Thermochemistry, bond energies and internal rotor barriers of methyl sulfinic acid, methyl sulfinic acid ester and their radicals

Abstract

AbstractSulfur–Oxygen containing hydrocarbons are formed in oxidation of sulfides and thiols in the atmosphere, on aerosols and in combustion processes. Understanding their thermochemical properties is important to evaluate their formation and transformation paths. Structures, thermochemical properties, bond energies, and internal rotor potentials of methyl sulfinic acid CH3S(O)OH, its methyl ester CH3S(O)OCH3 and radicals corresponding to loss of a hydrogen atom have been studied. Gas phase standard enthalpies of formation and bond energies were calculated using B3LYP/6‐311G (2d, p) individual and CBS‐QB3 composite methods employing work reactions to further improve accuracy of the ${\Delta} _{{\bf f}} H_{{\bf 298}}^{{\bf o}} $. Molecular structures, vibration frequencies, and internal rotor potentials were calculated. Enthalpies of the parent molecules CH3S(O)OH and CH3S(O)OCH3 are evaluated as −77.4 and −72.7 kcal mol−1 at the CBSQB3 level; Enthalpies of radicals C•H2S(O)OH, CH3S•(O)2, C•H2S(O)OCH3 and CH3S(O)OC•H2 (CBS‐QB3) are −25.7, −52.3, −22.8, and −26.8 kcal mol−1, respectively. The CH3C(O)O—H bond dissociation energy is of 77.1 kcal mol−1. Two of the intermediate radicals are unstable and rapidly dissociate. The CH3S(O)O. radical obtained from the parent CH3S(O)OH dissociates into methyl radical (${\bf CH}_{{\bf 3}}^{{\bf .}} $) plus SO2 with endothermicity (ΔHrxn) of only 16.2 kcal mol−1. The CH3S(O)OC•H2 radical dissociates into CH3S•=O and CH2=O with little or no barrier and an exothermicity of −19.9 kcal mol−1. DFT and the Complete Basis Set‐QB3 enthalpy values are in close agreement; this accord is attributed to use of isodesmic work reactions for the analysis and suggests this combination of B3LYP/work reaction approach is acceptable for larger molecules. Copyright © 2010 John Wiley & Sons, Ltd.