water–glycerol with the biradical polarizing agent TOTAPOL. We characterize the experimental parameters affecting the DNP efficiency: the magnetic field dependence, temperature dependence and polarization build-up times, microwave power dependence, sample heating effects, and spinning frequency dependence of the DNP signal enhancement. Stable system operation, including DNP performance, is also demonstrated over a 36 h period.

http://www.rsc.org/ej/CP/2010/c003685b.pdf

J. Phys. Chem B and C., vol. 114, Issues 22

2010-06-03T16:16:00.002-04:00

Multireference Ab Initio Calculations of g tensors for Trinuclear Copper Clusters in Multicopper Oxidases

Steven Vancoillie‡, Jakub Chalupsk§, Ulf Ryde, Edward I. Solomon, Kristine Pierloot‡, Frank Neese¶* and Lubomr Rulek§*

J. Phys. Chem. B, 2010, 114 (22), pp 7692–7702
DOI: 10.1021/jp103098r

Abstract: EPR spectroscopy has proven to be an indispensable tool in elucidating the structure of metal sites in proteins. In recent years, experimental EPR data have been complemented by theoretical calculations, which have become a standard tool of many quantum chemical packages. However, there have only been a few attempts to calculate EPR g tensors for exchange-coupled systems with more than two spins. In this work, we present a quantum chemical study of structural, electronic, and magnetic properties of intermediates in the reaction cycle of multicopper oxidases and of their inorganic models. All these systems contain three copper(II) ions bridged by hydroxide or O2− anions and their ground states are antiferromagnetically coupled doublets. We demonstrate that only multireference methods, such as CASSCF/CASPT2 or MRCI can yield qualitatively correct results (compared to the experimental values) and consider the accuracy of the calculated EPR g tensors as the current benchmark of quantum chemical methods. By decomposing the calculated g tensors into terms arising from interactions of the ground state with the various excited states, the origin of the zero-field splitting is explained. The results of the study demonstrate that a truly quantitative prediction of the g tensors of exchange-coupled systems is a great challenge to contemporary theory. The predictions strongly depend on small energy differences that are difficult to predict with sufficient accuracy by any quantum chemical method that is applicable to systems of the size of our target systems.

Solid-state NMR of Radioactive Materials

2010-05-25T14:12:00.003-04:00

A nice article in Chemical and Engineering News on solid-state NMR of radioactive materials.

http://pubs.acs.org/isubscribe/journals/cen/88/i21/html/8821sci1.html

J. Phys. Chem. C., vol. 114, Issue 20

2010-05-20T23:13:00.001-04:00

1H, 29Si, and 27Al MAS NMR as a Tool to Characterize Volcanic Tuffs and Assess Their Suitability for Industrial Applications

Piero Ciccioli†, Paolo Plescia‡ and Donatella Capitani*§

J. Phys. Chem. C, 2010, 114 (20), pp 9328–9343
DOI: 10.1021/jp103082h
Publication Date (Web): April 29, 2010

Abstract: The type and quality of the information provided by the direct analysis of volcanic tuffs by 1H, 29Si, and 27Al NMR were investigated. At this aim, five tuffs, characterized by different origin, bonding mechanism, and clast composition, were used as test materials. Results consistent with the different nature of the tuff matrix and mineral composition were obtained. While the relative content of Al in the crystal and amorphous phase was determined by 27Al MAS and 3Q MAS NMR, the prevalent glassy or zeolitic nature of the matrix was assessed by 29Si and 1H MAS NMR. Zeolites present at levels as low as 15% w/w were detected by 29Si MAS NMR, and in some tuffs, identification of their framework type was performed together with the determination of the Si/Al ratio and, for the first time, of their configurational entropy. Data obtained were coherent with those provided by X-ray fluorescence (XRF), X-ray powder diffraction (XPRD), thermogravimetric analysis (TGA), differential thermal gravimetry (DTG), cation exchange capacity (CEC) determinations, and scanning electron microscopy, used in both backscattering imaging mode (SEM) and for elemental analysis (SEM-EDS). Results show that, under favorable conditions, solid state NMR techniques can provide a comprehensive view of the chemical and physicochemical behavior of a tuff. A combined use of these techniques is suitable for characterization of tuffs on a routine basis, and can be particularly useful to decide if a material is suitable for industrial applications.

