Kinetic study of the pressure dependence of the reaction CF3O+NO2at 298 K Rate constant measurements (P=0.5–9Torr) 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.