Direct ab initio dynamics calculations of the rate constants for the reaction of CHF₂CF₂OCH₃ with Cl

A dual-level direct dynamics method is employed to reveal the dynamical properties of the reaction of CHF₂CF₂OCH₃ (HFE-254pc) with Cl atoms. The optimized geometries and frequencies of the stationary points and the minimum energy path (MEP) are calculated at the B3LYP/6-311G(d,p) level by using GAUSSIAN 98 program package, and energetic information is further refined by the G3(MP2) method. Two H-abstraction channels have been identified. For the reactant CHF₂CF₂OCH ₃ and the two products, CHF₂CF₂OCH₂ and CF₂CF₂OCH₃, the standard enthalpies of formation are evaluated with the values of -256.71 ± 0.88, -207.79 ± 0.12, and -233.43 ± 0.88 kcal/mol, respectively, via group-balanced isodesmic reactions. The rate constants of the two reaction channels are evaluated by means of canonical variational transition-state theory (CVT) including the small-curvature tunneling (SCT) correction over a wide range of temperature from 200 to 2000 K. The calculated rate constants agree well with the experimental data, and the Arrhenius expressions for the title reaction are fitted and can be expressed as k₁ = 9.22 × 10 ^-19 T^2.06 exp(219/T), k₂ = 4.45 × 10 ^-14 T^0.90 exp(-2220/T), and k = 4.71 × 10 ^-22 T^3.20) exp(543/T) cm³ molecule^-1 s^-1. Our results indicate that H-abstraction from -CH₃ group is the main reaction pathway in the lower temperature range, while H-abstraction from -CHF₂ group becomes more competitive in the higher temperature range.