Dehydrogenation of Ethene by Ti+ and V+:  Excited State Effects on the Mechanism for C−H Bond Activation from Kinetic Energy Release Distributions

• Published in: Journal of the American Chemical Society 1997-04, Vol.119 (17), p.3935-3941
• Main Authors: Gidden, Jennifer, van Koppen, Petra A. M, Bowers, Michael T
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja964377+

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 H2 loss on the metastable time frame (5−15 μs). Kinetic energy release distributions (KERDs) for H2 loss were measured. Loss of H2 from Ti(C2H4)+ exhibited a statistical KERD with an average release (Ē t) of 0.10 eV. In contrast, V(C2H4)+ gave a statistical release for H2 loss at low source pressures (Ē t = 0.021 eV) and a strongly non-statistical release at high source pressures (Ē t = 0.70 eV). The two statistical releases were assigned to ground state Ti+(4F) and V+(5D) ions while the non-statistical release was assigned to complexes originating from the V+(3F) excited state. Modeling the statistical KERDs using phase space theory yielded the bond energies, (Ti+−C2H2) = 51 ± 3 kcal/mol and (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.