The Comparison in Dehydrogenation Properties and Mechanism between MgCl2(NH3)/LiBH4 and MgCl2(NH3)/NaBH4 Systems

L. Gao†, Y. H. Guo†, Q. Li‡ and X. B. Yu*†

J. Phys. Chem. C, 2010, 114 (20), pp 9534–9540
DOI: 10.1021/jp103012t
Publication Date (Web): May 5, 2010

Abstract: The dehydrogenation properties and mechanism of MgCl2(NH3)/MBH4 (here, M is Li or Na) were investigated by thermogravimetric analysis and mass spectrometry, X-ray diffraction (XRD), solid-state 11B NMR, Fourier transform infrared, and differential scanning calorimetry (DSC). As for the MgCl2(NH3)/LiBH4 system, it was found that a new phase, namely, MgCl2(NH3)·LiBH4, to which the following dehydrogenation relates, is formed after ball milling. Judging from the reaction products, it is confirmed that MgCl2 is inclined to work as an ammonia carrier, and the ligand NH3, transferring from MgCl2, is able to combine with the LiBH4 to release H2 with a trace of ammonia at ca. 240 °C. With the increase of LiBH4 content in the mixture, the emission of ammonia was totally suppressed, and Mg(BH4)2 was produced by the decomposition reaction of MgCl2 with the excessive LiBH4 after the ligand NH3 was exhausted, resulting in an improved dehydrogenation in the whole system. As for the MgCl2(NH3)/NaBH4 system, no new phases are detected by XRD after ball milling. The MgCl2 works as a BH4− acceptor, and the ligand NH3 stays with Mg2+ to combine with the BH4−, which transfers from NaBH4 to Mg2+, resulting in a totally different decomposition route and thermal effects as compared with the MgCl2(NH3)/LiBH4 system. DSC results revealed that the decomposition of MgCl2(NH3)/LiBH4 presented an exothermic reaction with an enthalpy of −3.8 kJ mol−1 H2, while the MgCl2(NH3)/NaBH4 showed two apparent endothermic peaks associated with its two-step dehydrogenation with enthalpies of 8.6 and 2.2 kJ mol−1 H2, respectively. Moreover, the MS profiles of the MgCl2(NH3)/2NaBH4, with excessive BH4−, still released a trace of NH3, indicating that the NaBH4 is not so effective in suppressing the emission of NH3 as LiBH4 did.

Inorganic Chem

2010-05-19T13:57:00.000-04:00

Novel Cis- and Trans-Configured Bis(oxime)platinum(II) Complexes: Synthesis, Characterization, and Cytotoxic Activity

Yulia Yu. Scaffidi-Domianello, Kristof Meelich, Michael A. Jakupec, Vladimir B. Arion, Vadim Yu. Kukushkin, Markus Galanski and Bernhard K. Keppler

Inorg. Chem., Article ASAP
DOI: 10.1021/ic100584b
Publication Date (Web): May 11, 2010

Abstract
Novel cis- and trans-configured bis(oxime)platinum(II) complexes have been synthesized and characterized by elemental analyses, IR, electrospray ionization mass spectrometry, multinuclear (1H, 13C, and 195Pt) NMR spectroscopy, and, in five cases, by X-ray diffraction. Their cytotoxicity was studied in the cisplatin-sensitive CH1 cell line as well as in inherently cisplatin-resistant SW480 cancer cells. Remarkably, every single dihalidobis(oxime)platinum(II) complex (with either a cis or trans configuration) shows a comparable cytotoxic potency in both cell lines, indicating a capacity of overcoming cisplatin resistance. Particularly strong cytotoxicities were observed in the case of trans-[PtCl2(R2C═NOH)2] (R = Me, n-Pr, i-Pr) with IC50 values in the high nanomolar concentration range in both CH1 and SW480 cancer cells. These complexes are as potent as cisplatin in CH1 cells and up to 20 times more potent than cisplatin in SW480 cells. In comparison to transplatin, the novel compounds are up to 90 (CH1) and 120 times (SW480) more cytotoxic. The previously reported observation that the trans geometry yields a more active complex in the case of [PtCl2(Me2C═NOH)2] could be confirmed for at least two structural analogues.
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Cesium Hydroperoxostannate: First Complete Structural Characterization of a Homoleptic Hydroperoxocomplex

