Reactions of Ground-State Ti+ and V+ with Propane:  Factors That Govern C−H and C−C Bond Cleavage Product Branching Ratios

Reactions of Ti+ and V+ with C3H8, CH3CD2CH3, CD3CH2CD3, and C3D8 are studied to characterize the rate-limiting transition states and determine the factors that control the branching between H2 and CH4 elimination. For ground-state Ti+ reacting with propane, dehydrogenation and demethanation both occur at thermal energy with reaction efficiencies of 17% and less than 1%, respectively. For ground-state V+, dehydrogenation occurs at thermal energy with an efficiency of less than 1% whereas demethanation occurs with a 0.70 ± 0.06 eV threshold. Deuterium-labeling studies indicate that β-H(D) transfer to form the metal ethene dihydride complex or a multicenter elimination of H2 is the rate-limiting step for dehydrogenation, while reductive elimination of methane is shown to be rate limiting for demethanation. The product kinetic energy release distributions (KERDs) for H2 loss from Ti+(C3H8) and V+(C3H8) are both statistical. Modeling the experimental KERDs using statistical phase space theory yields D 0°(Ti+−C3H6) = 34.5 ± 3 kcal/mol and D 0°(V+−C3H6) = 30.7 ± 2 kcal/mol. To explain differences in the reactivity of Ti+ and V+, the potential energy surfaces of the reactions are discussed in some detail with an emphasis on the importance of spin−orbit-coupled crossings between surfaces of different spin.