Oxygen-17 cross-polarization nmr spectroscopy of inorganic solids

Abstract

We have obtained 17O nuclear magnetic resonance spectra of a variety of 17O-labeled solids (Mg(OH) 2, Ca(OH) 2, boehmite (AlO(OH)), talc (Mg 3Si 4O 10(OH) 2), (C 6H 5) 3SiOH, and amorphous SiO 2) using high-field static and “magic-angle” sample spinning techniques, together with 1H cross polarization and dipolar decoupling. Our results show that large cross-polarization enhancements can be obtained and that reliable second-order quadrupolar powder lineshapes can be observed under cross-polarization conditions. We have also investigated the dynamics of cross polarization for several samples, including measurements of cross-relaxation rates and 1H and 17O rotating-frame spin-lattice relaxation times. We show that rapid 17O rotating-frame spin-lattice relaxation reduces the cross-polarization enhancement in some cases and that differences in cross-relaxation rates can be used to “edit” spectra by selectively enhancing protonated oxygen resonances (in general, hydroxide versus oxide ions, in inorganic solids). When applied to high surface area metal oxides such as amorphous silica, this selectivity enables the observation of resonances from surface hydroxyl groups that are difficult to detect by conventional 17O NMR. Overall, the cross-polarization approach appears to have considerable utility for aiding in the interpretation of 17O NMR spectra of complex inorganic solids.