Kinetics and mechanism of the reactions of CH3CO and CH3C(O)CH2 radicals with O2. Low-pressure discharge flow experiments and quantum chemical computations

Abstract

The reactions CH₃CO + O₂ → products (1), CH₃CO + O₂ → OH +other products (1b) and CH₃C(O)CH₂ + O₂ → products (2) have been studied in isothermal discharge flow reactors with laser induced fluorescence monitoring of OH and CH₃C(O)CH₂ radicals. The experiments have been performed at overall pressures between 1.33 and 10.91 mbar of helium and 298 ± 1 K reaction temperature. OH formation has been found to be the dominant reaction channel for CH₃CO + O₂: the branching ratio, Γ_1b = k_1b/k₁, is close to unity at around 1 mbar, but decreases rapidly with increasing pressure. The rate constant of the overall reaction, k₂, has been found to be pressure dependent: the fall-off behaviour has been analysed in comparison with reported data. Electronic structure calculations have confirmed that at room temperature the reaction of CH₃C(O)CH₂ with O₂ is essentially a recombination-type process. At high temperatures, the further reactions of the acetonyl–peroxyl adduct may yield OH radicals, but the most probable channel seems to be the O₂-catalysed keto–enol transformation of acetonyl. Implications of the results for atmospheric modelling studies have been discussed.