Oxygen-17 cross-polarization nmr spectroscopy of inorganic solids

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 (Mg₃Si₄O₁₀(OH)₂), (C₆H₅)₃SiOH, and amorphous SiO₂) 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.