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 C2H3+H2→C2H4+H→C2H5 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⋅T2.48⋅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.