Kinetics of the combination reactions of chlorofluoromethylperoxy radicals with NO2 in the temperature range 233–373 K

Abstract

The rate parameters for the combination reactions of CCl₃O₂, CCl₂FO₂ and CF₃O₂ radicals with NO₂ were measured in the pressure range 1–10 Torr and from 233 to 373 K. Experiments were performed by pulsed laser photolysis and time-resolved mass spectrometry. Al reactions are in the fall-off region and are significantly faster than the equivalent reaction of CH₃O₂. They exhibit strong negative temperature coefficients and the rate constants increase in the series from CF₃O₂ to CCl₃O₂. The experimental equilibrium constant was obtained for the reaction of CCl₂FO₂ at 273 K, by using previously determined rate constants for the reverse reaction. This determination allowed us to obtain ΔH° of the reaction by calculating the structural and spectroscopic parameters by the semi-empirical MNDO method. Assuming that the value of ΔH° is the same for all reactions of the series it was possible to calculate the temperature dependence of all equilibrium constants and the bond dissociation energies D^°₂₉₈(CX₃O₂—NO₂)= 105 ± 5 kJ mol^–1(X = For Cl). ARRKM model was set up for these reactions and calibrated for the reaction of CCl₂FO₂. The model is shown to reproduce quite adequately the fall-off curves and with the substitution of chlorine for fluorine in the series of radicals investigated. This model was used to extrapolate the low-pressure data to high pressure, where no experimental data were available, and to calculate the kinetic parameters for the CClF₂O₂ reaction, which could not be investigated experimentally in the present study. Kinetic parameters are reported for the association reaction of the entire series of chlorofluoromethylperoxy radicals with NO₂.