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Solid-state (2)H-NMR studies of the structure and dynamics of hydroxyl groups on silica

Abstract

Solid-state, wide-line, $\sp2$H Nuclear Magnetic Resonance spectroscopy was used to study the structure and dynamics of exchange of deuterated hydroxyl groups on the surface of silica gel. Spectra were obtained from three different kinds of silica gel, and detailed studies were carried out using Aldrich 70-230 mesh, high-purity silica. In ambient temperature relaxation experiments, the longitudinal relaxation time, T$\sb1$ was determined to be 70 $\pm$ 30 msec. The transverse relaxation time, T$\sb2$ was found to be inhomogeneous, with minimum and maximum values of 50 $\mu$sec and 325 $\mu$sec, respectively. By means of a quadrupolar echo modification of a Carr-Purcell-Meibloom-Gill (CPMG) spin echo experiment, it was determined that deuterium nuclei migrate slowly across the silica surface and that the inherent spectral line width of the spin packets was less than 1 KHz. At 72 K, T$\sb1$ was estimated to be 0.1-0.5 seconds, and CPMG experiments indicated that some motion still existed on the surface. In experiments at 72 K and 92 K, NMR spectra were collected from samples of silica degassed under vacuum at various temperatures. Using the technique of spectral subtraction to isolate individual spin species, we compared the experimental data with simulated spectra. We found that at least four deuterium spin species exist on the silica surface; one of these is almost certainly bound D$\sb2$O, and the others are deuterated hydroxyl groups, which were discriminated by their quadrupolar coupling constants, asymmetry parameters, and exchange energy.

Subject Area

Chemistry|Molecules|Analytical chemistry

Recommended Citation

O'Donnell, John Michael, "Solid-state (2)H-NMR studies of the structure and dynamics of hydroxyl groups on silica" (1998). Doctoral Dissertations Available from Proquest. AAI9823762.
https://scholarworks.umass.edu/dissertations/AAI9823762

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