AB-INITIO MOLECULAR-ORBITAL STUDY OF POTENTIAL-ENERGY SURFACE FOR THE REACTION OF C2H3 WITH H-2 AND RELATED REACTIONS

The potential energy surface of the reaction C2H3+H-2-->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). T-2.48. exp(-3587/T) cm(3) molecule(-1) s(-1), is in good agreement with the experimental estimates. C2H5 cannot be formed directly by inserting C2H3 to H-2, 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. (C) 1995 American Institute of Physics.