Theoretical study of the hydrogen abstraction by chlorine atoms for CH₂BrCl and CHBrCl₂

The dynamical properties of the hydrogen abstraction reactions by chlorine atoms for bromochloromethane (CH₂BrCl) and bromodichloromethane (CHBrCl₂) are investigated theoretically. The optimized geometries and frequencies of the reactants, transition states, and products are calculated at the BH&H-LYP/6-311 + G(d,p) level. The minimum energy path is obtained by the intrinsic reaction coordinate method at the same level, and the energies along the MEPs are further refined at the QCISD(T)/6-311 + G(d,p) (single-point) level. It is shown that the vibrational adiabatic potential energy curves for both reactions have two barriers. The rate constants are calculated using the improved canonical variational transition state theory with small-curvature tunneling correction within the temperature range 200-500 K. The theoretical rate constants are quantitatively in good agreement with available experimental values, but the theoretical temperature dependence of the rate constants is somewhat steeper. Our results indicate that the variational effect is large for the Cl + CHBrCl₂ reaction, whereas it is small for the Cl + CH₂BrCl reaction. For both reactions, the tunneling effect is small. Our results give further understanding of the mechanism of hydrogen abstraction reactions.