DEHYDROGENATION OF ETHENE BY TI- EXCITED-STATE EFFECTS ON THE MECHANISM FOR C-H BOND ACTIVATION FROM KINETIC-ENERGY RELEASE DISTRIBUTIONS( AND V+ )

The energetics and mechanism for dehydrogenation of ethene by early tr ansition metals were examined. Reaction of Ti+ and V+ led to collision complexes that decomposed by H-2 loss on the metastable time frame (5 -15 mu s). Kinetic energy release distributions (KERDs) for H-2 loss w ere measured. Loss of H-2 from Ti(C2H4)(+) exhibited a statistical KER D with an average release ((E) over bar(t)) of 0.10 eV. In contrast, V (C2H4)(+) gave a statistical release for H-2 loss at low source pressu res ((E) over bar(t) = 0.021 eV) and a strongly non-statistical releas e at high source pressures ((E) over bar(t) 0.70 eV). The two statisti cal releases were assigned to ground state Ti+(F-4) and V+(D-5) ions w hile the non-statistical release was assigned to complexes originating from the V+(F-3) excited state. Modeling the statistical KERDs using phase space theory yielded the bond energies, D-0(0)(Ti+-C2H2) = 51 +/ - 3 kcal/mol and D-0(0)(V+-C2H2) = 41 +/- 2 kcal/mol. Why we observe t wo very different KERDs in the V(C2H4)(+) system at differing source p ressures is discussed. The results give important information about th e details of the potential energy surfaces of the two systems.