Density functional theory and ab initio direct dynamics studies on the hydrogen abstraction reactions of chlorine atoms with CHCl_3-nF_n (n = 0, 1, and 2) and CH₂Cl₂

The dynamical properties of the hydrogen abstraction reactions from halomethanes of CHCl_3-nF_n (n = 0-2) and CH₂Cl₂ with Cl atoms in the temperature range 200-700 K are investigated theoretically. The minimum energy paths (MEPs) of these 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 title reactions, the theoretical rate constants by using improved canonical variational transition state theory incorporating small-curvature tunneling correction are in good agreement with available experimental results. It is shown that the vibrational adiabatic potential energy curves for these reactions have two barriers, a situation similar to the analogous CH₃X + Cl (X = F, Cl, Br) reaction. For these reactions, the small-curvature tunneling effects are found to be small and the variational effects except for the CHCl₃ + Cl reaction are found to be small over the temperature range considered.