Kinetics and Thermochemistry of the OH Radical Reaction with CF₃CCl₂H and CF₃CFClH

Abstract

Rate coefficients are reported for the gas-phase reaction of hydroxyl (OH) radicals with CF 3 CCl 2 H ( k 1 ) and CF 3 CFClH ( k 2 ) over an extended temperature range. The measurements were performed using a laser photolysis/laser-induced fluorescence (LP/LIF) technique under slow flow conditions at a total pressure of 740 ± 10 Torr. The lower temperature measurements for k 1 and k 2 were in agreement with previous measurements using different techniques. Arrhenius plots of the data exhibit significant curvature and were best described by the following modified Arrhenius expressions (cm 3 molecule -1 s -1, 2σ error limits): k 1 (296−866 K) = (2.20 ± 0.25) × 10 -19 T 2.26±0.10 exp(−226 ± 51)/ T and k 2 (297−867 K) = (7.72 ± 0.60) × 10 -20 T 2.35±0.06 exp(−458 ± 30)/ T . The experimental measurements were analyzed within the context of transition state theory to provide accurate modified Arrhenius expressions that are applicable to flame conditions. Ab initio calculations were used to evaluate the thermochemical properties of the reactants and products and the partition functions of reactants and their activated complexes. Hydrogen bond dissociation energies (BDE) were also estimated based on calculated enthalpies of formation of reactants and products. An asymmetrical Eckart potential was performed to account for tunneling correction. The resulting TST-based modified Arrhenius expressions (cm 3 molecule -1 s -1 ) were k 1 (250−2000 K) = 3.25 × 10 -21 T 2.88 exp(−52/ T ) and k 2 (250−2000 K) = 4.52 × 10 -22 T 3.18 exp(−362/ T ). The TST-based modified Arrhenius expression is compared to previous TST and SAR predictions. The results of this study indicate that F substitution at the reaction site has a significant (reductive) effect on hydrochlorofluorocarbon (HCFC) reactivity. This result was also observed in our prior studies of CHFCl 2 and CHF 2 Cl. 4 A similar effect in HCFC reactivity was not observed for changes in F substitution β to the reaction site, 6 indicating that the electron-withdrawing effects of the F atoms are limited to the vicinity of the reaction site.