Aaron's Journal Update

J. Phys. Chem. C, 112 (14), 5552 -5562, 2008. 10.1021/jp7107973 S1932-7447(71)00797-0
Multinuclear Solid-State NMR Studies of Ordered Mesoporous Bioactive Glasses
Ekaterina Leonova, Isabel Izquierdo-Barba, Daniel Arcos, Adolfo López-Noriega, Niklas Hedin, Maria Vallet-Regí, and Mattias Edén*
Abstract: The local structures of highly ordered mesoporous bioactive CaO-SiO2-P2O5 glasses were investigated for variable Ca contents. 1H NMR revealed a diversity of hydrogen-bonded and "isolated" surface silanols as well as adsorbed water molecules. The structural roles of Si and P were explored using a combination of 29Si and 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) techniques; the proximities of Si and P to protons were studied through cross-polarization-based experiments, including 1H-29Si and 1H-31P hetero-nuclear two-dimensional correlation spectroscopy. The results are consistent with SiO2 being the main pore-wall component, whereas P is present as a separate amorphous calcium orthophosphate phase, which is dispersed over the pore wall as nanometer-sized clusters. The excess Ca that is not consumed in the phosphate phase modifies the silica glass network where it associates at/near the mesoporous surface. This biphasic structural model of the pore wall leads to the high accessibility of both Ca and P to body fluids, and its relation to the experimentally demonstrated high in vitro bioactivities of these materials is discussed.


J. Phys. Chem. C, 112 (14), 5672 -5678, 2008. 10.1021/jp7108708 S1932-7447(71)00870-7
Conductivity and Fluoride Ion Dynamics in -PbSnF4; 19F Field-Cycling NMR and Diffraction Studies
Eoin Murray, Dermot F. Brougham,* Jovan Stankovic, and Isaac Abrahams
Abstract: Fast-field cycling 19F NMR relaxometry has been applied to investigate fluoride ion dynamics in the layered anionic conductor PbSnF4. Two dynamic processes, on different timescales, were shown to drive the 19F relaxation. By considering the temperature dependencies of the NMR, conductivity, and diffraction data, a complete mechanism for fluoride transport can for the first time be proposed. The slower process is due to anion exchange between equivalent sites (F(2)) in the conducting fluoride plane, which lie between Sn and Pb layers. This is a diffusive process related directly to the mechanism of electrical conduction. The activation barriers for this motion agree closely with those determined from the temperature dependence of the DC conductivity. The faster process is due to non-diffusive exchange between the occupied sites (F(2)) and nominally vacant sites (F(1)), which lie between Sn planes. While this process does not directly limit conductivity, it generates vacancies on the F(2) sites and partial occupancy of the F(1) sites. Above 340 K, the fast process shows an increase in activation energy, as increased occupancy of F(1) sites necessitates the formation of a Frenkel defect on a significant proportion of the F(1) sites prior to an F(2)-F(1) jump. In this temperature range, the slow process shows a decrease in the activation energy, also observed in the conductivity data, due to increased numbers of vacancies in the F(2) sites which provide additional diffusive pathways through the conducting fluoride plane. The results demonstrate that -PbSnF4 is essentially a two-dimensional (anisotropic) conductor, in which nondiffusive fluoride exchange into sites normal to the conducting plane provides a high population of vacancies within the conducting plane, resulting in unusually high conductivity.


J. Phys. Chem. C, 112 (15), 6151 -6159, 2008. 10.1021/jp711438v S1932-7447(71)01438-9
Intermediate-Range Order of Alkali Disilicate Glasses and Its Relation to the Devitrification Mechanism
James G. Longstaffe, Ulrike Werner-Zwanziger, Jose F. Schneider, Marcio L. F. Nascimento, Edgar D. Zanotto, and Josef W. Zwanziger*
Abstract: There are two general mechanisms of devitrification in glass: heterogeneous nucleation of crystals from surfaces and impurities and homogeneous nucleation from the volume. It is thought that structural similarities between glass and crystal at the intermediate-range level influence the mechanism followed; however, there are scarce experimental studies to test this hypothesis. In this paper solid-state nuclear magnetic resonance spectroscopy is used to probe intermediate-range order in sodium and lithium disilicate glasses through measurement of the second moment of the distribution of dipolar couplings. These two glasses undergo heterogeneous and homogeneous nucleation, respectively. The second moments measured for the lithium glass closely follow the trend established by the layered structures of the isochemical crystalline phases, while the same measurements for the sodium glass do not. This observation supports the hypothesis that glasses capable of homogeneous nucleation are structurally more similar to the resulting crystalline phases than those glasses that exhibit only heterogeneous nucleation.



J. Phys. Chem. C, 112 (15), 6165 -6172, 2008. 10.1021/jp7114498 S1932-7447(71)01449-3
19F MAS NMR Investigation of Strontium Substitution Sites in Ca2+/Sr2+ Fluorapatite Solid Solutions
Gyunggoo Cho, # Chung-Nin Chau, and James P. Yesinowski*
Abstract: The partial replacement of Ca2+ by Sr2+ in the fluorapatite lattice results in additional peaks in the 19F MAS NMR spectra at 9.4 T other than the main resonance of Ca10F2(PO4)6 at 64.0 ppm (from hexafluorobenzene). The assignment of these peaks to specific structural configurations is possible in the sample containing the least strontium, with a composition of Ca8.97Sr1.03F2(PO4)6. The solid-solution character of this sample is established by the observation of spectral spin diffusion between various peaks in the SPARTAN (selective population anti-z and rate of transfer to adjacent nuclei) experiment. Calculations based upon modeling the 19F chemical shift tensor show that this process is facilitated for most crystallites by the close approach of two peaks' resonances during the rotor cycle. A peak and set of spinning sidebands with an isotropic chemical shift of 79.6 ppm is assigned to fluoride ions in the center of a triangle of Ca2Sr ions (the so-called Ca(2) sites occurring as Ca3F in the fluorapatite lattice). Smaller chemical shift perturbations observed by deconvolution of two shoulders at 61.2 and 58.8 ppm on the main 64.0 ppm resonance are assigned to a Ca3F configuration that has, respectively, either one or multiple Sr2+ neighbors in the adjacent Ca(2) sites. Quantitative peak intensity measurements relative to the main 64 ppm resonance of both the 79.6 ppm peak as well as the deconvoluted peaks separately indicate that Sr2+ ions preferentially occupy the Ca(2) site at a level 23% greater than that expected for random substitution in the above solid solution. A sample having the composition Ca4.95Sr5.05F2(PO4)6 has a qualitatively similar site preference and has peaks assigned to Ca2SrF at 70 ppm, CaSr2F at 87 ppm, and Sr3F at 105 ppm.




J. Phys. Chem. C, 112 (16), 6430 -6438, 2008. 10.1021/jp7119087 S1932-7447(71)01908-3
H2O and Cation Structure and Dynamics in Expandable Clays: 2H and 39K NMR Investigations of Hectorite
Geoffrey M. Bowers,* David L. Bish, and R. James Kirkpatrick
Abstract: Variable temperature 39K and 2H nuclear magnetic resonance (VT NMR) spectroscopy of K+-saturated hectorite, a prototypical smectite clay, provides new insight into the relationships between the structural and dynamical behavior of K+ and H2O in confinement and at surfaces. In d = 10 Å K-exchanged hectorite, interlayer K+ is rigidly held by the silicate rings, probably in 12-coordinate inner-sphere sites as in muscovite mica. In a 1/1.5 by weight hectorite/water paste, K+ occurs on interlayer and external surface sites that are indistinguishable by 39K NMR. The K+ environments experience changes in dynamical behavior over the temperature range from -50 to 60 C that are directly related to H2O dynamics. 39K NMR of the paste sample shows dynamic line narrowing at low temperatures due to modulation of the electric field gradient (EFG) at frequencies of the order of the static line width ( 20 kHz) and two "melting"-type dynamic transitions near -10 C, one for surface and one for confined K+. At and above 0 C, K+ remains closely associated with the clay surfaces and experiences motion at frequencies greater than 200 kHz and less than 10 MHz, as revealed by 39K T1 relaxation behavior, nutation behavior, and the 39K quadrupolar product. These data are consistent with rapid exchange between inner- and outer-sphere K+ sites reported previously for K-montmorillonite based on molecular dynamics simulations. Deuterium NMR shows the presence of two unique H2O environments in the system: one structurally and dynamically consistent with bulk water between particles and one attributable to H2O confined in the interlayer. Confined H2O experiences anisotropic motion between -50 and 0 C via fast rotation (>2 MHz) about a single axis oriented 127.5 ± 0.5 from the principal axis of the 2H EFG, potentially due to C2 rotation. This motion does not affect the 39K EFG significantly. Melting of free and confined H2O occurs between -10 and 0 C and near 0 C, respectively, similar to the melting behavior of K+ and likely reflecting the onset of molecular diffusion. At and above 10 C, all H2O environments experience motion near or in excess of 300 kHz through at least three NMR-indistinguishable mechanisms, including Brownian motion of free water, exchange of free and confined H2O near particle edges, and diffusive motion of H2O that remains confined on the experimental time scale. The correlation between the rates of 2H and 39K motion and the observed melting transitions for both spin populations strongly suggest that 39K melting and dynamics above the melting transition are linked to an increase in the motional freedom of H2O.



J. Phys. Chem. C, 112 (19), 7503 -7508, 2008. 10.1021/jp711762n S1932-7447(71)01762-X
Temperature Dependence of Spontaneous Polarization in Order-Disorder Pyridinium Periodate Extracted from 2H NMR Data
J. W sicki, A. Pajzderska,* and Z. Fojud
Abstract: On the basis of occupancy factor data for nitrogen atoms in the pyridinium cation obtained from the structural study of PyHIO4 by neutron diffraction,4 the shape of the potential for cation reorientation about the axis perpendicular to its plane was found. The 2H NMR spectra of d5PyHIO4 taken over a wide range of temperatures have allowed the determination of potential minima populations. These data were used to obtain the temperature dependence of spontaneous polarization. The agreement between the spontaneous polarization calculated in this way and the measured values is very good and indicates that the ferroelectric crystal is of the order-disorder type.



