Monday, July 12, 2010

Magn. Reson. Chem. - July 2010

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

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

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

Received: 31 March 2010; Revised: 17 May 2010; Accepted: 20 May 2010
Digital Object Identifier (DOI)

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

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

Received: 26 March 2010; Revised: 20 May 2010; Accepted: 24 May 2010
Digital Object Identifier (DOI)

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

Received: 20 April 2010; Revised: 2 June 2010; Accepted: 4 June 2010
Digital Object Identifier (DOI)

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

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

Received: 31 March 2010; Revised: 27 May 2010; Accepted: 2 June 2010
Digital Object Identifier (DOI)

No comments: