THEORETICAL AND SOLID-STATE NMR-STUDY OF ACETYLENE ADSORPTION ON NANO-SIZED MGO

We present a combined density functional theory, gauge-including atomi c orbital (GIAO), in situ C-13 MAS NMR, and in situ infrared study of acetylene adsorption on nano-sized MgO powder. Geometries were optimiz ed using the hybrid B3LYP exchange-correlation functional and triple-z eta basis sets. By utilizing models of the surface ranging in size fro m Mg4O4 to Mg8O8, we optimized geometries for a variety of surface-bou nd species on corner and edges sites. No stable dissociation products on the flat, (100)-like surface of an Mg12O12 cluster could be obtaine d theoretically, in agreement with a previous ultrahigh vacuum surface science experiment. We also present GIAO-restricted Hartree-Fock pred ictions of the C-13 chemical shifts of all species and their vibration al frequencies. The agreement between theory and experiment is outstan ding. Both the theoretical and experimental results strongly support t he formation of an acetylide on the MgO surface, while an alternative vinylidene species is not present. A minority species was also observe d, which both theory and experiment identify as an ethoxide. This work , the first combined theoretical and in situ NMR study of chemisorptio n on a (nonzeolitic) metal oxide surface, suggests that this approach will be very useful for the future study of catalytic phenomena on met al oxides.