J. Phys. Chem. B, 112 (13), 3927 -3930, 2008. 10.1021/jp711433d S1520-6106(71)01433-0
Water Diffusion in Nanoporous Glass: An NMR Study at Different Hydration Levels
Domenico Majolino,* Carmelo Corsaro, Vincenza Crupi, Valentina Venuti, and Ulderico Wanderlingh
Abstract: By pulsed field gradient nuclear magnetic resonance measurements, we investigated the translational diffusion of water confined in the 200 Å diameter pores of a sol-gel silica glass. The experiments, performed as a function of the hydration level, showed an enhancement of the self-diffusion coefficient when the water content corresponds to one or fewer monolayers. An explanation for this occurrence has been given in terms of a two-phase process involving a fast molecular exchange between the liquid and the vapor phase. Moreover, in partially filled pores, the surface water diffusion coefficient was measured, and was 4 times lower than the diffusion of liquid confined water in saturated spaces.



J. Phys. Chem. B, 112 (15), 4496 -4505, 2008. 10.1021/jp709739v S1520-6106(70)09739-0
Theoretical Predictions of 31P NMR Chemical Shift Threshold of Trimethylphosphine Oxide Absorbed on Solid Acid Catalysts
Anmin Zheng, Hailu Zhang, Xin Lu, Shang-Bin Liu,* and Feng Deng*
Abstract: The 31P NMR chemical shifts of adsorbed trimethylphosphine oxide (TMPO) and the configurations of the corresponding TMPOH+ complexes on Br nsted acid sites with varying acid strengths in modeled zeolites have been predicted theoretically by means of density functional theory (DFT) quantum chemical calculations. The configuration of each TMPOH+ complex was optimized at the PW91/DNP level based on an 8T cluster model, whereas the 31P chemical shifts were calculated with the gauge including atomic orbital (GIAO) approach at both the HF/TZVP and MP2/TZVP levels. A linear correlation between the 31P chemical shift of adsorbed TMPO and the proton affinity of the solid acids was observed, and a threshold for superacidity (86 ppm) was determined. This threshold for superacidity was also confirmed by comparative investigations on other superacid systems, such as carborane acid and heteropolyoxometalate H3PW12O40. In conjunction with the strong correlation between the MP2 and the HF 31P isotropic shifts, the 8T cluster model was extended to more sophisticated models (up to 72T) that are not readily tractable at the GIAO-MP2 level, and a 31P chemical shift of 86 ppm was determined for TMPO adsorbed on zeolite H-ZSM-5, which is in good agreement with the NMR experimental data.



J. Phys. Chem. B, 112 (16), 4943 -4947, 2008. 10.1021/jp7110082 S1520-6106(71)01008-3
Preparation and Characterization of New Glassy System As2P2S8-Ga2S3
Silvia H. Santagneli, José Schneider, Igor Skripachev, Sidney J. L. Ribeiro, and Younès Messaddeq*
Abstract: Glasses having the composition (100 - x)As2P2S8-xGa2S3 with x ranging from 0 to 50% were investigated to determine the compositional effect on properties and local structure. The glass transition temperature (Tg) and the stability parameter against crystallization (Tx - Tg) increased with the addition of Ga2S3. The structure of these glasses was probed by Raman scattering, Fourier transform infrared (FT-IR) and 31P nuclear magnetic resonance. On the basis of the observed vibrations and the strength of the 31P-31P homonuclear magnetic dipolar coupling, two scenarios can be proposed for the structural evolution induced by the addition of Ga2S3. For x 20% we may have the formation of GaS4E- groups (E = nonbonding electron), and for x 30% we have depolymerization of the As2P2S8 units and the formation of a network of GaPS4 units with each PS4/2 unit (Q4) species carrying a single positive formal charge



J. Phys. Chem. B, 112 (18), 5813 -5823, 2008. 10.1021/jp800580n S1520-6106(80)00580-9
51V NMR Chemical Shifts from Quantum-Mechanical/Molecular-Mechanical Models of Vanadium Bromoperoxidase
Mark P. Waller, K. R. Geethalakshmi, and Michael Bühl*
Abstract: According to quantum-mechanical/molecular-mechanical (QM/MM) optimizations, the active-site geometries of vanadium-dependent bromoperoxidase (VBPO) and vanadium-dependent chloroperoxidase (VCPO) are very similar. 51V NMR chemical shifts calculated from QM/MM-optimized models of VBPO are critically compared to VCPO and are found to be very similar for the two related proteins. The primary difference between these related structures, the presence of a His411 in VBPO whereas Phe397 is located at that position in VCPO, is studied via analysis of the respective theoretical 51V NMR spectra. The long-range electrostatic effects from more distal residues are also studied to establish their effect. Similar results are obtained for the two active sites of the VBPO homodimer. The experimentally observed shielding of the isotropic 51V NMR chemical shift on going from VCPO to VBPO is somewhat underestimated in the QM/MM models studied. NMR and NQC tensors of both enzymes are predicted to show noticeable differences, suggesting that precise solid-state 51V NMR data, when they become available, can be a sensitive probe for subtle differences in structural details between these enzymes.



J. Phys. Chem. B, 112 (19), 5881 -5882, 2008. 10.1021/jp801687s S1520-6106(80)01687-2
A Tribute to Attila Szabo
There are many biological NMR papers to be found in this special issue (Issue 19) for those who are interested.



J. Phys. Chem. B, 112 (19), 6114 -6121, 2008. 10.1021/jp076808o S1520-6106(07)06808-3
Theory of Stochastic Dipolar Recoupling in Solid-State Nuclear Magnetic Resonance
Robert Tycko*
Abstract: Dipolar recoupling techniques in solid-state nuclear magnetic resonance (NMR) consist of radio frequency (rf) pulse sequences applied in synchrony with magic-angle spinning (MAS) that create nonzero average magnetic dipole-dipole couplings under MAS. Stochastic dipolar recoupling (SDR) is a variant in which randomly chosen rf carrier frequency offsets are introduced to cause random phase modulations of individual pairwise couplings in the dipolar spin Hamiltonian. Several aspects of SDR are investigated through analytical theory and numerical simulations: (1) An analytical expression for the evolution of nuclear spin polarization under SDR in a two-spin system is derived and verified through simulations, which show a continuous evolution from coherent, oscillatory polarization exchange to incoherent, exponential approach to equilibrium as the range of random carrier offsets (controlled by a parameter fmax) increases; (2) in a many-spin system, polarization transfers under SDR are shown to be described accurately by a rate matrix in the limit of large fmax, with pairwise transfer rates that are proportional to the inverse sixth power of pairwise internuclear distances; (3) quantum mechanical interferences among noncommuting pairwise dipole-dipole couplings, which are a complicating factor in solid-state NMR studies of molecular structures by traditional dipolar recoupling methods, are shown to be absent from SDR data in the limit of large fmax, provided that coupled nuclei have distinct NMR chemical shifts.



J. Phys. Chem. B, 112 (20), 6285–6287, 2008. 10.1021/jp800646k
Fluorine-19 NMR Chemical Shift Probes Molecular Binding to Lipid Membranes
Eduard Y. Chekmenev,*†‡ Siu-Kei Chow,† Daniel Tofan,‡ Daniel P. Weitekamp,‡ Brian D. Ross,† and Pratip Bhattacharya†
Abstract: The binding of amphiphilic molecules to lipid bilayers is followed by 19F NMR using chemical shift and line shape differences between the solution and membrane-tethered states of −CF3 and −CHF2 groups. A chemical shift separation of 1.6 ppm combined with a high natural abundance and high sensitivity of 19F nuclei offers an advantage of using 19F NMR spectroscopy as an efficient tool for rapid time-resolved screening of pharmaceuticals for membrane binding. We illustrate the approach with molecules containing both fluorinated tails and an acrylate moiety, resolving the signals of molecules in solution from those bound to synthetic dimyristoylphosphatidylcholine bilayers both with and without magic angle sample spinning. The potential in vitro and in vivo biomedical applications are outlined. The presented method is applicable with the conventional NMR equipment, magnetic fields of several Tesla, stationary samples, and natural abundance isotopes.



Macromolecules, 41 (7), 2514–2519, 2008. 10.1021/ma702815k
Restricted Segmental Mobility Can Facilitate Medium-Range Chain Diffusion: A NMR Study of Morphological Influence on Chain Dynamics of Polyethylene
Y.-F. Yao,† R. Graf,† H. W. Spiess,*† and S. Rastogi†‡
Abstract: The influence of the morphology on the local motion in the noncrystalline regions and chain diffusion between crystalline and noncrystalline regions is studied in ultrahigh molecular weight linear polyethylene: The behaviors of samples of the same material crystallized from the melt and from solution are compared. The geometrical restrictions of the conformational transitions are probed via anisotropic NMR interactions, i.e., 13C−1H dipole–dipole couplings and 13C chemical shift anisotropy. As they are averaged out under MAS, recoupling techniques are applied to yield sideband patterns or quasi-static NMR spectra, from which residual anisotropies are determined. Chain diffusion is probed by 13C exchange NMR. As expected, the local conformational transitions are more restricted in the solution crystallized sample compared with the melt crystallized one. Moreover, the motional narrowing observed by both techniques indicates that the local mobility in the noncrystalline regions of the solution crystallized sample consists mainly of effectively axial motion of extended trans-conformers around their local chain axes. This facilitates the chain diffusion between the crystalline and the noncrystalline regions, being significantly faster in solution crystallized compared to melt crystallized samples, where the local mobility is much more isotropic. The implications of these results for the understanding of crystal thickening and cold drawing are discussed.

J. Am. Chem. Soc., 130 (18), 5922–5928, 2008.

Characterization of Alzheimer’s-like Paired Helical Filaments from the Core Domain of Tau Protein Using Solid-State NMR SpectroscopyOvidiu C.
Andronesi, Martin von Bergen, Jacek Biernat, Karsten Seidel, Christian Griesinger, Eckhard Mandelkow, and Marc Baldus

Abstract:
The polymerization of the microtubule-associated protein tau into paired helical filaments (PHFs) represents one of the hallmarks of Alzheimer’s disease. We employed solid-state nuclear magnetic resonance (NMR) to investigate the structure and dynamics of PHFs formed in vitro by the three-repeat-domain (K19) of protein tau, representing the core of Alzheimer PHFs. While N and C termini of tau monomers in PHFs are highly dynamic and solvent-exposed, the rigid segment consists of three major β-strands. Combination of through-bond and through-space ssNMR transfer methods with water-edited (15N,13C) and (13C,13C) correlation experiments suggests the existence of a fibril core that is largely built by repeat unit R3, flanked by surface-exposed units R1 and R4. Solid-state NMR, circular dichroism, and the fibrillization behavior of a K19 mutant furthermore indicate that electrostatic interactions play a central role in stabilizing the K19 PHFs.

