The CF3 + NO2 rate constant measured between 1.5 and 110 Torr and between 251 and 295 K by time resolved infrared emission

Abstract

The rate constant k₁ for reaction of CF₃ with NO₂ was investigated over the pressure range 1.5–110 Torr Ar or N₂ and at temperatures between 251 and 295 K. Time resolved emission from vibrationally excited CF₂O and FNO products was used as a detection method for CF₃ removal. The rate constant at room temperature shows no marked variation with pressure, with an average value of k₁ = (1.75 ± 0.26) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, where the errors are 2σ, but a small reduction at the lowest pressure indicates the possible contribution from a third body recombination process. We conclude that this process is minor, and if it is treated as a separate reaction channel it results in a limiting high pressure branching ratio of 13 ± 7%, which drops to half that value at 10 Torr. As the limiting high and low pressure rate constants are within the 2σ error bars of k₁, we recommend the average value as applying over the full pressure range. Combination with previous results shows that of the possible bimolecular channels, formation of CF₂O + FNO dominates (ca. 95%). No temperature dependence is seen, within experimental error. Pitfalls in the extraction of rate constants from the analysis of time resolved product emission are discussed, and a method is described to assign rate constants unambiguously to reactive formation and collisional quenching. CF₃ radicals were formed from the 248 nm photolysis of CF₃I, and observations of emission from the radical show previously unobserved excitation in the asymmetric C–F stretching mode ν₃.