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 .