Theoretical kinetic estimates for the recombination of hydrogen atoms with propargyl and allyl radicals

Abstract

Ab initio quantum chemical simulations are coupled with variational transition state theory in estimating rate constants for the H+C{sub 3}H{sub 3} and H+C{sub 3}H{sub 5} recombination reactions. The energy of interaction between the H atom and each of the radicals is 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 is 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 is separated into contributions correlating with the initial formation of allene and propyne. These theoretical results are compared with the available experimental data as well as with corresponding theoretical estimates for the H+C{sub 2}H{sub 3} and H+C{sub 2}H{sub 5} reactions. The H+propargyl and H+allyl total recombination rates are remarkably similar, with both being greater than the H+vinyl and H+ethyl rates, due to the presence of twice as many addition channels.