Direct ab initio dynamics studies on hydrogen-abstraction reactions of 1, 1, 1-trifluoroethane with hydroxyl radical

By means of ab initio direct dynamic method, the hydrogen abstraction reaction OH + CH₃CF₃ → CH₂CF₃ + H₂O is investigated theoretically to reveal its dynamic properties. The optimized geometries and frequencies are calculated at the MP2/6-31 + G(d) level for the reactants, products and transition state. With the aid of intrinsic reaction coordinate theory, the minimum energy path (MEP) is obtained at the same level, and the energies along the MEP are further refined by performing the single-point calculations at the MP4/6-311G(2df,2pd) level. The calculated forward potential barrier and the reaction enthalpy at the MP4/6-311G(2df,2pd)//MP2 level are 4.6 and -10.4 kcal mol^-1, respectively. Furthermore, the reaction rate constants in the temperature range 200-1000 K are evaluated by the canonical variational transition state theory (CVT) incorporating the small-curvature tunneling correction (SCT). The theoretical rate constants of forward reaction are in good agreement with the recent experimental values over the measured temperature ranges. Our results indicate that the variational effect on the calculation of rate constants is small over the whole temperature range, and the tunneling correction is important in the lower temperature range.