Reaction-path dynamics and theoretical rate constants for the CH _nF_4-n + O₃ → HOOO + CH_n-1F _4-n (n = 2,3) reactions

We present a theoretical study of the hydrogen abstraction reactions from CH₃F and CH₂F₂ by an ozone molecule. The geometries and harmonic vibrational frequencies of all stationary points are calculated at the MPW1K, BHandHLYP, and MPWB1K levels of theory. The energies of all of the stationary points were refined by using both higher-level (denoted as HL) energy calculations and QCISD(T)/6-311++G(2df,2pd) calculations based on the optimized geometries at the MPW1K/6-31+G(d,p) level of theory. The minimum energy paths (MEPs) were obtained by the MPW1KA5-31+G(d,p) level of theory. Energetic information of the points along the MEPs is further refined by the HL method. The rate constants were evaluated on the basis of the MEPs from the HL level of theory in the temperature range 200-2500 K by using the conventional transition-state theory (TST), the canonical variational transition-state theory (CVT), the microcanonical variational transition-state theory (μVT), the CVT coupled with the small-curvature tunneling (SCT) correction (CVT/SCT), and the μVT coupled with the Eckart tunneling correction (μVT/Eckart) based on the ab initio calculations. A general agreement was found among the TST, CVT, and μVT theories. The fitted three-parameter Arrhenius expressions of the calculated forward CVT/SCT and μVT/Eckart rate constants of the ozonolysis of fluoromethane are k^CVT/SCT(T) = 2.76 × 10^-34T ^5.81e^(-13975/T) and k^μVT/Eckart(T) = 1.15 × 10^-34T^5.97e^(-14530.7/T), respectively. The fitted three-parameter Arrhenius expressions of the calculated forward CVT/SCT and μVT/Eckart rate constants of the ozonolysis of difluoromethane are k^CVT/SCT(T) = 2.29 × 10^-36T^6.42e ^(-15451.6/T) and k^μVT/Eckart(T) = 1.31 × 10 ^-36T^6.45e^(-15465.8/T), respectively.