Rate Constants and Branching Ratios for the Self-Reaction of Acetyl Peroxy (CH3C(O)O2•) and Its Reaction with CH3O2

Self-Reaction of Acetonyl Peroxy Radicals and Their Reaction with Cl Atoms

The rate constant for the self-reaction of the acetonyl peroxy radicals, CH₃C(O)CH₂O₂, has been determined using laser photolysis/continuous wave cavity ring down spectroscopy (cw-CRDS). CH₃C(O)CH₂O₂ radicals have been generated from the reaction of Cl atoms with CH₃C(O)CH₃, and the concentration time profiles of four radicals (HO₂, CH₃O₂, CH₃C(O)O₂, and CH₃C(O)CH₂O₂) have been determined by cw-CRDS in the near-infrared. The rate constant for the self-reaction was found to be k = (5.4 ± 1.4) × 10^-12 cm³ s^-1, in good agreement with a recently published value (Zuraski, K., et al. J. Phys. Chem. A 2020, 124, 8128); however, the branching ratio for the radical path was found to be ϕ_1b = (0.6 ± 0.1), which is well above the recently published value (0.33 ± 0.13). The influence of a fast reaction of Cl atoms with the CH₃C(O)CH₂O₂ radical became evident under some conditions; therefore, this reaction has been investigated in separate experiments. Through the simultaneous fitting of all four radical profiles to a complex mechanism, a very fast rate constant of k = (1.35 ± 0.8) × 10^-10 cm³ s^-1 was found, and experimental results could be reproduced only if Cl atoms would partially react through H-atom abstraction to form the Criegee intermediate with a branching fraction of ϕ_Criegee = (0.55 ± 0.1). Modeling the HO₂ concentration-time profiles was possible only if a subsequent reaction of the Criegee intermediate with CH₃C(O)CH₃ was included in the mechanism leading to HO₂ formation with a rate constant of k = (4.5 ± 2.0) × 10^-14 cm³ s^-1.