Theoretical study of the reactions of CF₃OCHF₂ with the hydroxyl radical and the chlorine atom

Theoretical investigations are carried out on the reaction mechanism of the reactions of CF₃OCHF₂ (HFOC-125) with the OH radicals and Cl atoms, as well as the heats of formation of CF₃OCHF₂ and CF₃OCF₂. The electronic structure information on the potentia energy surface for each reaction is obtained at the B3LYP/6-311G(d,p) level, and energetic information is further refined by G3(MP2) theory. The direct dynamics calculation of the hydrogen abstraction reactions are also performed at the G3(MP2)//B3LYP/ 6-311G(d,p) level. The classical energy profile is corrected by interpolated single-point energies (ISPE) approach, incorporating the small-curvature tunnelling effect (SCT) calculated by the variational transition-state theory (VTST). The rate constants are in good agreement with the experimental data and are found to be k₁ = 4.95 × 10^-30 T^S.40 exp(-347/T) and k₂= 1.86 × 10^-23 T^3.43-exp(-1579/T) cm³ molecule^-1s^-1 over the temperature range 200-2000 K. The rate constants at 298 K for these two reactions are 3.38 × 10 ^-16 and 2.80 × 10^-17 cm³ molecule ^-1 s^-1, respectively. Using group-balanced isodesmic reactions as working chemical reactions, the standard enthalpies of formation for CF₃OCHF₂ and CF₃OCF₂ are -312.3 ± 1.0 and -257.3 ± 1.0 kcal mol^-1, respectively, evaluated by G3(MP2) theory based on the B3LYP/6-311G(d,p) geometries. The theoretical studies provide rate constants of the title reactions and the enthalpies of the species, which are important parameters in determining the atmospheric lifetime and the feasible pathways for the loss of HFOC-T25.