Determination of the orientation of 29Si chemical shift tensors using rotorsynchronized MAS NMR of single crystals: forsterite (Mg2SiO4)

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.