Theoretical study of the hydrogen abstraction reactions for CH₃R + Cl → CH₂R + HCl (R = Cl and Br)

The dynamical properties of the hydrogen abstraction reactions of chloromethane (CH₃Cl) and bromomethane (CH₃Br) with chlorine atoms in the temperature range 200-800 K are investigated theoretically. The minimum energy paths (MEPs) of both reactions are calculated at the BH&H-LYP/6-311G(d,p) level, and the energies along the MEPs are further refined at the QCISD(T)/6-311 + G(d,p) (single-point) level. For the CH₃Cl + Cl reaction, the theoretical rate constants are in good agreement with available experimental results, yet the Arrhenius slope is somewhat steeper. Compared with one previous theoretical investigation at a lower level, our calculated rate constants are closer to the experimental values. For the CH₃Br + Cl reaction that has no previous theoretical comparison, our calculated rate constants are in reasonable agreement with experiment, although again the Arrhenius slope is steeper than the experimental one. It is shown that the vibrational adiabatic potential energy curves for both reactions have two barriers, a situation similar to the analogous CH₃F + Cl reaction. For both reactions, the variational and small-curvature tunneling effects are found to be small over the temperature range considered.