Ab initio direct dynamics studies on the reactions of H atoms with CH3Cl and CH3Br

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The multiple channel reactions (1) H+CH3Cl→products and (2) H+CH3Br→products have been studied by ab initio direct dynamics method. The potential-energy surface information is calculated at the BH&H-LYP/6-311G(d,p) level of theory. Energies along the minimum energy paths are further improved by single-point energy calculations at the PMP4(SDTQ)/6-311+G(3df,2p) level of theory. For the two reactions, each with two reaction channels, hydrogen atom abstraction and halogen atom abstraction have been identified. The rate constants for each reaction channel are calculated by using improved canonical variational transition state theory incorporating the small-curvature tunneling correction in the temperature range 200–3000 K. The theoretical total rate constants, which are calculated from the sum of the individual rate constants, are in good agreement with the experimental data. For reaction (1), H-abstraction reaction will be preferred in the whole temperature range. For reaction (2), Br-abstraction reaction is the major channel, while as the temperature increases, the contribution of the H-abstraction reaction channel should be taken into account.