Kinetic studies of the reactions of hydroxyl (OH) radicals with hydrogen-containing chlorofluorocarbons (HCFC) over an extended temperature range using a laser photolysis/laser-induced fluorescence technique

Rate coefficients for the reaction of selected hydrogen-containing chlorofluorcarbon (HCFC) and hydroxyl (OH) radicals were systematically investigated using the laser photolysis/laser-induced fluorescence technique. The selected HCFC compounds included CHFCl{dollar}sb2{dollar} (k{dollar}sb1{dollar}), CHF{dollar}sb2{dollar}Cl (k{dollar}sb2{dollar}), CH{dollar}sb3{dollar}CF{dollar}sb2{dollar}Cl (k3), CH{dollar}sb2{dollar}ClCF{dollar}sb3{dollar} (k{dollar}sb4{dollar}), CH{dollar}sb2{dollar}ClCF{dollar}sb2{dollar}Cl (k{dollar}sb5{dollar}), CHCl{dollar}sb2{dollar}CF{dollar}sb3{dollar} (k{dollar}sb6{dollar}) and CHFClCF{dollar}sb3{dollar} (k{dollar}sb7{dollar}). Measurements were performed under slow flow conditions at a total pressure of {dollar}74pm10{dollar} torr. The rate coefficients increased dramatically over a wide temperature range, and were best described using the modified Arrhenius expression in the form of k(T)=AT{dollar}rmsp{lcub}B{rcub}{dollar}exp(-C/T). Two approaches were used to describe these modified Arrhenius expressions. One was a semi-empirical expression and the other was a purely empirical expression. Both approaches were used to fit our experimental data from 295 K to 900 K. We found the semi-empirical expression was superior to the purely empirical expression for k{dollar}sb1{dollar}, k{dollar}sb2{dollar}, k{dollar}sb3{dollar}, k{dollar}sb4{dollar}, k{dollar}sb5{dollar}, and k{dollar}sb6{dollar}. However, the purely empirical expression was found to be better than the semi-empirical expression for k{dollar}sb7{dollar}.; For the semi-empirical approach,'A' and 'B' were calculated from Benson's transition state theory (TST) model with the assistance of ab initio molecular orbital theory calculations at the level of MP2/6-31G(d). 'C' was obtained from a nonlinear least squares fit to the experimental data. The semi-empirical TST-based modified Arrhenius expressions were:{dollar}{dollar}eqalign{lcub}rm ksb1 (T)&=rm(3.13pm 0.20)sp* 10sp{lcub}-18{rcub}Tsp{lcub}(1.93pm 0.01){rcub}explbrack -(552pm 18)/Trbrackcr rm ksb2(T)&=rm (3.10pm 0.20)sp* 10sp{lcub}-18{rcub}Tsp{lcub}(1.94pm 0.01){rcub}explbrack -(1112pm 26)/Trbrackcr rm ksb3 (T)&=rm (1.23pm 0.32)sp* 10sp{lcub}-17{rcub}Tsp{lcub}(1.86pm 0.05){rcub}explbrack-(1493pm 128)/Trbrackcr rm ksb4 (T)&=rm(3.05pm 2.01)sp* 10sp{lcub}-18{rcub}Tsp{lcub}(1.91pm 0.02){rcub}explbrack -(644pm 157)/Trbrackcr rm ksb5 (T)&=rm(8.53pm2.03)sp*10sp{lcub}-19{rcub}Tsp{lcub}(2.28pm 0.09){rcub}explbrack -(937pm 148)/Trbrackcrrm ksb6 (T)&=rm(1.40pm0.35)sp*10sp{lcub}-17{rcub}Tsp{lcub}(1.7pm 0.04){rcub}explbrack-(543pm 73)/Trbrackcr{rcub}{dollar}{dollar}in units of cm{dollar}sp3{dollar}molecule{dollar}sp{lcub}-1{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar}. For the reaction of k{dollar}sb7{dollar}, a purely empirical fit was:{dollar}{dollar}rm ksb7 (T)=(1.07pm 4.64)sp* 10sp{lcub}-18{rcub}Tsp{lcub}(1.98pm 0.61){rcub}explbrack -(600pm 274)/Trbrackcr{dollar}{dollar}in units of cm{dollar}sp3{dollar}molecule{dollar}sp{lcub}-1{rcub}{dollar}s{dollar}sp{lcub}-1{rcub}{dollar}. The agreement between the modified Arrhenius expressions and experiments was better than 85% for all reactions.; This systematic investigation has led to a new methodology which is an alternative to Cohen's TST calculation to predict rate coefficients at stratospheric temperatures (200 K) and combustion temperatures (2000 K). This approach has revealed theoretically-based Arrhenius parameters in terms of the transition state thermochemical properties of each reaction. The transition state thermochemical properties of each reaction were then compared to the ab initio molecular orbital theory calculations at the levels of HF/6-31G(d) and MP2/6-31G(d). The uncertainty in {dollar}Deltarm Ssp{lcub}not={rcub}{dollar} was less than 2 gibbs/mole for all reactions.