J. Am. Chem. Soc., 130 (18), 5880–5882, 2008.

Mapping the Evolution of Adsorption of Water in Nanoporous Silica by in situ Solid-State 1H NMR Spectroscopy
Mingcan Xu, Kenneth D. M. Harris, and John Meurig Thomas

Abstract:
A recently developed technique for carrying out in situ solid-state NMR studies of adsorption processes in nanoporous materials is applied to map the evolution of the adsorption of water on the widely used siliceous nanoporous host material MCM-41. The technique allows the very earliest stages of the adsorption process to be probed, and using solid-state 1H NMR, the time resolution is of the order of a few hundred seconds. This work reveals that different water environments are populated at different stages of the adsorption process and reveals insights regarding both the sequence in which these different environments become populated and the exchange of water molecules between these environments. The results also provide access to information on the kinetics of the adsorption process, revealing an initial regime of rapid water adsorption up to ca. 1 wt % water, followed by a regime of slower water adsorption. In both regimes, the amount of water adsorbed increases linearly with time. As demonstrated by these results, the approach employed in this work creates new opportunities for investigating, in unprecedented detail, adsorption processes in nanoporous materials.

J. Am. Chem. Soc., 130 (18), 5870–5871, 2008.

2-Fluoro-ATP as a Versatile Tool for 19F NMR-Based Activity Screening
Brian J. Stockman

Abstract:
19F NMR-based methods have found utility in activity-based screening assays. However, because enzymes catalyze a diverse set of reactions, a large variety of fluorinated substrates would need to be identified to target each one separately. We have developed a more streamlined approach that is applicable to many enzymes that utilize ATP as a substrate. In this method, a fluorine-containing ATP analogue, 2-fluoro-ATP, is used to monitor the reaction. Applications are described for nicotinamide adenine dinucleotide synthetase and 3-phosphoinositide dependent kinase-1. Fragment screening results for the latter indicate that this technique can identify compounds that inhibit as well as activate reactions. The present results, together with previous biochemical studies from other laboratories, have shown that 2-fluoro-ATP can serve as a substrate for nine enzymes that are representative of three of the six enzyme subclasses, namely the transferases, hydrolases, and ligases. This suggests that 2-fluoro-ATP is suitable as a universal tool for screening ATP-requiring enzymes. Importantly, 2-fluoro-ATP has been determined to be a valid substrate for a variety of kinases, including both small molecule and protein kinases, suggesting that it may be useful for investigating the large number of pharmaceutically relevant kinases.

J. Am. Chem. Soc., 130 (20), 6306–6307, 2008.

NMR Crystallography of Zeolites: Refinement of an NMR-Solved Crystal Structure Using ab Initio Calculations of 29Si Chemical Shift Tensors
Darren H. Brouwer

Abstract:
An NMR structure refinement method for the NMR crystallography of zeolites is presented and demonstrated to give an NMR-determined crystal structure for the zeolite Sigma-2 that is in very good agreement with the single-crystal X-ray diffraction structure. The Si coordinates of the zeolite framework were solved from 29Si double-quantum NMR data obtained at a low magnetic field strength (7.0 T) and the Si and O coordinates were subsequently refined using the principal components of 29Si chemical shift tensors experimentally measured at an ultrahigh-field (21.1 T) and calculated using ab initio quantum chemical methods.

J. Am. Chem. Soc., 130 (17), 5618–5619, 2008.

Toward the Characterization of Peptidoglycan Structure and Protein−Peptidoglycan Interactions by Solid-State NMR Spectroscopy
Thomas Kern, Sabine Hediger,Patrick Müller,§ Cécile Giustini, Bernard Joris, Catherine Bougault,Waldemar Vollmer, and Jean-Pierre Simorre

Abstract:
Solid-state NMR spectroscopy is applied to intact peptidoglycan sacculi of the Gram-negative bacterium Escherichia coli. High-quality solid-state NMR spectra allow atom-resolved investigation of the peptidoglycan structure and dynamics as well as the study of protein−peptidoglycan interactions.

J. Am. Chem. Soc., 130 (16), 5459–5471, 2008.

Solid-State NMR Spectroscopy of Human Immunodeficiency Virus Fusion Peptides Associated with Host-Cell-Like Membranes: 2D Correlation Spectra and Distance Measurements Support a Fully Extended Conformation and Models for Specific Antiparallel Strand Registries
Wei Qiang, Michele L. Bodner, and David P. Weliky*

Abstract:
The human immunodeficiency virus (HIV) is “enveloped” by a membrane, and infection of a host cell begins with fusion between viral and target cell membranes. Fusion is catalyzed by the HIV gp41 protein which contains a functionally critical ~20-residue apolar “fusion peptide” (HFP) that associates with target cell membranes. In this study, chemically synthesized HFPs were associated with host-cell-like membranes and had “scatter-uniform” labeling (SUL), that is, only one residue of each amino acid type was U−13C, 15N labeled. For the first sixteen HFP residues, an unambiguous 13C chemical shift assignment was derived from 2D 13C/13C correlation spectra with short mixing times, and the shifts were consistent with continuous β-strand conformation. 13C−13C contacts between residues on adjacent strands were derived from correlation spectra with long mixing times and suggested close proximity of the following residues: Ala-6/Gly-10, Ala-6/Phe-11, and Ile-4/Gly-13. Specific antiparallel β-strand registries were further tested using a set of HFPs that were 13CO-labeled at Ala-14 and 15N-labeled at either Val-2, Gly-3, Ile-4, or Gly-5. The solid-state NMR data were fit with 50–60% population of antiparallel HFP with either Ala-14/Gly-3 or Ala-14/Ile-4 registries and 40–50% population of structures not specified by the NMR experiments. The first two registries correlated with intermolecular hydrogen bonding of 15–16 apolar N-terminal residues and this hydrogen-bonding pattern would be consistent with a predominant location of these residues in the hydrophobic membrane interior. To our knowledge, these results provide the first residue-specific structural models for membrane-associated HFP in its β-strand conformation.

J. Am. Chem. Soc., 130 (18), 5886–5900, 2008.

Dynamics of Silica-Supported Catalysts Determined by Combining Solid-State NMR Spectroscopy and DFT Calculations
Frédéric Blanc, Jean-Marie Basset, Christophe Copéret,† Amritanshu Sinha, Zachary J. Tonzetich, Richard R. Schrock, Xavier Solans-Monfort,Eric Clot,Odile Eisenstein, Anne Lesage, and Lyndon Emsley


Abstract:
The molecular dynamics of a series of organometallic complexes covalently bound to amorphous silica surfaces is determined experimentally using solid-state nuclear magnetic resonance (NMR) spectroscopy and density functional theory calculations (DFT). The determination is carried out for a series of alkylidene-based catalysts having the general formula [(SiO)M(ER)(CHtBu)(R′)] (M = Re, Ta, Mo or W; ER = CtBu, NAr or CH2tBu; R′ = CH2tBu, NPh2, NC4H4). Proton−carbon dipolar coupling constants and carbon chemical shift anisotropies (CSA) are determined experimentally by solid-state NMR. Room-temperature molecular dynamics is quantified through order parameters determined from the experimental data. For the chemical shift anisotropy data, we validate and use a method that integrates static values for the CSA obtained computationally by DFT, obviating the need for low-temperature measurements. Comparison of the room-temperature data with the calculations shows that the widths of the calculated static limit dipolar couplings and CSAs are always greater than the experimentally determined values, providing a clear indication of motional averaging on the NMR time scale. Moreover, the dynamics are found to be significantly different within the series of molecular complexes, with order parameters ranging from = 0.5 for [(SiO)Ta(CHtBu)(CH2tBu)2] and [(SiO)Re(CtBu)(CHtBu)(CH2tBu)] to = 0.9 for [(SiO)Mo(NAr)(CHtBu)(R′) with R′ = CH2tBu, NPh2, NC4H4. The data also show that the motion is not isotropic and could be either a jump between two sites or more likely restricted librational motion. The dynamics are discussed in terms of the molecular structure of the surface organometallic complexes, and the orientation of the CSAs tensor at the alkylidene carbon is shown to be directly related to the magnitude of the α-alkylidene CH agostic interation.

J. Am. Chem. Soc., 130 (17), 5798–5807, 2008.

Magic Angle Spinning NMR Experiments for Structural Studies of Differentially Enriched Protein Interfaces and Protein Assemblies
Jun Yang, Maria Luisa Tasayco, and Tatyana Polenova

Abstract:
Protein–protein interactions play vital roles in numerous biological processes. These interactions often result in formation of insoluble and noncrystalline protein assemblies. Solid-state NMR spectroscopy is rapidly emerging as a premier method for structural analysis of such systems. We introduce a family of two-dimensional magic angle spinning (MAS) NMR experiments for structural studies of differentially isotopically enriched protein assemblies. Using 1–73(13C,15N)/74–108(15N) labeled thioredoxin reassembly, we demonstrate that dipolar dephasing followed by proton-assisted heteronuclear magnetization transfer yields long-range 15N−13C correlations arising exclusively from the interfaces formed by the pair of differentially enriched complementary fragments of thioredoxin. Incorporation of dipolar dephasing into the 15N proton-driven spin diffusion and into the 1H−15N FSLG−HETCOR sequences permits 1H and 15N resonance assignments of the 74–108(15N) enriched C-terminal fragment of thioredoxin alone. The differential isotopic labeling scheme and the NMR experiments demonstrated here allow for structural analysis of both the interface and each interacting protein. Isotope editing of the magnetization transfers results in spectral simplification, and therefore larger protein assemblies are expected to be amenable to these experiments.

