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.