Single and Multiple Lewis Sites of MgO: A Combined IR and ab Initio Study with CD3CN as a Molecular Probe

CD 3 CN is used for the first time for characterizing Lewis sites of MgO ex-hydroxide in a combined IR and ab initio study. Four ν(CN) bands of coordinated CD 3 CN species are observed at 2317, 2302, 2293, and 2280 cm -1 at low coverages. The 2302 and 2293 cm -1 bands are attributed to CD 3 CN on Mg 3c and Mg 4c sites at the intersections of regular MgO faces of the (001) type. The 2317 and 2280 cm -1 bands are assigned to CD 3 CN stabilized by two neighboring Mg 4c sites on (110) defect faces and by two neighboring Mg 3c sites at the intersections of the (001) and (110) faces, respectively. This assignment is supported by the theoretical reproduction of the experimental Δν(CN) shifts to an accuracy of a few cm -1 . The only other detected ν(CN) band, which appears at high coverages, is the ν(CN) = 2261 cm -1 band of liquid CD 3 CN. This fact and results of frequency and energy analyses suggest the energy of CD 3 CN adsorption on Mg 5c sites of the (001) faces to be smaller than the energy of CD 3 CN liquefaction.