Physical Review B

Phys. Rev. B 77, 144404 (2008)
63,65Cu NMR and NQR evidence for an unusual spin dynamics in PrCu2 below 100 K
A. Sacchetti, M. Weller, J. L. Gavilano, R. Mudliar, B. Pedrini, K. Magishi, H. R. Ott, R. Monnier, B. Delley, and Y. Onuki
We report the results of a 63,65Cu NMR/NQR study probing the intermetallic compound PrCu2. The previously claimed onset of magnetic order at 65 K, indicated in a muon spin resonance study, is not confirmed. Based on our data, we discuss different possible reasons for this apparent discrepancy, including a non-negligible influence of the implanted muons on their environment. Competing dipolar and quadrupolar interactions lead to unusual features of the magnetic-ion/conduction-electron system, different from those of common intermetallics exhibiting structural or magnetic instabilities.

Phys. Rev. B 77, 144419 (2008)
7Li NMR study of heavy-fermion LiV2O4 containing magnetic defects
X. Zong, S. Das, F. Borsa, M. D. Vannette, R. Prozorov, J. Schmalian, and D. C. Johnston
We present a systematic study of the variations of the 7Li NMR properties versus magnetic defect concentration ndefect within the spinel structure of polycrystalline powder samples (ndefect=0.21, 0.49, and 0.83 mol %) and a collection of small single crystals (ndefect=0.38 mol %) of LiV2O4 in the temperature range from 0.5 to 4.2 K. We also report static magnetization measurements and ac magnetic susceptibility measurements at 14 MHz on the samples at low temperatures. Both the 7Li NMR spectrum and nuclear spin-lattice relaxation rate are inhomogeneous in the presence of the magnetic defects. The 7Li NMR data for the powders are well explained by assuming that (i) there is a random distribution of magnetic point defects, (ii) the same heavy Fermi liquid is present in the samples containing the magnetic defects as in magnetically pure LiV2O4, and (iii) the influences of the magnetic defects and of the Fermi liquid on the magnetization and NMR properties are separable. In the single crystals, somewhat different behaviors are observed, which are possibly due to a modification of the heavy Fermi liquid, to a lack of separability of the relaxation effects due to the Fermi liquid and the magnetic defects, to non-Fermi liquid behavior of the conduction electrons, and/or to quantum fluctuations of finite-size magnetic defects (magnetic droplets). Remarkably, the magnetic defects in the powder samples show evidence of spin freezing below T1.0 K, whereas in the single crystals with similar magnetic defect concentration, no spin freezing was found down to T=0.5 K. Thus, different types of magnetic defects and/or interactions between them appear to arise in the powders versus the crystals, which are possibly due to the substantially different synthesis conditions of the powders and crystals.

Journal of Magnetic Resonance - Vol 192 Issue 1

Through-space MP-CPMAS experiments between spin-1/2 and half-integer quadrupolar nuclei in solid-state NMR
B. Hu, J.P. Amoureux, J. Trébosc and S. Hafner
We present a new CPMAS method that allows the acquisition of through-space 2D HETCOR spectra between spin-1/2 nuclei and half-integer quadrupolar nuclei in the solid state. It uses rotor-synchronized selective pulses on the quadrupolar nucleus and continuous-wave RF irradiation on the spin-1/2 nucleus to create hetero-nuclear dipolar coherences. The method is more robust, more efficient, and easier to set up than the standard CPMAS transfer.

Spin state selective coherence transfer: A method for discrimination and complete analyses of the overlapped and unresolved 1H NMR spectra of enantiomers
Bikash Baishya, Uday Ramesh Prabhu and N. Suryaprakash
In general, the proton NMR spectra of chiral molecules aligned in the chiral liquid crystalline media are broad and featureless. The analyses of such intricate NMR spectra and their routine use for spectral discrimination of R and S optical enantiomers are hindered. A method is developed in the present study which involves spin state selective two dimensional correlation of higher quantum coherence to its single quantum coherence of a chemically isolated group of coupled protons. This enables the spin state selective detection of proton single quantum transitions based on the spin states of the passive nuclei. The technique provides the relative signs and magnitudes of the couplings by overcoming the problems of enantiomer discrimination, spectral complexity and poor resolution, permitting the complete analyses of the otherwise broad and featureless spectra. A non-selective 180° pulse in the middle of MQ dimension retains all the remote passive couplings. This accompanied by spin selective MQ–SQ conversion leads to spin state selective coherence transfer. The removal of field inhomogeneity contributes to dramatically enhanced resolution. The difference in the cumulative additive values of chemical shift anisotropies and the passive couplings, between the enantiomers, achieved by detecting Nth quantum coherence of N magnetically equivalent spins provides enhanced separation of enantiomer peaks. The developed methodology has been demonstrated on four different chiral molecules with varied number of interacting spins, each having a chiral centre.

Comparison of several hetero-nuclear dipolar recoupling NMR methods to be used in MAS HMQC/HSQC
B. Hu, J. Trébosc and J.P. Amoureux
We compare several hetero-nuclear dipolar recoupling sequences available for HMQC or HSQC experiments applied to spin-1/2 and quadrupolar nuclei. These sequences, which are applied to a single channel, are based either on the rotary resonance recoupling (R3) irradiation, or on two continuous rotor-synchronized modulations (SFAM1 and SFAM2), or on four symmetry-based sequences , or on the REDOR scheme. We analyze systems exhibiting purely hetero-nuclear dipolar interactions as well as systems where homo-nuclear dipolar interactions need to be canceled. A special attention is given to the behavior of these sequences at very fast MAS. It is shown that R3 methods behave poorly due to the narrowness of their rf-matching curves, and that the best methods are and SFAM (SFAM1 or SFAM2 if homo-nuclear interactions are not negligible). REDOR can also recouple efficiently hetero-nuclear dipolar interactions, provided the sequence is sent on the non-observed channel and homo-nuclear dipolar interactions are negligible. We anticipate that at ultra-fast spinning speed, SFAM1 and SFAM2 will be the most efficient methods.

A simple, small and low cost permanent magnet design to produce homogeneous magnetic fields
B. Manz, M. Benecke and F. Volke
A new portable, pocket-size NMR probe based on a novel permanent magnet arrangement is presented. It is based on a Halbach-type magnet design which mimics the field of a spherical dipole by using cylindrical bar and ring magnets. The magnet system is made up of only three individual magnets, and most field calculations and optimisations can be performed analytically. A prototype system has been built using a set of small, off the shelf commercially available permanent magnets. Proton linewidths of 50 ppm FWHM could be achieved at a field strength of 1 T. Calculations show that with custom-sized permanent magnets, linewidths of less than 1 ppm can be achieved over sample volumes of up to 1 mm3, which would in theory enable chemical shift resolved proton spectroscopy on mass-limited samples. But even with the achieved linewidth of 50 ppm, this can be a useful portable sensor for small amounts of liquid samples with restricted molecular mobility, like gels, polymers or high viscosity liquids.

High-performance solvent suppression for proton detected solid-state NMR
Donghua H. Zhou and Chad M. Rienstra
High-sensitivity proton detected experiments in solid-state NMR have been recently demonstrated in proton diluted proteins as well as fully protonated samples under fast magic-angle spinning. One key element for performing successful proton detection is effective solvent suppression achieved by pulsed field gradients (PFG) and/or saturation pulses. Here we report a high-performance solvent suppression method that attenuates multiple solvent signals simultaneously by more than a factor of 10,000, achieved by an optimized combination of homospoil gradients and supercycled saturation pulses. This method, which we call Multiple Intense Solvent Suppression Intended for Sensitive Spectroscopic Investigation of Protonated Proteins, Instantly (MISSISSIPPI), can be applied without a PFG probe. It opens up new opportunities for two-dimensional heteronuclear correlation spectroscopy of hydrated proteins at natural abundance as well as high-sensitivity and multi-dimensional experimental investigation of protein–solvent interactions.

Am. Chem. Soc., 130 (15), 5014–5015, 2008.

Hadamard Amino-Acid-Type Edited NMR Experiment for Fast Protein Resonance Assignment
Ewen Lescop, Rodolfo Rasia, and Bernhard Brutscher

Abstract:
An original Hadamard-encoding scheme allows discrimination among seven amino acid types in a single two-dimensional NMR experiment. Combined with hyperdimensional NMR techniques, this presents a promising new method for fast, automated backbone resonance assignment

Chem. Mater., 20 (7), 2583–2591, 2008.

Dispersion of Silicate in Tricalcium Phosphate Elucidated by Solid-State NMR
A. Rawal,† X. Wei,‡ M. Akinc,‡ and K. Schmidt-Rohr*†

Abstract:
The dispersion of silicate in tricalcium phosphate, a resorbable bioceramics for bone replacement, has been investigated by various solid-state nuclear magnetic resonance (NMR) methods. In samples prepared with 5 and 10 mol % of both 29SiO2 and ZnO, three types of silicate have been detected: (i) SiO44− (Q0 sites) with long longitudinal (T1,Si) relaxation times (~10 000 s), which substitute for ~1% of PO43−; (ii) silicate nanoinclusions containing Q2, Q1, and Q0 sites with T1,Si ~ 100 s, which account for most of the silicon; and (iii) crystalline Q4 (SiO2) with long T1,Si. Sensitivity was enhanced >100-fold by 29Si enrichment and refocused detection. The inclusions in both samples have a diameter of ~8 nm, as proved by 29Si{31P} REDOR dephasing on a 30-ms time scale, which was simulated using a multispin approach specifically suited for nanoparticles. 29Si CODEX NMR with 30-s 29Si spin diffusion confirms that an inclusion contains >10 Si (consistent with the REDOR result of >100 Si per inclusion). Overlapping signals of silicate Q2, Q1, and Q0 sites were spectrally edited based on their J-couplings, using double-quantum filtering. The large inhomogeneous broadening of the Q2, Q1, and Q0 29Si subspectra indicates that the nanoinclusions are amorphous.

Phys. Rev. Lett. 100, 133002 (2008)

Nuclear Magnetic Resonance Chemical Shift in an Arbitrary Electronic Spin StateTeemu O. Pennanen and Juha Vaara

Abstract
We present a general and systematic electronic structure theory of the nuclear magnetic resonance shielding tensor and the associated chemical shift for paramagnetic atoms, molecules, and nonmetallic solids. The approach is for the first time rigorous for an arbitrary spin state as well as arbitrary spatial symmetry and is formulated without reference to spin susceptibility. The leading-order magnetic-field dependence of shielding is derived. The theory is demonstrated by first principles calculations of organometallic molecules.

