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

Abstract

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.

Computed Pre-reactive Complex Association Lifetimes Explain Trends in Experimental Reaction Rates for Peroxy Radical Recombinations

“…On the other hand, acetonyl peroxy collisions had a significantly higher chance to remain associated for a longer time. We noted that this was consistent with the fact that the self-reaction rate of acetonyl peroxy radicals has been experimentally found to be ∼10–20× higher than that of methyl peroxy radicals. ,, On the basis of both computations and experimentally determined effective activation energies, acetonyl peroxy, like methyl peroxy, is found to have a negligible energy barrier, while t -butyl peroxy has a barrier of several kcal/mol.…”

“…We noted that this was consistent with the fact that the self-reaction rate of acetonyl peroxy radicals has been experimentally found to be ∼10−20× higher than that of methyl peroxy radicals. 1,17,18 On the basis of both computations 13 and experimentally determined effective activation energies, 1 acetonyl peroxy, like methyl peroxy, is found to have a negligible energy barrier, while t-butyl peroxy has a barrier of several kcal/mol. The behavior of pre-reactive complexes is implicitly included in theoretical descriptions of chemical reactions, e.g., in the pre-exponential factor A in the Arrhenius equation.…”

Acetonyl Peroxy and Hydroperoxy Self- and Cross-Reactions: Temperature-Dependent Kinetic Parameters, Branching Fractions, and Chaperone Effects

“…Temperatures below 270 K were not used due to an observed increase in absorption in the UV kinetic trace ascribed to possible aerosol formation. Additional reactions suggested to be important for this chemical mechanism in the recent work by Assali and Fittschen 28 were tested in a sensitivity analysis but were not found to be relevant under our experimental conditions (see the Supporting Information). For R1 and R2, the CH 3 C(O)CH 2 O 2 , HO 2 , and OH kinetic data were fit simultaneously using a Levenberg−Marquardt algorithm 40,43,44 to optimize the kinetic rate coefficients, branching fractions, and rate enhancement terms for R4.…”

Collection on the Spectroscopy, Structure, and Reactivity of Stabilized Criegee Intermediates

“…38−41 However, previously unexpected routes to sCIs involving reactions of peroxy radicals have also recently been proposed. 42,43 The papers included in this Collection are selected from recent publications in The Journal of Physical Chemistry A and The Journal of Physical Chemistry Letters to reflect the continuing worldwide interest in the structures, properties, and reactivity of this unusual class of molecules. Trends apparent in these publications include extension of the experimental and computational investigations to larger, structurally more complex sCIs than formaldehyde oxide and acetaldehyde oxide, and to the study of Criegee intermediate chemistry occurring in water and at the air−water interface.…”