Direct ab initio dynamics and theoretical rate constants studies for hydrogen-abstraction reactions of chlorine atoms with dibromomethane and tribromomethane

The dynamical properties for the two hydrogen-abstraction reactions of dibromomethane (CH₂Br₂) and tribromomethane (CHBr₃) with chlorine atoms are theoretically investigated. The optimized geometries and frequencies of the reactants, transition states, and products are calculated at the BH&H-LYP/6-311G(d,p) level. The minimum energy path is obtained by the intrinsic reaction coordinate method at the same level, and is further refined by single-point energy corrections at the QCISD(T)/6-311 + G(d,p) level. The thermal rate constants are also calculated using the improved canonical variational transition state theory with zero-curvature tunneling and small-curvature tunneling corrections within the temperature range 200-500 K. The theoretical rate constants are quantitatively in good agreement with available experimental values, yet the theoretical temperature dependence of rate constants is somewhat steeper than the experimental result. Our results indicate that the variational effect is large for the CHBr₃ reaction, whereas it is small for the CH₂Br₂ reaction. For both reactions, the tunneling effect is small.