SSNMR up to March 2008 issue - IUPAC Chemical shift recommendations

SSNMR up to March 2008 issue

Solid State Nuclear Magnetic Resonance
Volume 33, Issue 3, March 2008, Pages 41-56
doi:10.1016/j.ssnmr.2008.02.004 How to Cite or Link Using DOI (Opens New Window)

Further conventions for NMR shielding and chemical shifts IUPAC recommendations 2008star, open
Prepared for publication by, Robin K. Harris, Edwin D. Becker, Sonia M. Cabral De Menezes, Pierre Granger, Roy E. Hoffman and Kurt W. Zilm
Available online 20 February 2008.

Abstract
IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem. 73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the 1H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a part per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS).

This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating 13C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids are reviewed in some detail, and recommendations are given for best practice.

Keywords: Nuclear magnetic resonance; Recommendations; Chemical shifts; Conventions; IUPAC physical and biophysical chemistry division; Shielding tensors

Joel's Journal Updates

Effects of Guanidinium-Phosphate Hydrogen Bonding on the Membrane-Bound Structure and Activity of an Arginine-Rich Membrane Peptide from Solid-State NMR Spectroscopy
Ming Tang, Alan J. Waring, Robert I. Lehrer, Mei Hong
Angwandte (2008)47, 3202.
Summary:
Barreling through: Guanidinium-phosphate hydrogen bonding significantly affects the structure and activity of the antimicrobial peptide PG-1. Solid-state NMR data show that a mutant of PG-1, having dimethylated Arg residues, adopts an in-plane orientation, interfacial location, and fast uniaxial motion around the membrane normal (see scheme). The less active mutant thus disrupts the membrane by in-plane diffusion, in contrast to the more active wild-type PG-1, which forms immobile transmembrane b-barrels to cause toroidal-pore membrane defects.

Application of Ultrahigh-Field 59Co Solid-State NMR Spectroscopy in the Investigation of the 1,2-Polybutadiene Catalyst [Co(C8H13)(C4H6)]
Patrick Crewdson, David L. Bryce, Frank Rominger, Peter Hofmann
Angwandte (2008)47, 3454.
Summary:
Highly suitable: The 1,2-polybutadiene catalyst [Co(3:2-C8H13)(4-C4H6)] (1) was isolated and structurally characterized by ultrahigh-field 59Co solid-state NMR spectroscopy, demonstrating the utility of this technique. It can be applied to study the formation mechanism of syndiotactic 1,2-polybutadiene.

Determination of NMR Lineshape Anisotropy of Guest Molecules within Inclusion Complexes from Molecular Dynamics Simulations
Saman Alavi, Peter Dornan, Tom K. Woo
ChemPhysChem(2008)9(6),911.
Abstract:
Nonspherical cages in inclusion compounds can result in non-uniform motion of guest species in these cages and anisotropic lineshapes in NMR spectra of the guest. Herein, we develop a methodology to calculate lineshape anisotropy of guest species in cages based on molecular dynamics simulations of the inclusion compound. The methodology is valid for guest atoms with spin 1/2 nuclei and does not depend on the temperature and type of inclusion compound or guest species studied. As an example, the nonspherical shape of the structure I (sI) clathrate hydrate large cages leads to preferential alignment of linear CO2 molecules in directions parallel to the two hexagonal faces of the cages. The angular distribution of the CO2 guests in terms of a polar angle and azimuth angle and small amplitude vibrational motions in the large cage are characterized by molecular dynamics simulations at different temperatures in the stability range of the CO2 sI clathrate. The experimental 13C NMR lineshapes of CO2 guests in the large cages show a reversal of the skew between the low temperature (77 K) and the high temperature (238 K) limits of the stability of the clathrate. We determine the angular distributions of the guests in the cages by classical MD simulations of the sI clathrate and calculate the 13C NMR lineshapes over a range of temperatures. Good agreement between experimental lineshapes and calculated lineshapes is obtained. No assumptions regarding the nature of the guest motions in the cages are required.

Proton Spin Diffusion in Polyethylene as a Function of Magic-Angle Spinning Rate. A Phenomenological Approach.
O. Dmitrenko, Shi Bai, Peter A. Beckmann, Scott van Bramer, Alexander J. Vega, and C. Dybowski
JPCA(2008)112,3046.
Abstract:
The analysis of heavy-metal solids with NMR spectroscopy provides a means of investigating the electronic environment through the dependence of the chemical shift on structure. We have investigated the relation of the 207Pb NMR isotropic chemical shift, span, and skew of a series of solid Pb(II) compounds to lattice parameters. Complementary relativistic spin-orbit density functional calculations on clusters such as PbI64- that model the local environment in the dihalides show a dependence of NMR properties on the local structure in good agreement with experimental results.

Low-Temperature NMR Studies of Zn Tautomerism and Hindered Rotations in Solid Zincocene Derivatives
Juan Miguel Lopez del Amo, Gerd Buntkowsky, Hans-Heinrich Limbach,* Irene Resa, Rafael Fernndez, and Ernesto Carmona
JPCA(2008)112,3557.
Abstract:
Using a combination of NMR methods we have detected and studied fluxional motions in the slip-sandwich structure of solid decamethylzincocene (I, [(5-C5Me5)Zn(1-C5Me5)]). For comparison, we have also studied the solid iminoacyl derivative [(5-C5Me5)Zn(1-C(NXyl)C5Me5)] (II). The variable temperature 13C CPMAS NMR spectra of I indicate fast rotations of both Cp* rings in the molecule down to 156 K as well as the presence of an order-disorder phase transition around 210 K. The disorder is shown to be dynamic arising from a fast combined Zn tautomerism and 1/5 reorganization of the Cp* rings between two degenerate states A and B related by a molecular inversion. In the ordered phase, the degeneracy of A and B is lifted; that is, the two rings X and Y are inequivalent, where X exhibits a larger fraction of time in the 5 state than Y. However, the interconversion is still fast and characterized by a reaction enthalpy of H = 2.4 kJ mol-1 and a reaction entropy of S = 4.9 J K-1 mol-1. In order to obtain quantitative kinetic information, variable temperature 2H NMR experiments were performed on static samples of I-d6 and II-d6 between 300 and 100 K, where in each ring one CH3 is replaced by one CD3 group. For II-d6, the 2H NMR line shapes indicate fast CD3 group rotations and a fast "5 rotation", corresponding to 72 rotational jumps of the 5 coordinated Cp* ring. The latter motion becomes slow around 130 K. By line shape analysis, an activation energy of the 5 rotation of about 21 kJ mol-1 was obtained. 2H NMR line shapes analysis of I-d6 indicates fast CD3 group rotations at all temperatures. Moreover, between 100 and 150 K, a transition from the slow to the fast exchange regime is observed for the 5-fold rotational jumps of both Cp* rings, exhibiting an activation energy of 18 kJ mol-1. This value was corroborated by 2H NMR relaxometry from which additionally the activation energies 6.3 kJ mol-1 and 11.2 kJ mol-1 for the CD3 rotation and the molecular inversion process were determined.

Phys. Rev. B

57Fe NMR and spin structure of manganese ferriteH. Štěpánková, B. Sedlák, and V. Chlan, P. Novák and Z. Šimša

Abstract
NMR of 57Fe in five MnFe2O4 single crystals with different degrees of inversion was measured in liquid He temperature. At the zero external field, two lines originating from Fe3+ ions on the octahedral sites are observed at 68.7 and 71.1 MHz, while the line at 72.0 MHz, the amplitude of which increases with increasing inversion, is ascribed to Fe3+ ions on the tetrahedral sites. Measurement in the external field shows that the spin structure is in accord with the Goodenough-Kanamori rules. This contradicts to an abnormal spin structure which Shim et al. [Phys. Rev. B 75, 134406 (2007)] proposed recently on the basis of 57Fe NMR measured in polycrystalline manganese ferrite. Reinterpretation of the NMR in polycrystalline compounds is given.


Phys. Rev. B 77, 104301 (2008)

6Li and 7Li NMR line-shape and stimulated-echo studies of lithium ionic hopping in LiPO3 glass
Sandra Faske, Hellmut Eckert, and Michael Vogel

Abstract
6Li and 7Li NMRs are used to investigate the lithium ion dynamics in LiPO3 glass. In particular, 6Li NMR stimulated-echo experiments are used to provide straightforward access to two-time correlation functions characterizing the lithium ionic hopping motion in the millisecond regime in a glassy ion conductor. Temperature-dependent measurements serve to separate the spin diffusion contribution and the dynamic contribution to the stimulated-echo decays. The 6Li NMR correlation functions of LiPO3 glass describing the lithium ionic motion show pronounced nonexponential decays, which can be well described by a stretched exponential function with a temperature-independent small stretching parameter =0.27, indicating the complex nature of the lithium dynamics. The temperature dependence of the mean correlation times resulting from these stimulated-echo experiments is described by an activation energy Ea=0.66 eV. The values of are in good agreement with time constants from previous electrical and mechanical relaxation studies. At appropriate temperatures, the 6Li and 7Li NMR spectra are superpositions of a broad and a narrow spectral component, which result from slow and fast lithium ions, respectively, on the NMR time scale. A detailed analysis of the temperature dependence of these line shapes provides information about the distribution of correlation times.

Phys. Rev. B 77, 115130 (2008)

Electronic properties of Sc5M4Si10 (M=Co,Rh,Ir) probed by NMR and first-principles calculations
C. S. Lue, R. F. Liu, Y. F. Fu, and C. Cheng

We report a systematic study of 45Sc NMR measurements on the Sc5Co4Si10-type silicides Sc5M4Si10 (M=Co,Rh,Ir). From the central transition line shapes, three nonequivalent Sc sites have been identified. We thus measured the Knight shift and spin-lattice relaxation time (T1) for each of the three crystallographic sites. Results of experimental Knight shift and T1 together with theoretical band structure calculations provide evidence that orbital electrons are responsible for the observed shifts as well as the relaxation rates. In addition, we found no correlation between the Fermi-level density of states and the superconducting transition temperature of the studied materials. Further analyses clearly indicate that the effect of electron-phonon coupling plays a significant role for the superconductivity of Sc5M4Si10, and these materials should be classified as moderate-coupling superconductors.

