Journal updates

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

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

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.

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

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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

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

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.

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

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.

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.

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

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

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

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.