A THEORETICAL ANALYSIS OF THE CH3 _ H REACTION: ISOTOPE EFFECTS, THE HIGH-PRESSURE LIMIT, AND TRANSITION STATE RECROSSING

摘要

The reaction of methyl radicals with hydrogen atoms is studied with a combination of ab initio quantum chemistry, variational transition state theory, and classical trajectory simulations. The interaction between the two radicals, including the umbrella mode of the methyl radical, is examined at the CAS+1+2 level using an augmented correlation consistent polarized valence triple zeta basis set. The implementation of an analytic representation of the ab initio data within variable reaction coordinate transition state theory yields predictions for the zero-pressure limit isotopic exchange rate constants that are about 15% greater than the available experimental data. Trajectory simulations indicate that the transition state recrossing factor for the capture process is 0.90, essentially independent of temperature and isotope. The dynamically corrected theoretical prediction for the CH{sub}3+H high-pressure rate coefficient is well reproduced by the expression 1.32×10{sup}-10T{sup}0.153 exp(-15.1/RT) cm{sup}3 molecule{sup}-1 s{sup}-1, where R=1.987 cal mol{sup}-1 K{sup}-1, for temperatures between 200 and 2400 K. This prediction is in good agreement with the converted experimental data for all but the one measurement at 200 K. Calculations for the triplet abstraction channel suggest that it is unimportant. Methyl umbrella mode variations have surprisingly little effect on the predicted rate coefficients.