J. Am. Chem. Soc.

J. Am. Chem. Soc., 130 (14), 4584 -4585, 2008.
Bolalipid Membrane Structure Revealed by Solid-State 2H NMR Spectroscopy
David P. Holland, Andrey V. Struts, Michael F. Brown, and David H. Thompson*

Abstract:
Membranes made from three specifically deuterium-labeled ether-linked bolalipids, [1',1',20',20'-2H4]C20BAS-PC, [2',2',19',19'-2H4]C20BAS-PC, or [10',11'-2H2]C20BAS-PC, were analyzed by 2H NMR spectroscopy. Unlike more common monopolar, ester-linked phospholipids, C20BAS-PC exhibits a high degree of orientational order throughout the membrane and the sn-1 chain of the lipid initially penetrates the bilayer at an orientation different from that of the bilayer normal, resulting in inequivalent deuterium atoms at the C1 position. The approximate hydrophobic layer thickness and area per lipid are 18.4 Å and 60.4 Å2, respectively, at 25 C, and their respective thermal expansion coefficients are within 20% of the monopolar phospholipid, DLPC.

J. Am. Chem. Soc., 130 (14), 4757 -4766, 2008.
Orientation of Fluorinated Cholesterol in Lipid Bilayers Analyzed by 19F Tensor Calculation and Solid-State NMR
Nobuaki Matsumori, Yusuke Kasai, Tohru Oishi, Michio Murata, and Kaoru Nomura


Abstract:
6-F-cholesterol was reported to exhibit biological and interfacial properties similar to unmodified cholesterol. We have also found that 6-F-cholesterol mimicked the cholesterol activity observed in the systems of amphotericin B and lipid rafts. However, to use 6-F-cholesterol as a molecular probe to explore molecular recognition in membranes, it is indispensable to have detailed knowledge of the dynamic and orientation properties of the molecule in membrane environments. In this paper, we present the molecular orientation of 6-F-cholesterol (30 mol %) in dimyristoylphosphatidylcholine (DMPC) bilayers revealed by combined use of 19F chemical shift anisotropy (CSA), 2H NMR, and C-F rotational echo double resonance (REDOR) experiments. The axis of rotation of 6-F-cholesterol was shown to be in a similar direction to that of cholesterol in DMPC bilayers, which is almost parallel to the long axis of the molecular frame. The molecular order parameter of 6-F-cholesterol was determined to be ca. 0.85, which is within the range of reported values of cholesterol. These findings suggest that the dynamic properties of 6-F-cholesterol in DMPC are quite similar to those of unmodified cholesterol; therefore, the introduction of a fluorine atom at C6 has virtually no effect on cholesterol dynamics in membranes. In addition, this study demonstrates the practical utility of theoretical calculations for determining the 19F CSA principal axes, which would be extremely difficult to obtain experimentally. The combined use of quantum calculations and solid-state 19F NMR will make it possible to apply the orientation information of 19F CSA tensors to membrane systems.

Applied Magnetic Resonance Special Issue: Applications of Solid-State NMR to Characterizing Materials

Issue edited by John V. Hanna and Mark E. Smith:

Calcium Phosphates and Hydroxyapatite: Solid-State NMR Experiments and First-Principles Calculations
F. Pourpoint, C. Gervais , L. Bonhomme-Coury, T. Azaïs, C. Coelho, F. Mauri, B. Alonso, F. Babonneau and C. Bonhomme
Various calcium phosphates and hydroxyapatite (HAp) have been fully characterized by one- and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments and first principles calculations of NMR parameters, such as chemical shift anisotropy (CSA) and electric field gradient tensors for all nuclei. Such compounds act as useful biocompatible materials. The projector augmented wave (PAW) and gauge including PAW methods allowed the complete assignment of spectra, including 1H magic-angle spinning (MAS) spectra for which ultimate resolution is not attained experimentally. 1H CSA tensors and orientation of the principal axes systems have been also discussed. 17O parameters have been calculated for a large variety of oxo-bridges and terminal oxygen atoms, including P–O–Si fragments characteristic for silicophosphate phases. The (δiso, C Q) sets of values allowed the clear distinction between the various oxygen atoms in a calculated 17O 3-quantum MAS experiment. Such an approach should be of great help for the description of interfaces in complex materials, in terms of structure and chemical composition.

Solid-State NMR Studies on Ionic closo -Dodecaborates
I. Tiritiris, T. Schleid and K. Müller
Alkali metal dodecahydro-closo-dodecaborates M2[B12H12] (M = K, Rb, Cs, NH4, N(CH3)4) and the perhalogenated cesium salts Cs2[B12X12] (X = Cl, Br, I) are studied by solid-state 11B nuclear magnetic resonance (NMR) spectroscopy as well as X-ray diffraction (XRD) and differential scanning calorimetry. The present work addresses the molecular dynamics of the anionic [B12X12]2− icosahedra which is examined by variable-temperature 11B NMR line shape studies between 120 and 370 K. Characteristic line shape effects are observed which strongly depend on the actual substituent X and the counterion M+. All alkali metal dodecahydro-closo-dodecaborates M2 [B12H12] exhibit at elevated temperatures 11B NMR spectra with a single isotropic line which proves the presence of an efficient molecular process, resulting in dynamic (rotational) disorder along with vanishing dipolar and quadrupolar interactions. The positional order of the boron clusters, however, remains unaffected, as shown by the XRD data. At lower temperatures, the underlying motions are frozen on the NMR timescale resulting in characteristic 11B NMR spectra with a dominant homonuclear 11B–11B dipolar splitting. The per-halogenated cesium salts Cs2[B12X12] behave differently. Hence, from the experimental 11B NMR spectra at room temperature a substantial mobility is only seen for the [B12Cl12]2− anion. Obviously, the degree of anion mobility depends on the size of the substituent X in the [B12X12]2− clusters (X = H, Cl, Br, I). A quantitative analysis of the experimental 11B NMR spectra of the alkali metal dodecahydro-closo-dodecaborates M2 [B12H12] is achieved by line shape simulations, considering [B12H12]2− ions undergoing reorientational jumps between icosahedral sites. From the motional correlation times the activation energies are derived. It is found that a correlation exists between the activation energies, the motional correlation times and the lattice constant. Hence, the activation energies and correlation times strongly increase with decreasing size of the cation M+, which reflects an increasing sterical hindrance due to a decreasing crystallo-graphic lattice constant in the same direction.

NMR Insights into Wasteforms for the Vitrification of High-Level Nuclear Waste
D. Holland , B. G. Parkinson, M. M. Islam, A. Duddridge, J. M. Roderick, A. P. Howes and C. R. Scales
Magic-angle spinning nuclear magnetic resonance of 11B, 29Si and 27Al has been used to study the distribution of nonbridging oxygen atoms (NBO) in an alkali borosilicate glass to which surrogate oxides for high-level radioactive waste have been added. The properties of such glasses are shown to depend on the fraction N 4 of four-coordinated boron atoms (B4) and on the fraction of silicate tetrahedra possessing one NBO, Q3. The aqueous corrosion rate increases with Q3 content, as does weight loss due to evaporation from the melt. The activation energy for direct current conduction scales with N 4. Values of N 4 obtained for these glasses deviate from those predicted by the currently accepted model and are strongly affected by the modifier or intermediate nature of the surrogate oxide and also by its effect on the distribution of NBO between the silicate and borate polyhedra.

MAS NMR Strategies for the Characterization of Supported Molybdenum Catalysts
J.-B. d'Espinose de Lacaillerie and Z. Gan
After recalling a few advances made by 1H, 29Si, and 27Al on the understanding of hydro-desulfuration (HDS) molybdenum-based catalysts supported on amorphous oxides, we critically evaluate the potential of 95Mo magic-angle spinning nuclear magnetic resonance (MAS NMR) for gaining further insight into the structure of uncalcined precursors and sulfided HDS catalysts. It is shown that when performed at a very high field (19.6 T), it is indeed sensitive to the nature of the molybdenum–support interaction. In particular, a wide distribution of the molybdenum environment present on the surface was evidenced. However, the possibility to characterize sulfided catalysts by 95Mo MAS NMR appeared still an unmet challenge.

Solid-State MAS NMR Studies of Sulfonic Acid-Functionalized SBA-15
R. Kanthasamy, I. K. Mbaraka, B. H. Shanks and S. C. Larsen
Solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) was used to characterize propylsulfonic acid-functionalized SBA-15 materials prepared by the cocondensation method. The propylsulfonic acid-group concentration and distribution was systematically varied from 2.5 to 10%. As a way of coupling two acid sites and controlling the acid site spatial distribution, tethered propylsulfonic acid groups were incorporated into SBA-15. 13C, 29Si and 1H MAS NMR were used to determine the extent of functionalization. Probe molecules, such as triethylphosphine oxide and diphenyldiphosphines were used to evaluate acid site strength and acid site distributions, respectively.

Multinuclear MAS NMR Investigation of Sol-Gel and Ball-Milled Nanocrystalline Ga2O3
L. A. O'Dell, S. L. P. Savin, A. V. Chadwick and M. E. Smith
71Ga magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to characterize the structural evolution of nanocrystalline Ga2O3 samples prepared by sol-gel and ball-milling techniques. 29Si and 27Al MAS NMR have also been used to characterize silica and alumina Zener pinning phases. 71Ga NMR parameters are reported for the α- and β-Ga2O3 phases, and more tentatively for the δ-Ga2O3 phase. By simulating the octahedrally coordinated gallium NMR line of β-Ga2O3 using Gaussian distributions in χQ, the extent of disorder in the Ga2O3 crystallites has been quantified. The ball-milled samples contain much more inherent disorder than the sol-gel samples in the nano-phase, which was observed from simulations of the 71Ga MAS NMR spectra. The silica pinning phase produced highly crystalline and densely aggregated nanocrystalline Ga2O3, as well as the smallest nanocrystal sizes.

