Assessing the Viability of the Methylsulfinyl Radical-Ozone Reaction

Although integral to remote marine atmospheric sulfur chemistry, the reaction between methylsulfinyl radical (CH₃ SO) and ozone poses challenges to theoretical treatments. The lone theoretical study on this reaction reported an unphysically large barrier of 66 kcal mol^-1 for abstraction of an oxygen atom from O₃ by CH₃ SO. Herein, we demonstrate that this result stems from improper use of MP2 with a single-reference, unrestricted Hartree-Fock (UHF) wavefunction. We characterized the potential energy surface using density functional theory (DFT), as well as multireference methodologies employing a complete active-space self-consistent field (CASSCF) reference. Our DFT PES shows, in contrast to previous work, that the reaction proceeds by forming an addition adduct [CH₃ S(O₃ )O] in a deep potential well of 37 kcal mol^-1 . An O-O bond of this adduct dissociates via a flat, low barrier of 1 kcal mol^-1 to give CH₃ SO₂ +O₂ . The multireference computations show that the initial addition of CH₃ SO+O₃ is barrierless. These results provide a more physically intuitive and accurate picture of this reaction than the previous theoretical study. In addition, our results imply that the CH₃ SO₂ formed in this reaction can readily decompose to give SO₂ as a major product, in alignment with the literature on CH₃ SO reactions.