The rate constant for the self-reaction of the acetonyl peroxy radicals, CHC(O)CHO, has been determined using laser photolysis/continuous wave cavity ring down spectroscopy (cw-CRDS). CHC(O)CHO radicals have been generated from the reaction of Cl atoms with CHC(O)CH, and the concentration time profiles of four radicals (HO, CHO, CHC(O)O, and CHC(O)CHO) have been determined by cw-CRDS in the near-infrared. The rate constant for the self-reaction was found to be = (5.4 ± 1.4) × 10 cm s, in good agreement with a recently published value (Zuraski, K., et al. , , 8128); however, the branching ratio for the radical path was found to be ϕ = (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 CHC(O)CHO 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 = (1.35 ± 0.8) × 10 cm s 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 = (0.55 ± 0.1). Modeling the HO concentration-time profiles was possible only if a subsequent reaction of the Criegee intermediate with CHC(O)CH was included in the mechanism leading to HO formation with a rate constant of = (4.5 ± 2.0) × 10 cm s.
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