H 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 of¹H chemical shielding anisotropies from polycrystalline powders. By virtue of high-order dipolar decoupling, large spectral width, resonance offset stability, and the absence of artifacts fromtilted-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 of¹H 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.