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

The rate constant for the self-reaction of the acetonyl peroxy radicals, CH ₃ C(O)CH ₂ O ₂ , has been determined using laser photolysis/continuous wave cavity ring down spectroscopy (cw-CRDS). CH ₃ C(O)CH ₂ O ₂ radicals have been generated from the reaction of Cl atoms with CH ₃ C(O)CH ₃ , and the concentration time profiles of four radicals (HO ₂ , CH ₃ O ₂ , CH ₃ C(O)O ₂ , and CH ₃ C(O)CH ₂ O ₂ ) 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 cm ³ 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 CH ₃ C(O)CH ₂ O ₂ 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 cm ³ 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 HO ₂ concentration-time profiles was possible only if a subsequent reaction of the Criegee intermediate with CH ₃ C(O)CH ₃ was included in the mechanism leading to HO ₂ formation with a rate constant of k = (4.5 ± 2.0) × 10 ^-14 cm ³ s ^-1 .