Wednesday, February 09, 2011

Phys. Chem. Chem. Phys., 2011

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

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

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

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

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

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.

Monday, January 31, 2011

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.

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

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).

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

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

from RSC - J. Mater. Chem. latest articles

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 → Na0.5NBH0.5 + 0.5NaH + 2.0H2. The hypothetical phase Na0.5NBH0.5 is amorphous, where the basic molecular unit of the original NaAB is polymerized into a [–B[double bond, length as m-dash]N–]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).

Monday, January 17, 2011

17O Central Transition NMR

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

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

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).

Monday, January 10, 2011

Solid-State NMR

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 Tdf 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.

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

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

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 PtII chemical shift range).

J. Phys. Chem. A

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

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

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.

Friday, January 07, 2011

J. Chem. Phys.

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

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 0.1o–10o range above the critical temperature.

Phys. Rev. Lett.

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

Friday, December 17, 2010

Phys. Rev. B.

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 To≃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 To, 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 La0.67Ca0.33MnO3, is not a random uncorrelated process but the signature of the formation of a genuine spin-glass state, which forT<Tc consolidates with the ferromagnetic (FM) state into a single thermodynamic phase. Comparison with NMR measurements performed onLa1−xCaxMnO3 systems for 0.0≤x≤0.41 and ODM La0.70Sr0.30MnO3, 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 Tc a discontinuous first-orderlike change in both the FM and the “glassy” Edwards-Anderson order parameters.

Thursday, December 16, 2010

Phys. Rev. Lett.

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

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}

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.