Kinetic study of the pressure dependence of the reaction CF3O + NO2at 298 K Rate constant measurements (P = 0.5–9 Torr) and competition between association (CF3ONO2)and disproportionation (CF2O + FNO2)channels

Abstract

The kinetics of the reaction of the CF₃O radical with NO₂ has been studied at 298 K at low pressure (0.5–9 Torr of helium) by fast-flow–laser-induced fluorescence (LIF), pyrolysis of a dilute mixture of CF₃OOCF₃ and He being used as the source of the CF₃O radical. In good agreement with the results of Zellner and co-workers (obtained in the pressure range 5–100 Torr), the rate constant has been found to be pressure dependent. All the experimental data were analysed by a multichannel RRKM procedure using the results of abinitio calculations as input data. This revealed that two reaction channels, the association CF₃O + NO₂ → CF₃ONO₂ (1a) and the disproportionation CF₃O + NO₂ → CF₂O + FNO₂ (1b), must be invoked and they proceed via a common energized adduct CF₃ONO₂^*. The pressure dependence of the branching ratio was predicted by this calculation: the disproportionation channel would be negligible near atmospheric pressure and becomes the major channel at pressure below ca. 0.3 Torr, with a calculated second-order limiting low-pressure value of the rate equal to 3.2 × 10⁻¹² cm³ molecule⁻¹ s⁻¹. The high-pressure limit rate constant obtained is k_∞ = (1.65 ± 0.2) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. An analytical representation of the pressure dependence of the rate constant at 298 K is proposed using the conventional Troe expression with an added constant.