Theoretical Kinetic Estimates for the Recombination of Hydrogen Atoms with Propargyl and Allyl Radicals

Ab initio quantum chemical simulations were coupled with variational transition state theory in estimating rate constants for the H C3H3 and H C3H5 recombination reactions. The energy of interaction between the H atom and each of the radicals was evaluated at the CAS 1 2 level for the range of separations and relative orientations spanning the transition state region. An analytic representation of these interaction energies was then implemented in variable reaction coordinate transition state theory calculations of the high pressure limit recombination rate constant for temperatures ranging from 200 to 2000 K. For the propargyl reaction, the overall addition rate was separated into contributions correlating with the initial formation of allene and propyne. These theoretical results were compared with the available experimental data as well as with corresponding theoretical estimates for the H C2H3 and H C2H5 reactions. The H propargyl and H allyl total recombination rates were remarkably similar, with both being greater than the H vinyl and H ethyl rates, due to the presence of twice as many addition channels.