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

Sulfur-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 CH ₃S(=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 Δ_fH ₂₉₈^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 CH ₃-S(=O)-OH dissociates into methyl radical (CH₃-S ^•) plus SO₂ with endothermicity (ΔH _rxn) 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.