Rate Constants and Branching Ratios for the Self-Reaction of Acetyl Peroxy (CH3C(O)O2•) and Its Reaction with CH3O2

Abstract

The self-reaction of acetylperoxy radicals (CH3C(O)O2•) (R1) as well as their reaction with methyl peroxy radicals (CH3O2•) (R2) have been studied using laser photolysis coupled to a selective time resolved detection of three different radicals by cw-CRDS in the near-infrared range: CH3C(O)O2• was detected in the Ã-X˜ electronic transition at 6497.94 cm−1, HO2• was detected in the 2ν1 vibrational overtone at 6638.2 cm−1, and CH3O2• radicals were detected in the Ã-X˜ electronic transition at 7489.16 cm−1. Pulsed photolysis of different precursors at different wavelengths, always in the presence of O2, was used to generate CH3C(O)O2• and CH3O2• radicals: acetaldehyde (CH3CHO/Cl2 mixture or biacetyle (CH3C(O)C(O)CH3) at 351 nm, and acetone (CH3C(O)CH3) or CH3C(O)C(O)CH3 at 248 nm. From photolysis experiments using CH3C(O)C(O)CH3 or CH3C(O)CH3 as precursor, the rate constant for the self-reaction was found with k1 = (1.3 ± 0.3) × 10−11 cm3s−1, in good agreement with current recommendations, while the rate constant for the cross reaction with CH3O2• was found to be k2 = (2.0 ± 0.4) × 10−11 cm3s−1, which is nearly two times faster than current recommendations. The branching ratio of (R2) towards the radical products was found at 0.67, compared with 0.9 for the currently recommended value. Using the reaction of Cl•-atoms with CH3CHO as precursor resulted in radical profiles that were not reproducible by the model: secondary chemistry possibly involving Cl• or Cl2 might occur, but could not be identified.

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

“…In our recent work on the CH 3 C­(O)­O 2 self-reaction we have shown that CH 3 O 2 is not selective anymore because absorption–time profiles registered on and off the peak wavelength clearly do not have the same shape. From different experiments it was concluded that CH 3 C­(O)­O 2 is still absorbing in the CH 3 O 2 wavelength range . This also has to be considered in this work because CH 3 C­(O)­O 2 is a reaction product of the CH 3 C­(O)­CH 2 O 2 self-reaction.…”

“…The self-reaction of CH 3 C­(O)­CH 2 O 2 is thought to have three reaction paths, whereof () and () lead to stable products, not influencing the kinetic measurements of our experiments, while path () leads to the formation of radicals through rapid decomposition of the initially formed alkoxy radical which in the presence of O 2 will rapidly lead to the formation of the acetyl peroxy radical, CH 3 C­(O)­O 2 , as well as to the formation of a few percent of OH and HO 2 radicals: , The CH 3 C­(O)­CH 2 O 2 decay will subsequently be perturbed by complex secondary chemistry with an estimated rate constant of k 7 = (5.0 ± 2.0) × 10 –12 cm 3 s –1 and a branching ratio of ϕ 7 = (0.5 ± 0.2) . The product of (), CH 3 C­(O)­CH 2 O, will rapidly decompose to regenerate the CH 3 C­(O)­O 2 radicals while the other product, CH 3 C­(O)­O, rapidly decomposes and leads to the formation of CH 3 O 2 radicals.…”

“…The spectrum of this radical has been measured several times − and is very structured. A highly selective quantification can be obtained by removing the contribution of possible broadband absorbers (CH 3 C­(O)­O 2 in this system) by measuring profiles on top of the absorption line (red circles in Figure a) and at a wavelength just next to it (black circles in Figure a). HO 2 profiles are then obtained by subtracting online–offline (red dots in Figure a).…”

“…From different experiments it was concluded that CH 3 C(O)O 2 is still absorbing in the CH 3 O 2 wavelength range. 21 This also has to be considered in this work because…”

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

“…However, there is clearly an outlier in the case of MeOO + AcOO. Recent experiments 26 have measured a surprisingly high reactivity for this cross-reaction (k = 200 × 10 −13 cm −3 molecule −1 s −1 ), even higher than the reactivity of the AcOO self-reaction (k = 130 × 10 −13 cm −3 molecule −1 s −1 ) Our modeling would instead be consistent with a value of k for the cross-reaction in between the values for MeOO and AcOO self-reactions. This intuitive prediction is also consistent with our recent calculations of the energetics in these systems.…”