Ab initio and RRKM studies of the reactions of C, CH, and ¹CH₂ with acetylene

We present ab initio calculations of geometries, energies, and normal mode frequencies for complexes and saddle points along the minimum energy reaction path for the reaction C + HCCH → C₃H₂ → C₃H + H. We also present ab initio calculations along the minimum energy reaction path in the entrance channel for the reactions C + HCCH, CH + HCCH, and ¹CH₂ + HCCH. These results and those presented earlier by Walch¹ are used to calculate rate constants for the reactions of C, CH, and ¹CH₂ with acetylene, using variational RRKM theory. The rate constants obtained agree well with experimental results for all three reactions. Unimolecular lifetimes for intermediate complexes associated with each reaction path are also presented. Among the more stable C₃ isomers are propargyl and propargylene, which have lifetimes of 25 and 1900 ps, respectively, under thermal reaction conditions.