A Computational Re-examination of the Criegee Intermediate–Sulfur
Dioxide Reaction
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Abstract
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 CH<sub>2</sub>OOS(O)O
heteroozonide (HOZ) adduct formed in the CI + SO<sub>2</sub> reaction
converts SO<sub>2</sub> to SO<sub>3</sub>. 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 <i>endo</i> and <i>exo</i> conformers of the HOZ. Although ∼75% of the parent CI + SO<sub>2</sub> reaction is initiated by formation of the <i>exo</i> HOZ, hyperconjugation preferentially stabilizes many of the <i>endo</i> intermediates and transition structures by 1–5
kcal mol<sup>–1</sup>. Our quantum chemical calculations, in
conjunction with statistical rate theory models, predict a rate coefficient
for the parent CI + SO<sub>2</sub> reaction of 3.68 × 10<sup>–11</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>, in good agreement with recent experimental measurements.
RRKM/master equation simulations based on our quantum chemical data
predict a prompt carbonyl + SO<sub>3</sub> yield of >95% for the
reaction of both the parent and dimethyl CI with SO<sub>2</sub>. The
existence of concerted cycloreversion transition structures 10–15
kcal mol<sup>–1</sup> higher in energy than the HOZ accounts
for most of the predicted SO<sub>3</sub> formation