Covalency Measurements via NMR in Lithium Metal Phosphates
S. L. Wilcke, Y.-J. Lee, E. J. Cairns and J. A. Reimer
31P nuclear magnetic resonance (NMR) shifts on the order of thousands of parts per million are observed for olivine LiMPO4 (M = Mn, Fe, Co, Ni) samples, a promising class of Li ion rechargeable battery electrode materials. Variable-temperature 31P NMR measurements of shift are used to determine that the supertransferred hyperfine interaction is the dominant mechanism giving rise to these unusually large observed 31P shifts. Various models for predicting 31P and 7Li shifts in LiMPO4 (M = Mn, Fe, Co, Ni) were investigated. Alloys of LiFe1−x Mn x PO4, where x varies from 0 to 1, were also investigated by 7Li NMR. Covalency constants, calculated from variable-temperature NMR shifts and magnetic susceptibility data, are determined for the P–O–M bonds in LiMPO4 (M = Mn, Fe, Co, Ni) and compared to the covalency constants of the Li–O–M bond. The sign and relative magnitude of the covalency constants are discussed in terms of positive and negative spin densities at the nuclei of interest. The covalency constants for the Li–O–M and P–O–M bonds were measured for Li1.8Na0.2FeMn2(PO4)3 and compared to the covalency constants measured in the olivine LiMPO4 (M = Mn, Fe, Co, Ni) samples. The Li1.8Na0.2FeMn2(PO4)3 structure has a volume per transition metal atom and Li–O–M bond distances that are similar to those of the olivine LiMPO4 (M = Mn, Fe, Co, Ni) samples.

The Challenge of Paramagnetism in Two-Dimensional 6,7Li Exchange NMR
L. S. Cahill, R. P. Chapman, C. W. Kirby and G. R. Goward
6,7Li fast magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy is used to study LiMn2O4 and Li3V2(PO4)3. The presence of paramagnetic transition metal centers in these materials has a profound effect on the resulting NMR spectra. Lithium ion mobility has been studied by two-dimensional (2-D) exchange spectroscopy (EXSY) in Li3V2(PO4)3 but an absence of lithium ion exchange was observed for LiMn2O4. Several differences between the two materials are explored to explain these results. LiMn2O4 experiences a greater donation of electron spin density to the Li nucleus via the Fermi-contact interaction when compared with Li3V2(PO4)3. This contributes to a greater hyperfine chemical shift and a larger dependence of chemical shift on temperature. The delocalized electrons in LiMn2O4 cause temperature-independent T 1 relaxation rates and shorter relative T 2 values. The relative rates of ionic conductivity and spin–lattice or spin–spin relaxation in LiMn2O4 and Li3V2(PO4)3 are contrasted to illustrate the constraints on the use of 2-D EXSY to characterize ion dynamics in paramagnetic materials.

Shift Anisotropy Tensors in Amorphous Natural-Abundance Solids: High-Resolution 29Si Chemical Shift Anisotropy Distributions under Very Slow Sample Rotation
D. Sakellariou and T. Charpentier
The magic-angle turning technique is applied to amorphous natural-abundance silicate materials and high-resolution silicon-29 correlations between the isotropic and anisotropic chemical shifts are obtained. Very narrow tilted spinning sideband patterns are resolved in the two-dimensional spectra whose line width corresponds to the natural line width of the silicon nuclei. Various numerical approaches are implemented to extract the distribution of the chemical shift anisotropy tensors in these materials and their results are compared.

NMR Studies of Heat-Induced Transitions in Structure and Cation Binding Environments of a Strontium-Saturated Swelling Mica
G. M. Bowers, M. C. Davis, R. Ravella, S. Komarneni and K. T. Mueller
In this work, we combine 27Al, 29Si, 19F, and 23Na magic-angle spinning (MAS) nuclear magnetic resonance (NMR) to characterize the structure and interlayer cation environments in a strontium-saturated member of the swelling mica family before and after a heat-induced collapse of the interlayer space. The 27Al and 29Si MAS NMR demonstrate that the sample consists mainly of swelling mica, though the composition does not match the ideal structural formula. Aluminum NMR also shows that a portion of the aluminum shifts from a tetrahedral to an octahedral coordination environment upon heating. Changes in the 29Si and 19F NMR after heating are consistent with a structural rearrangement of the tetrahedral sheet to permit the binding of larger cations in the ditrigonal cavity. The 23Na MAS NMR results indicate the presence of three unique sodium environments before and after heating. The heat-invariant resonance is consistent with the presence of sodium carbonate. The other two resonances are associated with interlayer sodium and reflect a migration of sodium to a dominantly anhydrous ditrigonal binding structure with heating. Quantitative elemental analysis and NMR data presented here suggest strontium is bound deep within the ditrigonal cavity of the collapsed micas.

NMR Study of a Rare-Earth Aluminoborosilicate Glass with Varying CaO-to-Na2O Ratio
A. Quintas, T. Charpentier, O. Majérus, D. Caurant, J.-L. Dussossoy and P. Vermaut
The effect of substituting two Na+ by one Ca2+ in a rare-earth aluminoborosilicate glass is investigated by multinuclear magic-angle spinning (MAS) and multiple-quantum (MQ)MAS nuclear magnetic resonance (NMR) spectroscopy. Quantitative analysis of the 23Na and 27Al MAS/MQMAS data along with the 11B MAS NMR data provides complementary information enabling to cast light on different structural key points. A strong decrease of the N 4 = BO4/(BO3 + BO4) ratio is observed consecutively to this substitution, indicating that sodium is more favorable than calcium to the formation of BO4 units. The experimental N 4 ratio is compared to the Dell and Bray model prediction and it is shown that several adjustments, due to the presence in our glass of Nd and Zr, are necessary to obtain acceptable agreement with experimental data. 29Si MAS NMR data also put in evidence an effect of the substitution on the polymerization degree. Glass in glass phase separation is clearly detected when the ratio of CaO to Na2O is greater than 1 and a different evolution of NMR parameters is observed for the ratio of CaO to Na2O being less than or equal to 1. Concerning aluminum charge compensation, it is demonstrated that, as long as no phase separation is detected, the negative charge of AlO4 − entities is almost exclusively balanced by sodium cations. Finally, changes of the sodium ions organization within the glass network are also evidenced by spin–lattice relaxation and spin echo decay measurements.

Laser-Heated High-Temperature NMR: A Time-Resolution Study
R. Winter, A. Jones, R. Shaw-West, M. Wolff, P. Florian and D. Massiot
The time resolution achievable in in situ high-temperature nuclear magnetic resonance experiments is investigated using laser heating of refractory materials. Three case studies using 27Al in alumina nanoparticles, 29Si in silicon carbide and 23Na in a glass-forming mixture of sodium carbonate and quartz have been conducted to distinguish the cases of (a) a fast-relaxing, high natural abundance nucleus, (b) a probe nucleus with low abundance and low spin–lattice relaxation rate, and (c) a complex and changing system of industrial relevance. The most suitable nucleus for in situ high-temperature studies is one with high abundance but slow relaxation because the differential relaxation time between hot and cold parts of the sample effectively removes the signal from the cold material. There is no "in situ penalty" from the diminishing Boltzmann polarization at high temperature since this effect is balanced by a corresponding increase of the spin–lattice relaxation rate.

Crystalline Aluminium Borates with the Mullite Structure: A 11B and 27Al Solid-State NMR Study
K. J. D. MacKenzie, M. E. Smith, T. F. Kemp and D. Voll
27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were acquired at 8.45, 14.1 and 16.45 T for a series of aluminium borates with the mullite structure (Al6−x B x O9, where x has nominal values of 1 to 4) augmented with 27Al multiple-quantum MAS NMR spectra at 8.45 T. Even though the 27Al NMR spectra are complex, simulation of the combined set of data produced a relatively well-defined set of parameters (e.g., quadrupolar interaction, isotropic chemical shift, etc.) for each site. The 11B MAS NMR spectra of the same compounds were also acquired at 14.1 T. Linear changes in the X-ray a-, b- and c-cell parameters with composition suggest that these compounds constitute a continuous series. Based on a Rietveld structural refinement of the compound synthesized as Al4B2O9, the resulting site occupancies and relative site distortions allow the identification of particular sites with specific NM resonances. Changes in the 27Al and 11B MAS NMR spectra of the related compounds with x = 1–4 show at the lowest Al contents a greater degree of asymmetry in the Al sites of the octahedral chains. A fairly distorted cross-linking tetrahedral site, which persists throughout the composition range, is accompanied in the lower Al compositions by two 5-fold coordinated Al–O units which are replaced by two more-regular tetrahedral Al–O sites as the Al content increases. In the compounds of lowest Al composition (i.e., highest B content) both the tetrahedral and trigonal cross-linking sites are distinguishable, but as the Al content increases, the BO4 units progressively disappear.

29Si, 27Al, 1H and 23Na MAS NMR Study of the Bonding Character in Aluminosilicate Inorganic Polymers
M. R. Rowles, J. V. Hanna , K. J. Pike, M. E. Smith and B. H. O'Connor
29Si, 27Al, 1H and 23Na solid-state magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to relate nominal composition, bonding character and compressive strength properties in aluminosilicate inorganic polymers (AIPs). The 29Si chemical shift varies systematically with Si-to-Al ratio, indicating that the immediate structural environment of Si is altering with nominal composition. Fast 1H MAS and 29Si T SiH/T 1ρ relaxation measurements demonstrated that occluded pore H2O mobility within the disordered cavities is slow in comparison with H2O mobility characteristics observed within the ordered channel structures of zeolites. The 27Al MAS NMR data show that the Al coordination remains predominantly 4-coordinate. In comparison with the 29Si MAS data, the corresponding 27Al MAS line shapes are relatively narrow, suggesting that the AlO4 tetrahedral geometry is largely unperturbed and the dominant source of structural disorder is propagated by large distributions of Si–O bond angles and bond lengths. Corresponding 23Na MAS and multiple-quantum MAS NMR data indicate that Na speciation is dominated by distributions of hydration states; however, more highly resolved 23Na resonances observed in some preparations supported the existence of short-range order. New structural elements are proposed to account for the existence of these Na resonances and an improved model for the structure of AIPs has also been proposed.

