REACTIONS OF GROUND-STATE TI- FACTORS THAT GOVERN C-H AND C-C BOND-CLEAVAGE PRODUCT BRANCHING RATIOS( AND V+ WITH PROPANE )

Reactions of Ti+ and V+ with C3H8, CH3CD2CH3, CD3CH2CD3, and C3D8 are studied to characterize the rate-limiting transition states and determ ine the factors that control the branching between H-2 and CH4 elimina tion. 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+, dehydrogen ation occurs at thermal energy with an efficiency of less than 1% wher eas demethanation occurs with a 0.70 +/- 0.06 eV threshold. Deuterium- labeling studies indicate that beta-H(D) transfer to form the metal et hene dihydride complex or a multicenter elimination of H-2 is the rate -limiting step for dehydrogenation, while reductive elimination of met hane is shown to be rate limiting for demethanation. The product kinet ic energy release distributions (KERDs) for H-2 loss from Ti+(C3H8) an d V+(C3H8) are both statistical. Modeling the experimental KERDs using statistical phase space theory yields D(0)degrees(Ti+-C3H6) = 34.5 +/ - 3 kcal/mol and D(0)degrees(V+-C3H6) = 30.7 +/- 2 kcal/mol. To explai n differences in the reactivity of Ti+ and V+, the potential energy su rfaces of the reactions are discussed in some detail with an emphasis on the importance of spin-orbit-coupled crossings between surfaces of different spin.