1H chemical shielding anisotropies from polycrystalline powders using MSHOT-3 based CRAMPS

It is demonstrated that combined rotation and multiple-pulse spectroscopy (CRAMPS) based on MSHOT-3 homonuclear multiple-pulse decoupling represents a powerful method for determination of H-1 chemical shielding anisotropies from polycrystalline powders. By virtue of high-order dipolar decoupling, large spectral width, resonance offset stability, and the absence of artifacts from tilted-axis precession, MSHOT-3-based CRAMPS enables straightforward sampling of high-quality spectra. Comparison with explicit calculations, taking the effect of the multiple-pulse sequence into account, shows that the spectra may be simulated and iteratively fitted using standard software for the calculation of magic-angle spinning spectra influenced by chemical shielding anisotropy with the shielding interaction reduced by the scaling factor of the MSHOT-3 decoupling sequence. The method is demonstrated by experimental determination of H-1 chemical shielding anisotropies for adipic acid, Ca(OH)(2), malonic acid, and KHSO4. The data are compared with those determined previously from single-crystal NMR studies. (C) 1998 Academic Press.