Hydrogen abstraction reactions of OH radicals with CF2ClCClXH (X = F, Cl) and CFCl2CClXH (X = F, Cl): a mechanistic and kinetic study

Abstract

The reactions of OH (OD) radicals with CF2ClCClFH (R1), CF2ClCCl2H (R2), CFCl2CClFH (R3), and CFCl2CCl2H (R4) have been investigated theoretically by a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the MPW1K/6-311+G(d,p) level. To improve the reaction enthalpy and potential barrier of each reaction channel, the single-point energy calculation is made by the MC-QCISD method. The enthalpies of formation of the species CF2ClCClFH, CF2ClCCl2H, CFCl2CClFH, CFCl2CCl2H, CF2ClCClF, CF2ClCCl2, CFCl2CClF, and CFCl2CCl2 are evaluated by two sets of isodesmic reactions. Using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) method, the rate constants of OH and OD radicals with CF2ClCClXH (X = F, Cl) and CFCl2CClXH (X = F, Cl) are evaluated over a wide temperature range of 100–2,000 K at the MC-QCISD//MPW1K/6-311+G(d,p) level. The calculated CVT/SCT rate constants are consistent with available experimental data. The results show that the tunneling correction has an important contribution in the calculation of rate constants at lower temperatures. For the above-mentioned four reactions, the kinetic isotope effects are also calculated. Finally, the effect of fluorine or chlorine substitution on reactivity of the C–H bond is discussed.