Determination of Chemical Shift Anisotropy Lineshapes in a Two Dimensional Magic Angle Spinning NMR Experiment.

We describe a technique for measuring chemical shift anisotropy (CSA) lineshapes in NMR spectra of polycrystalline or amorphous solids. CSA lineshapes provide a means of studying molecular motion and molecular orientation, but they typically can not be measured in conventional one dimensional spectra because of the extensive overlap of lines from different inequivalent nuclei. By combining magic angle spinning (MAS) with a radiofrequency pulse sequence synchronized with the sample rotation in one time period of a two dimensional experiment, we obtain two dimensional spectra in which the CSA lineshapes appear along one axis and the normal MAS spectrum appears along the other axis. The CSA lines are thereby resolved as long as the inequivalent nuclei have resolved isotropic chemical shifts in the MAS spectrum. Our technique differs from previous, related techniques in that our CSA lineshapes are precisely the same as those obtained from non-spinning samples in the absence of spectral overlap; the analysis of the spectra is thus simplified substantially. We describe the theory and experimental implementation of the technique in detail, and present resolved sup 13 C CSA lineshapes for methyl-alpha-D-glucopyranoside.