H-1 CHEMICAL SHIELDING ANISOTROPIES FROM POLYCRYSTALLINE POWDERS USING MSHOT-3 BASED CRAMPS

It is demonstrated that combined rotation and multiple-pulse spectrosc opy (CRAMPS) based on MSHOT-3 homonuclear multiple-pulse decoupling re presents a powerful method for determination of H-1 chemical shielding anisotropies from polycrystalline powders. By virtue of high-order di polar decoupling, large spectral width, resonance offset stability, an d the absence of artifacts from tilted-axis precession, MSHOT-3-based CRAMPS enables straightforward sampling of high-quality spectra. Compa rison with explicit calculations, taking the effect of the multiple-pu lse sequence into account, shows that the spectra may be simulated and iteratively fitted using standard software for the calculation of mag ic-angle spinning spectra influenced by chemical shielding anisotropy with the shielding interaction reduced by the scaling factor of the MS HOT-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 d etermined previously from single-crystal NMR studies. (C) 1998 Academi c Press.