Issue 31, 2007

Association rate constants for reactions between resonance-stabilized radicals: C3H3 + C3H3, C3H3 + C3H5, and C3H5 + C3H5

Abstract

Reactions between resonance-stabilized radicals play an important role in combustion chemistry. The theoretical prediction of rate coefficients and product distributions for such reactions is complicated by the fact that the initial complex-formation steps and some dissociation steps are barrierless. In this paper direct variable reaction coordinate transition state theory (VRC-TST) is used to predict accurately the association rate constants for the self and cross reactions of propargyl and allyl radicals. For each reaction, a set of multifaceted dividing surfaces is used to account for the multiple possible addition channels. Because of their resonant nature the geometric relaxation of the radicals is important. Here, the effect of this relaxation is explicitly calculated with the UB3LYP/cc-pvdz method for each mutual orientation encountered in the configurational integrals over the transition state dividing surfaces. The final energies are obtained from CASPT2/cc-pvdz calculations with all π-orbitals in the active space. Evaluations along the minimum energy path suggest that basis set corrections are negligible. The VRC-TST approach was also used to calculate the association rate constant and the corresponding number of states for the C6H5 + H → C6H6 exit channel of the C3H3 + C3H3 reaction, which is also barrierless. For this reaction, the interaction energies were evaluated with the CASPT2(2e,2o)/cc-pvdz method and a 1-D correction is included on the basis of CAS+1+2+QC/aug-cc-pvtz calculations for the CH3 + H reference system. For the C3H3 + C3H3 reaction, the VRC-TST results for the energy and angular momentum resolved numbers of states in the entrance channels and in the C6H5 + H exit channel are incorporated in a master equation simulation to determine the temperature and pressure dependence of the phenomenological rate coefficients. The rate constants for the C3H3 + C3H3 and C3H5 + C3H5 self-reactions compare favorably with the available experimental data. To our knowledge there are no experimental rate data for the C3H3 + C3H5 reaction.

Graphical abstract: Association rate constants for reactions between resonance-stabilized radicals: C3H3 + C3H3, C3H3 + C3H5, and C3H5 + C3H5

Article information

Article type
Paper
Submitted
05 Mar 2007
Accepted
13 Apr 2007
First published
18 May 2007

Phys. Chem. Chem. Phys., 2007,9, 4259-4268

Association rate constants for reactions between resonance-stabilized radicals: C3H3 + C3H3, C3H3 + C3H5, and C3H5 + C3H5

Y. Georgievskii, J. A. Miller and S. J. Klippenstein, Phys. Chem. Chem. Phys., 2007, 9, 4259 DOI: 10.1039/B703261G

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