Alkane oxidation by VO2+ in the gas phase:: A unique dependence of reactivity on the chain length

The oxidation of small alkanes by the vanadium oxide cation VO2+ in the gas phase has been studied by Fourier transform ion-cyclotron resonance mass spectrometry (FTICR-MS). Sophisticated mass spectrometric experiments in conjunction with investigation of deuterium-labeled substances are used to elucidate the mechanistic details of the reactions. In marked contrast to oxidative dehydrogenation followed by elimination of ethene in the reaction of VO2+ with ethane, the corresponding reaction with propane mainly affords elimination of dihydrogen concomitant with an ionic product assigned to the allyl complex (eta(3)-C3H5)V(O)(OH)(+). In the case of n-butane/VO2+, a combined loss of H-2 and H2O provides yet another product channel. Branching ratios, reaction rates, and some mechanistic aspects of the oxidation of propane, n-butane, and isobutane by VO2+ are reported. The experiments are complemented by a computational study of the potential energy surface of propane/VO2+ by means of density functional theory.