Oxidation properties of the early transition-metal dioxide cations MO2+ (M= Ti, V, Zr, Nb) in the gas-phase

Fourier-transform mass spectrometry is used to characterize the gas-phase r eactivities of metal dioxide cations MO2+ (M = Ti, V, Zr, Nb) towards simpl e hydrocarbons. Thus, II-atom abstraction from water and methane by TiO2+ a nd ZrO2+ demonstrate the radical-type reactivity of these species. In marke d contrast, VO2+ and NbO2+ behave as closed-shell species in that no radica l losses occur. These reactivity patterns are in accord with a description of the bonding schemes of the metal dioxides. B3LYP calculations predict si nglet ground states for VO2+ and NbO2+ and doublet ground states for TiO2and ZrO2+ the latter can be described as oxygen-centered radicals. In addit ion, the oxidations of ethene and ethane by vanadyl cation VO2+ are studied by ab initio theory. The oxidation of ethene by VO2+ is suggested to proce ed via a conceptually new mechanism which involves a direct oxygen transfer from the metal to the olefin to afford acetaldehyde complexed to VO+ as re action product. (C) 1999 Elsevier Science B.V.