Investigation of the radical product channel of the CH3C(O)CH2O2 + HO2 reaction in the gas phase

Abstract

The reaction of CH₃C(O)CH₂O₂ with HO₂ has been studied at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl₂/acetone/methanol/air mixtures. The branching ratio for the reaction channel forming CH₃C(O)CH₂O, OH and O₂ (reaction (3b)) was investigated in experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The observed prompt formation of phenol under conditions when CH₃C(O)CH₂O₂ reacts mainly with HO₂ indicates that this reaction proceeds partially by channel (3b), which forms OH both directly and indirectly, by virtue of secondary generation of CH₃C(O)O₂ (from CH₃C(O)CH₂O) and its reaction with HO₂ (reaction (6c)). The secondary generation of OH radicals was confirmed by the observed formation of CH₃C(O)OOH, a well-established product of the CH₃C(O)O₂ + HO₂ reaction (via channel (6a)). A number of delayed sources of OH also contribute to the observed phenol formation, such that full characterisation of the system required simulations using a detailed chemical mechanism. The dependence of the phenol and CH₃C(O)OOH yields on the initial peroxy radical precursor reagent concentration ratio, [methanol]₀/[acetone]₀, were well described by the mechanism, consistent with a small but significant fraction of the reaction of CH₃C(O)CH₂O₂ with HO₂ proceeding via channel (3b). This allowed a branching ratio of k_3b/k₃ = 0.15 ± 0.08 to be determined. The results therefore provide strong indirect evidence for the participation of the radical-forming channel of the title reaction.