Franck-Condon Dominated Chemistry. Formation and Dissociations of the Dimethylhydroxysulfuranyl Radical

Abstract

The structure and energetics of the hydroxyl radical adduct to dimethyl sulfide (DMS) was revisited using high level ab initio calculations. Density functional theory B3LYP/6-31++G(2d,p) and perturbational MP2(FULL)/6-31++G(2d,p) calculations found a weakly bound structure, (CH3)2SOH•, with a long S-O bond that was a local energy minimum. Single point calculations at the effective QCISD(T)/6-311++G(3df,2p) level of theory, denoted as G2++(MP2), found the (CH3)2S-OH• bonding energy to be 40 kJ mol-1 at 298 K. The standard heat of formation of (CH3)2SOH• was assessed from dissociation and isodesmic reactions as -45 ± 4 kJ mol-1. No other local minima corresponding to C2H7OS radicals were found at the present level of theory that could be derived from DMS or dimethyl sulfoxide (DMSO). A very weak complex, CH3S(H)-•OCH3, was found that was bound by mere 4 kJ mol-1 against dissociation to CH3SH and •OCH3. Vertical electron capture by (CH3)2SOH+ is predicted to form (CH3)2SOH• with a highly non-relaxed geometry corresponding to a vibrational excitation of 138 kJ mol-1 above the local minimum and 88 kJ mol-1 above the dissociation threshold to DMS and OH•. Unimolecular dissociation of (CH3)2SOH• to methanesulfenic acid (CH3SOH) and •OCH3 faces an energy barrier that diminishes at shorter S-O distances. The dipole-allowed electronic excitation in (CH3)2SOH• was calculated with CIS/6-311++G(2df,p) to have λmax = 248 nm in the gas phase. The resulting B state represents a charge-transfer complex of (CH3)2S+• and OH-. The present computational results allowed us to explain the existing controversy between the experimental results obtained by gas-phase flow kinetics, radiolysis in aqueous solution, and neutralization-reionization mass spectrometry.