A Theoretical Study of 51V Electric Field Gradient Tensors in Pyrovanadates and Metavanadates
A. Y. H. Lo, J. V. Hanna and R. W. Schurko
A computational study of the 51V electric field gradient (EFG) tensors in pyrovanadates, α-Zn2V2O7, Cd2V2O7, β-Mg2V2O7 and BaCaV2O7, and the metavanadates, LiVO3, α-NaVO3, KVO3, ZnV2O6 and MgV2O6, is presented. Restricted Hartree–Fock and hybrid density functional theory calculations have been used to investigate the effects of the size of vanadium-oxygen clusters, basis set size, proton-termination and embedded cluster techniques on the accuracy of the calculated EFG tensors. Good agreement between theory and experiment is obtained for most of the vanadates. A sound methodology is suggested for calculating the EFG tensor in pyrovanadates which contain isolated V2O7 4− clusters. For metavanadates, the charges of the bridging oxygen atoms can be differentiated from those of terminal oxygen atoms by terminating the former with hydrogen atoms, and embedded cluster molecular orbital calculations are useful in accounting for the long-range electrostatic interactions which influence the EFG tensor components. EFG tensor orientations vary for different pyrovanadate structural types, and individual components are confined by symmetry elements in the metavanadates. A preliminary comparison is made between 51V EFG tensors calculated with ab initio and plane wave methods. Theoretical EFG tensor components and orientations, in combination with experimental 51V solid-state nuclear magnetic resonance data, are demonstrated to be useful tools for prediction of molecular structure.

Chem. Mater

Chem. Mater., 20 (6), 2205–2217, 2008.
A Solid-State NMR, X-ray Diffraction, and Ab Initio Investigation into the Structures of Novel Tantalum Oxyfluoride Clusters
Todd M. Alam,*† Jacalyn S. Clawson,†‡ François Bonhomme,† Steven G. Thoma,† Mark A. Rodriguez,† Shaohui Zheng,§ and Jochen Autschbach§

Abstract
A series of tantalum oxyfluoride materials containing the [Ta4F16O4]4− and [Ta8F24O12]8− anion clusters have been synthesized and characterized using X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. The structure of both tantalum oxyfluoride materials display octahedrally bonded tantalum atoms with bridging oxygen and terminal fluoride atoms. The [Ta4F16O4]4− cluster is an eight-membered ring, whereas the [Ta8F24O12]8- cluster forms a cagelike structure. Solid-state dynamics of these clusters were explored by monitoring the impact of temperature on the one-dimensional (1D) 19F magic angle spinning (MAS) NMR, 13C cross-polarization (CP) MAS NMR, and two-dimensional (2D) double quantum (DQ) 19F MAS NMR spectra. The DQ 19F NMR correlation experiments allowed the through space connectivity between the different resolved fluorine environments to be determined, thus aiding in the spectral assignment and structural refinement of these materials. Ab initio 19F NMR chemical shift calculations were used to assist in the interpretation of the 19F NMR spectra. The influence of scalar relativistic and Ta−F spin–orbit coupling on the 19F NMR shielding calculation arising from bonding to tantalum atoms is also addressed.

Chem. Mater

Chem. Mater., 20 (6), 2412–2422, 2008.
Direct Synthesis and Solid-State NMR Characterization of Cubic Mesoporous Silica SBA-1 Functionalized with Phenyl Groups
Hsien-Ming Kao,*† Chia-Hsiu Liao,† Tzu-Ti Hung,† Yu-Chi Pan,† and Anthony S. T. Chiang‡

Abstract:
Well-ordered mesoporous silicas SBA-1 (cubic Pm3n symmetry) functionalized with phenyl groups have been synthesized via co-condensation of tetraethoxysilane (TEOS) and phenyltriethoxysilane (PhTES) under acidic conditions. The synthesis parameters such as temperature, type of surfactant, and synthesis composition have been systematically investigated as a function of PhTES contents. The phenyl-containing units are incorporated quantitatively and reach a maximum PhTES loading up to 33 mol % (based on silicon) without a significant degradation of the structural ordering of the Pm3n mesophase. A combination of multinuclear (1H, 13C, 29Si) solid-state NMR and two-dimensional (2D) solid-state NMR correlation techniques such as 13C{1H} and 29Si{1H} HETCOR (heteronuclear correlation) and 1H-1H exchange NMR has been used to establish framework locations of phenyl functional groups that are incorporated in the mesoporous structure and their interactions with the surfactant molecules. 2D 13C{1H} HETCOR NMR experiments reveal that the phenyl moieties are in close spatial proximity to the trimethylammonium headgroups of the cationic surfactant species in the as-synthesized materials, suggesting that there are some specific interactions between them to maintain the surfactant packing parameter (g) smaller than 1/3 necessary for the formation of the cubic mesophase. The detection of couplings between the protons associated with various 29Si species via 29Si{1H} HETCOR NMR established that the T silicon species due to the phenyl groups incorporated are in closer proximity to the Q4 silicon species than to the Q3 silicon species. This observation also provides direct molecular-level evidence for the co-condensation of PhTES and TEOS in the synthesis of mesoporous organosilicas.

J. Am. Chem. Soc.

J. Am. Chem. Soc., 130 (12), 3722 -3723, 2008.

Direct Observation of the Active Center for Methane Dehydroaromatization Using an Ultrahigh Field 95Mo NMR Spectroscopy
Heng Zheng, Ding Ma, Xinhe Bao,* Jian Zhi Hu, Ja Hun Kwak, Yong Wang, and Charles H. F. Peden

Abstract:
The use of an ultrahigh magnetic field spectrometer and 95Mo isotope enrichment facilitate the direct observation of the local structure of Mo species on Mo/zeolite catalysts by 95Mo NMR. Top trace: The experimental 95Mo NMR spectrum of 6Mo/HZSM-5. Bottom traces: The simulated overall spectrum (orange), the spectral component corresponding to MoO3 (purple), and the component corresponding to the exchanged Mo species (green). The exchanged Mo species proved to be the active center for the methane dehydroaromatization (MDA) reaction.

J. Chem. Phys.

Solid-state NMR covariance of homonuclear correlation spectra
J. Chem. Phys. 128, 134502 (2008)
Bingwen Hu, Jean-Paul Amoureux, Julien Trebosc, Michael Deschamps, and Gregory Tricot

Direct covariance NMR spectroscopy, which does not involve a Fourier transformation along the indirect dimension, is demonstrated to obtain homonuclear correlation two-dimensional (2D) spectra in the solid state. In contrast to the usual 2D Fourier transform (2D-FT) NMR, in a 2D covariance (2D-Cov) spectrum the spectral resolution in the indirect dimension is determined by the resolution along the detection dimension, thereby largely reducing the time-consuming indirect sampling requirement. The covariance method does not need any separate phase correction or apodization along the indirect dimension because it uses those applied in the detection dimension. We compare in detail the specifications obtained with 2D-FT and 2D-Cov, for narrow and broad resonances. The efficiency of the covariance data treatment is demonstrated in organic and inorganic samples that are both well crystallized and amorphous, for spin −1/2 nuclei with 13C, 29Si, and 31P through-space or through-bond homonuclear 2D correlation spectra. In all cases, the experimental time has been reduced by at least a factor of 10, without any loss of resolution and signal to noise ratio, with respect to what is necessary with the 2D-FT NMR. According to this method, we have been able to study the silicate network of glasses by 2D NMR within reasonable experimental time despite the very long relaxation time of the 29Si nucleus. The main limitation of the 2D-Cov data treatment is related to the introduction of autocorrelated peaks onto the diagonal, which does not represent any actual connectivity.

Inorg. Chem. - 2D 45Sc NMR of ScAgSn

Inorg. Chem., 46 (3), 771 -779, 2007. 10.1021/ic061691o S0020-1669(06)01691-0

New Stannide ScAgSn: Determination of the Superstructure via Two-Dimensional 45Sc Solid State NMR

C. Peter Sebastian, Long Zhang, Constanze Fehse, Rolf-Dieter Hoffmann, Hellmut Eckert,* and Rainer Pöttgen*

Institut für Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany, and Institut für Physikalische Chemie and NRW Graduate School of Chemistry, Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany

Received September 7, 2006

Abstract:

The new stannide ScAgSn was synthesized by induction melting of the elements in a sealed tantalum tube and subsequent annealing. ScAgSn crystallizes with a pronounced subcell structure: ZrNiAl type, P 2m, a = 708.2(2) pm, c = 433.9(1) pm, wR2 = 0.1264, 321 F2 values, and 14 variables. The Guinier powder pattern reveals weak superstructure reflections pointing to a TiFeSi-type structural arrangement: I2cm, a = 708.1(1) pm, b = 1225.2(2) pm, c = 869.9(1) pm, wR2 = 0.0787, 5556 F2 values, and 49 variables. So far the growth of high-quality single crystals failed. Determination of the superstructure was partly based on merohedral triplet X-ray data augmented by 119Sn Mössbauer spectroscopy and 119Sn and 45Sc solid-state NMR data. In particular, the observation of three crystallographically inequivalent sites in 45Sc NMR triple quantum magic-angle spinning (TQ-MAS) NMR spectra provided unambiguous proof of the superstructure proposed. The ScAgSn structure consists of a three-dimensional [AgSn] network (with Ag-Sn distances between 273 and 280 pm) in which the scandium atoms are located in distorted hexagonal channels, each having five tin and two silver nearest neighbors. Both crystallographically independent tin sites have a tricapped trigonal prismatic coordination, that is, [Sn1Sc6Ag3] and [Sn2Ag6Sc3] environments, which are well distinguished in the 119Sn NMR and Mössbauer spectra because of their different site symmetries.

JACS - 95Mo NMR of metal sites in zeolites

J. Am. Chem. Soc., 130 (12), 3722 -3723, 2008.

10.1021/ja7110916

Direct Observation of the Active Center for Methane Dehydroaromatization Using an Ultrahigh Field ⁹⁵Mo NMR Spectroscopy

Heng Zheng, Ding Ma, Xinhe Bao,* Jian Zhi Hu, Ja Hun Kwak, Yong Wang, and Charles H. F. Peden

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China, and Pacific Northwest National Laboratory, P. O. Box 999, MS K8-98, Richland, Washington 99352

xhbao@dicp.ac.cn

Received December 14, 2007

Abstract:

The use of an ultrahigh magnetic field spectrometer and ⁹⁵Mo isotope enrichment facilitate the direct observation of the local structure of Mo species on Mo/zeolite catalysts by ⁹⁵Mo NMR. Top trace: The experimental ⁹⁵Mo NMR spectrum of 6Mo/HZSM-5. Bottom traces: The simulated overall spectrum (orange), the spectral component corresponding to MoO₃ (purple), and the component corresponding to the exchanged Mo species (green). The exchanged Mo species proved to be the active center for the methane dehydroaromatization (MDA) reaction.