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.