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 ϕ1b =
(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 ϕCriegee = (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.