1 research outputs found
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 CH<sub>2</sub>OOSÂ(O)ÂO
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
CCSDÂ(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