Dehydrogenation of ethene by Ti+ and V+: Excited state effects on the mechanism for C-H bond activation from kinetic energy release distributions

The energetics and mechanism for dehydrogenation of ethene by early transition 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 were measured. Loss of H-2 from Ti(C2H4)(+) exhibited a statistical KERD 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 pressures ((E) over bar(t) = 0.021 eV) and a strongly non-statistical release at high source pressures ((E) over bar(t) 0.70 eV). The two statistical releases were assigned to ground state Ti+(F-4) and V+(D-5) ions while 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 two very different KERDs in the V(C2H4)(+) system at differing source pressures is discussed. The results give important information about the details of the potential energy surfaces of the two systems.