Kinetics of the Association Reactions of NO with the Series of Chlorofluoromethyl Radicals CFxCl3-x

Article abstract of DOI:10.1021/j100007a027

The kinetics of the reactions CF2Cl+NO+M->CF2ClNO+M and CFCl2+NO+M->CFCl2NO+M have been investigated using pulsed laser photolysis/time-resolved mass spectrometry in the pressure range 1-12 Torr and the temperatures of 233, 298, 333 K, using N2 as the buffer gas.The results, along with those obtained in preceding works for the homologous reactions CF3+NO+M->CF3NO+M and CCl3+NO+M->CCl3NO+H (refs 5 and 7), were analyzed by a variational RRKM method.This analysis shows that all kinetic, structural, and thermochemical data of this series of species and of their corresponding reactions are internally consistent within the framework of the variational RRKM model used.By fitting the usual Troe's expression to the RRKM calculated rate constants values, themselves based on the experimental data, the broadening factor F_cca.0.6 was obtained.Setting F_c=0.6 for all reactions, the following rate expressions have been obtained (units of cm⁶ molecule^-2 s^-1 for k₀ and cm³ molecule^-1 s^-1 for k_<*>: k₀(CF3)=(2.0+/-0.4) x 1E-29(T/298)^-3.2; k_<*>(CF3)=(1.9+/-0.4) x 1E-11(T/298)^-1.0; k₀(CF2Cl)=(1.8+/-)0.35) x 1E-29(T/298)^-4.4; k_<*>(CF2Cl)=(1.9^+1.0-0.5) x 1E-11(T/298)^-1.5; k₀(CFCl2)= (1.0+/-0.25) x 1E-29(T/298)^-4.8; k_<*>(CFCl2)=(1.9^+1.0-0.5) x 1E-11(T/298)^-1.5; k₀(CCl3)=(0.85+/-0.2) x 1E-29(T/298)^-5.5; k_<*>(CCl3)=(0.55+/-0.12) x 1E-11(T/298)^-1.6.It appears that in the low-pressure part of the falloff, the RRKM theory may be used in a predictive way for association reactions forming nitroso compounds: from the experimental determination of k₀, a reliable evaluation of the enthalpy of reaction can be obtained (within +/-8 kJ mol^-1), using β_cca.0.2 and vice versa.The values of k_<*> have been found equal within uncertainties for those species containing fluorine.This is to be compared to the almo st equal enthalpies of their corresponding reactions.In the case of CCl3, both k_<*> and the reaction enthalpy have lower values than those obtained for the other species of the series.In contrast, k₀ decreases regulary in the series, from CF3 to CCl3, and this trend is well accounted for by the calculations.