Quantum chemical and master equation modeling of the acetylperoxy + HO2 reaction

Abstract:

Peroxy radicals are ubiquitous in tropospheric chemistry. Under low-NO_x conditions, the reaction of peroxy radicals with HO₂ becomes highly significant. The acetylperoxy + HO₂ reaction has a particularly large and diverse atmospheric impact in that it can cause radical termination (1a), indirect radical propagation via O₃ formation (1b), or direct radical propagation (1c):

CH₃C(O)OO + HO₂ → CH₃C(O)OOH + O₂ (1a)

CH₃C(O)OO + HO₂ → CH₃C(O)OH + O₃ (1b)

CH₃C(O)OO + HO₂ → CH₃C(O)O + OH + O₂ (1c)

Early theoretical studies established that reaction 1a proceeds in a single step on the triplet potential energy surface, whereas reactions 1b and 1c proceed via the formation and either rearrangement or decomposition of a hydrotetraoxide intermediate on the singlet potential energy surface. However, these studies were limited by the use of lower levels of electronic structure theory and treatment of only the ground state conformers of minima and transition structures (TSs). These limitations required unreasonable assumptions about the stability of acetylperoxy-HO₂ complexes and could not reproduce experimental rate constants.

In this study, we use the B3LYP and ωB97X-D density functionals with the cc-pVTZ+d basis set to compute optimized geometries and harmonic frequencies for all minima and TSs and Weizmann-1 Brueckner Doubles (W1BD) theory for computing highly accurate relative energies. On both the singlet and triplet surfaces, the acetylperoxy-HO₂ pre-reactive complexes range in stability from 4 to 7 kcal mol^-1 and conformers of the covalently bound hydrotetraoxide are between 16 and 20 kcal mol^-1 more stable than the separated CH₃C(O)OO + HO₂ radicals. RRKM/master equation predictions of the rate constants of pathway 1a agree to within 20% of the recent experimental measurements of Hui et al. from 230 to 300 K. This agreement requires applications of long-range transition state theory to three triplet entrance channels. Treating pathways 1b and 1c poses greater electronic structure challenges since the TSs involved in the formation and decomposition of the hydrotetroxide contain significant singlet diradical character. A broken-spin-symmetry version of W1BD theory predicts TS energies that need to be lowered by 2-3 kcal mol^-1 to obtain a predicted total rate constant and 298 K branching fractions for 1b and 1c in good agreement with Hui et al.