Hydrolysis on transition metal oxide clusters and the stabilities of M-O-Mbridges

Water addition to molecular single, double and triple M-O-M bridges (M = Sc , Ti, V, Cc, and Mn) were considered, and the stabilities toward stepwise h ydrolysis of the oxygen bridges were studied by means of quantum chemistry. The M-O bond distances for the studied systems were compared to experiment for demonstration of the applicability of the B3LYP functional to the inve stigated systems. While substantial exothermicities were found for the hydr olysis of double and triple M-O-M bridges, addition of water to a single br idge was generally found to be slightly endothermic. The lack of enthalpy d rive for the (OH)(y)OxM-O-MOx(OH)(y) + H2O --> 2MO(x-1)(OH)(y+2) reaction w as taken to suggest that entropy increase and the formation of mononuclear water complexe, would be decisive factors for the dissociation. A mechanism was proposed for the observed erosion of the protective chromium oxide sca le on high-temperature alloys at elevated temperatures and high humidities, based on the formation of CrO2(OH)(2)(g).