Ab initio calculation of the transition-state properties and addition rate constants for H + C₂H₂ and selected isotopic analogues

We present GVB-POL-CI ab initio calculations of the geometries, energetics, and normal mode frequencies of C₂H₂, C₂H₃, and the transition state for the addition reaction of H + C₂H₂. In addition, normal mode frequencies for the isotopic variants D + C₂D₂, D + C₂H₂, and H + C₂D₂ are presented. These results are compared to experimental values for C₂H₂ and to ab initio values of Nagase and Kern, and semiempirical values of Keil, Lynch, Cowfer, and Michael. The results are also used to calculate the apparent bimolecular addition rate constant using conventional RRKM theory for chemical activation. The calculated rate constants and their isotopic variants are compared as a function of temperature and pressure to available experimental information. The agreement is little different from that obtained by Keil et al. with a similar calculation using semiempirical values for acetylene, transition-state, and vinyl radical properties. In particular, the calculated high-pressure limit of the rate constant appears to be at least 1 order of magnitude higher than the experimental limit Several possible reasons for this discrepancy are discussed.