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 (η3-C3H5)V(O)(OH)+. In the case of n-butane/VO2+, a combined loss of H2 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.