Andrei V. Churakov, Sergey Sladkevich, Ovadia Lev, Tatiana A. Tripol’skaya and Petr V. Prikhodchenko

Inorg. Chem., Article ASAP
DOI: 10.1021/ic100554u
Publication Date (Web): May 11, 2010

Abstract

The crystal structure of cesium hexahydroperoxostannate Cs2Sn(OOH)6 is presented. The compound was characterized by single crystal and by powder X-ray diffraction, FTIR, 119Sn MAS NMR, and TG-DTA. Cs2Sn(OOH)6 crystallizes in the trigonal space group P, a = 7.5575(4), c = 5.1050(6) Å, V = 252.51(4) Å3, Z = 1, R1 = 0.0120 (I > 2σ(I)), wR2 = 0.0293 (all data), and comprises cesium cations and slightly distorted octahedral [Sn(OOH)6]2− anions lying on the threefold axis. The [Sn(OOH)6]2− unit forms 12 interanion hydrogen bonds resulting in anionic chains spread along the c-axis. All six hydroperoxo ligands are crystallographically equivalent; O−O distances are 1.482(2), only slightly longer than the O−O distance in hydrogen peroxide. FTIR and 119Sn MAS NMR reveal the similarity between all alkali hydroperoxostannates.
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J. Phys. Chem. C., v114, Issue 19

2010-05-14T10:07:00.002-04:00

Solution State NMR Techniques Applied to Solid State Samples: Characterization of Benzoic Acid Confined in MCM-41

Thierry Azais*, Geoffrey Hartmeyer†, Sandrine Quignard, Guillaume Laurent and Florence Babonneau
J. Phys. Chem. C, 2010, 114 (19), pp 8884–8891
DOI: 10.1021/jp910622m

Abstract:In this paper we present an NMR methodology to characterize small organic molecules confined in mesoporous materials. In particular, we demonstrate that NMR techniques issued from solution state NMR are well suited to characterize benzoic acid encapsulated in hexagonally ordered mesoporous silica MCM-41 possessing two different averaged pore sizes (30 and 100 Å). As evidenced by differential scanning calorimetry, entrapped benzoic acid molecules are highly mobile at room temperature due to confinement effect and possess a glass phase transition temperature around −55 °C. Thus, the 13C NMR characterization of encapsulated molecules has to be adapted to that particular behavior. In particular, the cross-polarization technique traditionally used in solid state NMR to record 13C magic angle spinning (MAS) spectra is of poor efficiency due to weak 1H−13C dipolar interaction. Nevertheless, the presence of 1H−13C cross-relaxation phenomenon (nuclear Overhauser effect, NOE) allows us to record 13C spectra through power-gated techniques, routinely used in solution state NMR, in order to enhance the 13C signal through NOE. Furthermore, the long T2′(1H) values (up to 22 ms) are compatible with the setup of J-coupling-based experiments such as MAS refocused {1H}−13C INEPT experiments allowing us to characterize the sample through chemical bonds. These results combined with those of MAS 1H NOESY experiments lead us to distinguish unambiguously different benzoic acid populations within the large pore sample. Finally, we show that cooling down the samples at −35 °C diminishes the mobility and allows the reintroduction of the 1H−13C dipolar interaction. Thus, 2D MAS {1H}−13C HETCOR experiments can be performed at low temperature to explore spatial proximities.

Spin Canting of Maghemite Studied by 57Fe NMR and In-Field Mssbauer Spectrometry

T. Jean Daou†, Jean-Marc Greneche‡, Seong-Joo Lee§, Soonchil Lee§, Christophe Lefevre†, Sylvie Bgin-Colin† and Genevive Pourroy*†

J. Phys. Chem. C, 2010, 114 (19), pp 8794–8799
DOI: 10.1021/jp100726c

Abstract: Local probe techniques, 57Fe in-field Mssbauer, and 57Fe NMR spectrometries have been combined to describe the magnetic structure of maghemite nanoparticles of 39 (±5) nm in size and commercial maghemite. Maghemite nanoparticles were obtained from oxidation of magnetite nanoparticles. Commercial maghemite consists of nanostructured rods, and the size of crystalline domain fairly compares to that of nanoparticles. The structure of the two samples is a partially disordered spinel structure. Both local probe techniques agree that Fe magnetic moments of octahedral and tetrahedral sites are canted in both systems. It was concluded that the canting originates not only from surface effects but also from the bulk resulting from the disordered spinel structure and the frustrated cationic topology, giving rise to reversed Fe moments.

