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 3 S(O)OH, its methyl ester CH 3 S(O)OCH 3 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 CH 3 S(O)OH and CH 3 S(O)OCH 3 are evaluated as −77.4 and −72.7 kcal mol −1 at the CBSQB3 level; Enthalpies of radicals C • H 2 S(O)OH, CH 3 S • (O) 2 , C • H 2 S(O)OCH 3 and CH 3 S(O)OC • H 2 (CBS‐QB3) are −25.7, −52.3, −22.8, and −26.8 kcal mol −1 , respectively. The CH 3 C(O)O—H bond dissociation energy is of 77.1 kcal mol −1 . Two of the intermediate radicals are unstable and rapidly dissociate. The CH 3 S(O)O. radical obtained from the parent CH 3 S(O)OH dissociates into methyl radical ( ${\bf CH}_{{\bf 3}}^{{\bf .}} $ ) plus SO 2 with endothermicity (Δ H rxn ) of only 16.2 kcal mol −1 . The CH 3 S(O)OC • H 2 radical dissociates into CH 3 S • =O and CH 2 =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.