Friday, December 14, 2007

Joel's journal updates

Advances in Structural Analysis of Fluoroaluminates Using DFT Calculations of 27Al Electric Field Gradients
M. Body, C. Legein, J.-Y. Buzaré, G. Silly, P. Blaha, C. Martineau, and F. Calvayrac
Based on the analysis of 23 aluminum sites from 16 fluoroaluminates, the present work demonstrates the strong potential of combining accurate NMR quadrupolar parameter measurements, density functional theory (DFT)-based calculations of electric field gradients (EFG), and structure optimizations as implemented in the WIEN2k package for the structural and electronic characterizations of crystalline inorganic materials. Structure optimizations are essential for compounds whose structure was refined from usually less accurate powder diffraction data and provide a reliable assignment of the 27Al quadrupolar parameters to the aluminum sites in the studied compounds. The correlation between experimental and calculated EFG tensor elements leads to the proposition of a new value of the 27Al nuclear quadrupole moment Q(27Al) ) 1.616 ((0.024)  10-29 m2. The DFT calculations provide the orientation of the 27Al EFG tensors in the crystal frame. Electron density maps support that the magnitude and orientation of the 27Al EFG tensors in fluoroaluminates mainly result from the asymmetric distribution of the Al 3p orbital valence electrons. In most cases, the definition of relevant radial and angular distortion indices, relying on EFG orientation, allows correlations between these distortions and magnitude and sign of the Vii.

K-39 Quadrupolar and Chemical Shift Tensors for Organic Potassium Complexes and Diatomic Molecules
Philip K. Lee, Rebecca P. Chapman, Lei Zhang, Jiaxin Hu, Leonard J. Barbour, Elizabeth K. Elliott, George W. Gokel, and David L. Bryce
Solid-state potassium-39 NMR spectra of two potassium complexes of crown-ether-based organic ligands (1·KI and 2) have been acquired at 11.75 and 21.1 T and interpreted to provide information on the 39K quadrupolar and chemical shift tensors. The analyses reveal a large potassium chemical shift tensor span of 75 ± 20 ppm for 1·KI. This appears to be the first such measurement for potassium in an organic complex, thereby suggesting the utility of potassium chemical shift tensors for characterizing organic and biomolecular K+ binding environments. Compound 2 exhibits a cation- interaction between K+ and a phenyl group, and therefore, the 39K NMR tensors obtained for this compound must be partly representative of this interaction. Analyses of potassium-39 spin-rotation data for gaseous 39K19F and 39K35Cl available from molecular beam experiments performed by Cederberg and co-workers reveal the largest potassium CS tensor spans known to date, 84.39 and 141 ppm, respectively. Collectively, the results obtained highlight the potential of ultrahigh-field potassium-39 solid-state NMR spectroscopy and, in particular, the wide range of the anisotropy of the potassium CS tensor when organic and diatomic systems are considered.

Molecular Dynamics in Paramagnetic Materials as Studied by Magic-Angle Spinning 2H NMR Spectra
Motohiro Mizuno,* You Suzuki, Kazunaka Endo, Miwa Murakami, Masataka Tansho, and Tadashi Shimizu
A magic-angle spinning (MAS) 2H NMR experiment was applied to study the molecular motion in paramagnetic compounds. The temperature dependences of 2H MAS NMR spectra were measured for paramagnetic [M(H2O)6][SiF6] (M = Ni2+, Mn2+, Co2+) and diamagnetic [Zn(H2O)6][SiF6]. The paramagnetic compounds exhibited an asymmetric line shape in 2H MAS NMR spectra because of the electron-nuclear dipolar coupling. The drastic changes in the shape of spinning sideband patterns and in the line width of spinning sidebands due to the 180 flip of water molecules and the reorientation of [M(H2O)6]2+ about its C3 axis were observed. In the paramagnetic compounds, paramagnetic spin-spin relaxation and anisotropic g-factor result in additional linebroadening of each of the spinning sidebands. The spectral simulation of MAS 2H NMR, including the effects of paramagnetic shift and anisotropic spin-spin relaxation due to electron-nuclear dipolar coupling and anisotropic g-factor, was performed for several molecular motions. Information about molecular motions in the dynamic range of 102 s-1 k 108 s-1 can be obtained for the paramagnetic compounds from the analysis of 2H MAS NMR spectra when these paramagnetic effects are taken into account.

Carbon-13 Chemical Shift Tensors of Disaccharides: Measurement, Computation and Assignment
Limin Shao, Jonathan R. Yates, and Jeremy J. Titman*
A recently developed chemical shift anisotropy amplification solid-state nuclear magnetic resonance (NMR) experiment is applied to the measurement of the chemical shift tensors in three disaccharides: sucrose, maltose, and trehalose. The measured tensor principal values are compared with those calculated from first principles using density functional theory within the planewave-pseudopotential approach. In addition, a method of assigning poorly dispersed NMR spectra, based on comparing experimental and calculated shift anisotropies as well as isotropic shifts, is demonstrated.