;o

2010-05-13T10:18:00.000-04:00

Natural abundance high field 43Ca solid state NMR in cement science

Igor L. Moudrakovski, Rouhollah Alizadeh and James J. Beaudoin

Phys. Chem. Chem. Phys., 2010 DOI: 10.1039/c000353k

Abstract

This work is a systematic attempt to determine the possibilities and the limitations of the 43Ca high field solid state NMR in the study of cement-based materials. The low natural abundance (0.135%) and small gyromagnetic ratio of 43Ca present a serious challenge even in a high magnetic field. The NMR spectra of a number of cement compounds of known structure and composition are examined. The spectra of several phases important in cement science, e.g., anhydrous beta di-calcium silicate (-C2S) and tri-calcium (C3S) silicate were obtained for the first time and the relation of spectroscopic and structural parameters is discussed. The method was also applied to the hydrated C3S and synthetic calcium silicate hydrates (C–S–H) of different composition in order to understand the state of calcium and transformations in the structure during hydrolysis. The spectra of hydrated C3S reveals a calcium environment similar to that of the C–S–H samples and 11 Å Tobermorite. These observations support the validity of using layered crystalline C–S–H systems as structural models for the C–S–H that forms in the hydration of Portland cement.

Solid-State NMR Literature Blog

Phys. Chem. Chem. Phys., 2011

17O Central Transition NMR

Quadrupole Central Transition 17O NMR Spectroscopy of Biological Macromolecules in Aqueous Solution

Solid-State NMR

Inorg. Chem.

Solvent Effects and Dynamic Averaging of 195Pt NMR Shielding in Cisplatin Derivatives

J. Phys. Chem. A

Crystal Structure Based Design of Signal Enhancement Schemes for Solid-State NMR of Insensitive Half-Integer Quadrupolar Nuclei

J. Chem. Phys.

Phys. Rev. Lett.

Phys. Rev. B.

Author(s): N. Panopoulos, D. Koumoulis, G. Diamantopoulos, M. Belesi, M. Fardis, M. Pissas, and G. Papavassiliou

Phys. Rev. Lett.

Journal of chemical physics

Proceedings of National Academy of Sciences, Early Edition

J. Phys. Chem. B and C, vol. 114, Issues 37-42

J. Phys. Chem. B and C, volume 114, Issues 43 - 45 + November ASAPs

Bryan's Blog Update, Part II

Bryan's Blog Update, Part I

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Journal updates

Journal of magnetic resonance

Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization

J. Chem. Phys.

J. Phys. Chem. B and C, vol. 114, Issues 36

J. Phys. Chem. B and C, v114, Issues 32 - 35

J. Chem. Phys.

Inorganic Chemistry

J. Chem. Phys.

J Phys Chem A

J. Phys. Chem. C, vol.114, Issue 29

J. Phys. Chem C. v. 114, issue 28

Journal of Magnetic Resonance ASAP

Magn. Reson. Chem. - July 2010

Prog. in NMR Spectrosc. - July 2010 - Floquet theory

Journal Update

Hiyam's Journal Update

J. Phys. Chem. A

Journal of Materials Chemistry

Probing the local structures and protonic conduction pathways in scandium substituted BaZrO3 by multinuclear solid-state NMR spectroscopy

Journal of Physical Chemistry B and C, v114, Issues 23

Angewandte Chemie International Edition

Phys. Chem. Chem. Phys. 2010 Vol. 12 Issue 22

J. Phys. Chem B and C., vol. 114, Issues 22

Solid-state NMR of Radioactive Materials

J. Phys. Chem. C., vol. 114, Issue 20

Inorganic Chem

J. Phys. Chem. C., v114, Issue 19

;o

Solvent Effects and Dynamic Averaging of ¹⁹⁵Pt NMR Shielding in Cisplatin Derivatives

Probing the local structures and protonic conduction pathways in scandium substituted BaZrO₃ by multinuclear solid-state NMR spectroscopy