Direct dynamics studies on the hydrogen abstraction reactions of an F atom with CH₃X (X) F, Cl, and Br)

The hydrogen abstraction reactions of F + CH₃F (R1), F + CH ₃Cl (R2), and F + CH₃Br (R3) are investigated by the dual-level direct dynamics method. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path (MEP) are obtained at the MP2/6-311G(d, p) level of theory, and then the energy profiles are refined at the CCSD(T)/6-311++G(3df, 2pd) level of theory. The basis set superposition error (BSSE) on the energy changes is corrected by means of the counterpoise method. Using the variational transition state theory (VTST) with the inclusion of the small-curvature tunneling correction, the rate constants are calculated over a wide temperature range of 189-2000 K. It is found that the activation energies for the title reactions are on the order of R1 > R2 > R3 and the rate constants exhibit just the opposite order of k₃ > k₂ > k₁. Both the activation energies and the rate constants show the clear-cut linear correlations with the hardness η of the halomethane molecules. Good agreement between the calculated and experimental rate constants is obtained at the measured temperatures. Furthermore, we hope that the theoretical studies for these compounds can give further information concerning the effects of halogen substitution on the rate constants of this class of hydrogen abstraction reactions.