Direct dynamics studies on hydrogen abstraction reactions of CH ₃CHFCH₃ and CH₃CH₂CH₂F with OH radicals

The hydrogen abstraction reactions of CH₃CHFCH₃ and CH₃CH₂CH₂F with the OH radicals have been studied theoretically by a dual-level direct dynamics method. The geometries and frequencies of all the stationary points are optimized by means of the DFT calculation. There are complexes at the reactant side or exit route, indicating these reactions may proceed via indirect mechanisms. To improve the reaction enthalpy and potential barrier of each reaction channel, the single point energy calculation is performed by the MC-QCISD/3 method. The rate constants are evaluated by canonical variational transition state theory (CVT) with the small-curvature tunneling correction method (SCT) over a wide temperature range 200-2000 K. The canculated CVT/SCT rate constants are consistent with available experimental data. The results show that both the variation effect and the SCT contribution play an important role in the calculation of the rate constants. For reactions CH₃-CHFCH₃ and CH₃CH ₂CH₂F with OH radicals, the channels of H-abstraction from -CHF- and -CH₂-groups are the major reaction channels, respectively, at lower temperature. Furthermore, to further reveal the thermodynamics properties, the enthalpies of formation of reactants CH₃CHFCH ₃, CH₃CH₂CH₂F, and the product radicals CH₃CFCH₃, CH₃CHFCH₂, CH₃CH₂CHF, CH₃CHCH₂F, and CH ₂CH₂CH₂F are studied using isodesmic reactions.