A Computational Re-examination of the Criegee Intermediate–Sulfur Dioxide Reaction

The atmospheric oxidation of sulfur dioxide by the parent and dimethyl Criegee intermediates (CIs) may be an important source of sulfuric acid aerosol, which has a large impact on radiative forcing and therefore upon climate. A number of computational studies have considered how the CH₂OOS­(O)­O heteroozonide (HOZ) adduct formed in the CI + SO₂ reaction converts SO₂ to SO₃. In this work we use the CBS-QB3 quantum chemical method along with equation-of-motion spin-flip CCSD­(dT) and MCG3 theories to reveal new details regarding the formation and decomposition of the endo and exo conformers of the HOZ. Although ∼75% of the parent CI + SO₂ reaction is initiated by formation of the exo HOZ, hyperconjugation preferentially stabilizes many of the endo intermediates and transition structures by 1–5 kcal mol^–1. Our quantum chemical calculations, in conjunction with statistical rate theory models, predict a rate coefficient for the parent CI + SO₂ reaction of 3.68 × 10^–11 cm³ molecule^–1 s^–1, in good agreement with recent experimental measurements. RRKM/master equation simulations based on our quantum chemical data predict a prompt carbonyl + SO₃ yield of >95% for the reaction of both the parent and dimethyl CI with SO₂. The existence of concerted cycloreversion transition structures 10–15 kcal mol^–1 higher in energy than the HOZ accounts for most of the predicted SO₃ formation.