Improved computational modeling of the kinetics of the acetylperoxy + HO₂ reaction

The acetylperoxy + HO₂ reaction has multiple impacts on the troposphere, with a triplet pathway leading to peracetic acid + O₂ (reaction (1a)) competing with singlet pathways leading to acetic acid + O₃ (reaction (1b)) and acetoxy + OH + O₂ (reaction (1c)). A recent experimental study has reported branching fractions for these three pathways (α_1a, α_1b, and α_1c) from 229 K to 294 K. We constructed a theoretical model for predicting α_1a, α_1b, and α_1c using quantum chemical and Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) simulations. Our main quantum chemical method was Weizmann-1 Brueckner Doubles (W1BD) theory; we combined W1BD and equation-of-motion spin-flip coupled cluster (SF) theory to treat open-shell singlet structures. Using RRKM/ME simulations that included all conformers of acetylperoxy-HO₂ pre-reactive complexes led to a 298 K triplet rate constant, k_1a = 5.11 × 10^-12 cm³ per molecule per s, and values of α_1a in excellent agreement with experiment. Increasing the energies of all singlet structures by 0.9 kcal mol^-1 led to a combined singlet rate constant, k_1b+1c = 1.20 × 10^-11 cm³ per molecule per s, in good agreement with experiment. However, our predicted variations in α_1b and α_1c with temperature are not nearly as large as those measured, perhaps due to the inadequacy of SF theory in treating the transition structures controlling acetic acid + O₃ formation vs. acetoxy + OH + O₂ formation.