Determination Of Chemical Shift Tensors Research Articles

Orientational order in liquid crystals by combiningH2andC13nuclear magnetic resonance spectroscopy and density functional theory calculations

Structural and orientational order properties of the liquid crystal 4,4'-bis-heptyl-azoxybenzene (HAB) have been obtained in its nematic and smectic- A phases by simultaneously analyzing several observables extracted from 2H and 13C nuclear magnetic resonance (NMR) spectra, i.e., 2H quadrupolar, 2H-1H and 13C-2H dipolar couplings, as well as 13C chemical shift anisotropy. 13C experiments required the application of high-resolution solid-state NMR techniques like 1H high-power decoupling and cross polarization, as well as the independent determination of chemical shift tensors by means of density functional theory (DFT) calculations, here performed taking into account the effect of the anisotropic medium by the polarizable continuum model method. The approach, consisting in the simultaneous analysis of all the 2H and 13C experimental data to derive orientational order parameters, and in the use of geometrical parameters determined by DFT methods, allows more detailed and reliable results to be obtained with respect to the traditional approach based on the sole analysis of 2H experiments.

Determination of the orientation of 29 Si chemical shift tensors using rotorsynchronized MAS NMR of single crystals: forsterite (Mg 2 SiO 4 )

A novel approach is presented to determine both the main axis values and the orientation of 29Si chemical shift tensors using slow rotation of single crystals at the magic angle (MAS). Provided that the MAS frequency is less than the chemical shift anisotropy and that the radiofrequency (r.f.) pulse excitation is rotorsynchronized the single crystal MAS spectra consist of a mixture of absorptive and dispersive line shape contributions to each MAS sideband. Changing systematically the timing of the r.f. excitation with respect to the rotor position a set of spectra is obtained which allows a precise determination of the chemical shift tensors and their orientations with respect to the crystal axis system despite MAS. This method offers both large resolution enhancement and a considerable time saving, in comparison to the traditional determination of chemical shift tensors, where the angular dependence of the resonance frequency at three orthogonal crystal orientations is measured. Both methods are compared using forsterite as test sample.