Methanol Behviour in Direct Methanol Fuel Cells

Younkee Paik, Seong-Soo Kim, Oc Hee Han
Angwandte (2007)47, 94.
Methanol crossover and reaction intermediates are studied in a direct methanol fuel cell (DMFC) by means of 2D and 13C MAS NMR spectroscopy. A membrane electrode assembly (MEA) composed of three polymer electrolyte membrane (PEM) layers is used in the DMFC, and the middle PEM film is extracted to perform the NMR measurements.

Theory and Computation of Nuclear Magnetic Resonance Parameters
J. Vaara
The art of quantum chemical electronic structure calculation has over the last 15 years reached a point where systematic computational studies of magnetic response properties have become a routine procedure for molecular systems. One of their most prominent areas of application are the spectral parameters of nuclear magnetic resonance (NMR) spectroscopy, due to the immense importance of this experimental method in many scientific disciplines. This article attempts to give an overview on the theory and state-of-the-art of the practical computations in the field, in terms of the size of systems that can be treated, the accuracy that can be expected, and the various factors that would influence the agreement of even the most accurate imaginable electronic structure calculation with experiment. These factors include relativistic effects, thermal effects, as well as solvation/environmental influences, where my group has been active. The dependence of the NMR spectra on external magnetic and optical fields is also briefly touched on.

Structural characterization of an anhydrous polymorph of paclitaxel by solid-state NMR
Elizabeth M. Heider, James K. Harper and David M. Grant
The three-dimensional structure of a unique polymorph of the anticancer drug paclitaxel (Taxol®) is established using solid state NMR (SSNMR) tensor (13C & 15N) and heteronuclear correlation (1H–13C) data. The polymorph has two molecules per asymmetric unit (Z = 2) and is thus the first conformational characterization with Z > 1 established solely by SSNMR. Experimental data are correlated with structure through a series of computational models that extensively sample all conformations. For each computational model, corresponding tensor values are computed to supply comparisons with experimental information which, in turn, establishes paclitaxels structure. Heteronuclear correlation data at thirteen key positions provide shift assignments to the asymmetric unit for each comparison. The two distinct molecules of the asymmetric unit possess nearly identical baccatin III moieties with matching conformations of the C10 acetyl moiety and, specifically, the torsion angle formed by C30–O–C10–C9. Additionally, both are found to exhibit an extended conformation of the phenylisoserine sidechain at C13 with notable differences in the dihedral angles centered around the rotation axes of O–C13, C2–C1 and C3–C2.

Microscopic Li self-diffusion parameters in the lithiated anode material Li4+xTi5O12 (0 x 3) measured by 7Li solid state NMR
Martin Wilkening, Wojciech Iwaniak, Jessica Heine, Viktor Epp, Alexandra Kleinert, Malte Behrens, Gerhard Nuspl, Wolfgang Bensch and Paul Heitjans
The microscopic Li diffusion parameters in the lithiated spinel Li4+xTi5O12, which is on its way to become a commercially used anode material in Li ion batteries, are probed for the first time via nuclear magnetic resonance spectroscopy.

A high-field solid-state 35/37Cl NMR and quantum chemical investigation of the chlorine quadrupolar and chemical shift tensors in amino acid hydrochlorides
Rebecca P. Chapman and David L. Bryce
A series of six L-amino acid hydrochloride salts has been studied by 35/37Cl solid-state NMR spectroscopy (at 11.75 and 21.1 T) and complementary quantum chemical calculations. Analyses of NMR spectra acquired under static and magic-angle-spinning conditions for the six hydrochloride salts, those of aspartic acid, alanine, cysteine, histidine, methionine and threonine, allowed the extraction of information regarding the chlorine electric field gradient (EFG) and chemical shift tensors, including their relative orientation. Both tensors are found to be highly dependent on the local environment, with chlorine-35 quadrupolar coupling constants (CQ) ranging from –7.1 to 4.41 MHz and chemical shift tensor spans ranging from 60 to 100 ppm; the value of CQ for aspartic acid hydrochloride is the largest in magnitude observed to date for an organic hydrochloride salt. Quantum chemical calculations performed on cluster models of the chloride ion environment demonstrated agreement between experiment and theory, reproducing CQ to within 18%. In addition, the accuracy of the calculated values of the NMR parameters as a function of the quality of the input structure was explored. Selected X-ray structures were determined (L-Asp HCl; L-Thr HCl) or re-determined (L-Cys HCl·H2O) to demonstrate the benefits of having accurate crystal structures for calculations. The self-consistent charge field perturbation model was also employed and was found to improve the accuracy of calculated quadrupolar coupling constants, demonstrating the impact of the neighbouring ions on the EFG tensor of the central chloride ion. Taken together, the present work contributes to an improved understanding of the factors influencing 35/37Cl NMR interaction tensors in organic hydrochlorides.

No comments: