Mechanistic and kinetic study of alkane activation by Ti⁺ and V⁺ in the gas phase. Lifetimes of reaction intermediates

Abstract

The reactions of Ti⁺ and V⁺ with several deuterium-labeled alkanes are studied by using an ion beam apparatus. The dominant reactions observed for both of these metal ions are single and double dehydrogenations. Alkane loss reactions are also observed for Ti⁺ but may be due to electronically excited states. The dehydrogenation mechanisms are investigated by using partially deuterated alkanes. The results are consistent with 1,2-eliminations for both V⁺ and Ti⁺, where deuterium scrambling may occur in the latter case. It is proposed that some 1,3-elimination of hydrogen also occurs in the reaction of Ti⁺+ with n-butane. Although the dehydrogenation reactions of V⁺ and Ti⁺+ appear to be similar to those of Ru⁺ and Rh⁺, there are some important differences in the reactivity of V⁺. Extensive adduct formation and large deuterium isotope effects are consistent with reaction intermediates which are relatively long-lived for V⁺ in comparison to Ti⁺, Ru⁺, and Rh⁺. Collisional stabilization studies are used to estimate dissociation rates of reaction intermediates formed when Ti⁺ and V⁺ interact with n-butane. The measured upper limits to the unimolecular decomposition rates are 1.47 x 10⁵ s⁻¹ and 1.23 x 10⁷ s⁻¹ for V⁺ and Ti⁺, respectively. Model RRKM calculations are able to reproduce these rates and provide an explanation of isotope effects observed when n-butane-d₁₀ is employed as the neutral reactant. The slower rate for V⁺ is suggested to arise from the inability of V⁺ to form two strong a bonds due to the 3d⁴ electronic configuration of the ground-state ion. This renders C-H bond insertion energetically much less favorable for V⁺ than for the other metal ions and limits the excitation energy of reaction intermediates.