Assessing the Viability of the Methylsulfinyl Radical‐Ozone Reaction

• Published in: Chemphyschem Vol. 21; no. 12; pp. 1289 - 1294
• Main Authors: Estep, Marissa L., Moore, Kevin B., Schaefer, H. F.
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 16-06-2020
• Subjects: ab initio calculations; Decomposition reactions; density functional calculations; Density functional theory; Marine chemistry; methylsulfinyl radical calculations; Ozone; Potential energy; sulfur chemistry; Wave functions; ab initio calculations; density functional calculations; sulfur chemistry; methylsulfinyl radical calculations; ozone
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.202000188

Although integral to remote marine atmospheric sulfur chemistry, the reaction between methylsulfinyl radical (CH3SO) 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 ion of an oxygen atom from O3 by CH3SO. 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 [CH3S(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 CH3SO2+O2. The multireference computations show that the initial addition of CH3SO+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 CH3SO2 formed in this reaction can readily decompose to give SO2 as a major product, in alignment with the literature on CH3SO reactions.
Match for experiments: The reaction between the methylsulfinyl radical and ozone is important to atmospheric sulfur chemistry, but the published theoretical mechanism for this reaction includes too large a barrier to align with available experimental results. Our computations indicate that the reactants form an adduct that proceeds to dissociate without a significant barrier.