Ab initio molecular orbital study of potential energy surface for the reaction of C₂H₃ with H₂ and related reactions

The potential energy surface of the reaction C₂H ₃+H₂→C₂H₄+H→C ₂H₅ has been investigated using various theoretical methods including QCISD(T), CCSD(T), RCCSD(T), Gaussian-2 (G2), and the density-functional B3LYP approach. The reaction of the vinyl radical with molecular hydrogen is shown to take place through the hydrogen atom abstraction channel leading to the formation of C₂H₄+H with the activation energy of 10.4 kcal/mol at all the G2, QCISD(T)/ 6-311+G(3df, 2p), and CCSD(T)/6-311+G(3df, 2p) levels. The rate constant, calculated using the variational transition state theory with tunneling correction, k=3.68·10^-20·T^2.48·exp(-3587/T) cm³ molecule^-1 s^-1, is in good agreement with the experimental estimates. C₂H₅ cannot be formed directly by inserting C₂H₃ to H₂, but can only be produced by addition of H to C₂H₄, with a barrier of 4.5-4.7 kcal/mol calculated at high levels of theory. In order to match the experimental rate constant, the activation energy needs to be adjusted to 2.8 kcal/mol. Generally, the B3LYP method is found to predict well the geometries and vibrational frequencies of various species. However, it is less reliable for energy calculations than the QCISD(T) and CCSD(T) methods.