Reactions of oxygen atoms with hydrocarbon radicals: a priori kinetic predictions for the CH3+O, C2H5+O, and C2H3+O reactions

The reactions of oxygen atoms with methyl, ethyl, and vinyl radicals are studied with a combination of ab initio quantum chemistry, variational transition state theory, and classical trajectory simulations. The interaction between the two radicals is examined with multi-reference configuration-interaction calculations employing augmented double zeta and augmented triple zeta basis sets. The implementation of analytic representations of the ab initio data within variable-reaction -coordinate (VRC) transition state theory (TST) yields predictions for the high-pressure limit addition rate coefficients. The dynamically corrected theoretical predictions for the CH3 + O, C2H5 + O, and C2H3 + O high-pressure rate coefficients are well reproduced by the expressions 9.20 x 10(-11) T-0.050 exp(136/RT), 5.26x 10(-11)T(0.032) exp 394/RT), and 1.71 x 10(-11)T(0.205) exp (427/RT) cm(3) molecule(-1) s(-1), respectively, where R = 1.987 cal mol(-1) K-1, for temperatures between 200 and 2500 K. For the CH3 + O reaction, these predictions are in remarkably good agreement with the extensive experimental data, while for the C2H5 + O and C2H3 + O reactions the theoretical predictions appear somewhat lower and higher, respectively, than the rather limited experimental data. VRC-TST analyses also suggest that the abstraction reactions to produce C2H4 + OH and C2H2 + OH have rate coefficients that are about 10% of the corresponding addition rate. Notably, the latter predictions have a significantly greater uncertainty, probably about a factor of 2, than do those for the addition. For comparison, the abstraction was experimentally observed to be about 21 +/- 8% of the total for C2H5 + O, but was not observed in C2H3 + O. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.