Franck-Condon dominated chemistry. Formation and dissociations of the dimethylhydroxysulfuranyl radical

The structure and energetics of the hydroxyl radical adduct to dimethyl sul fide (DMS) was revisited using high level ab initio calculations. Density f unctional theory B3LYP/6-31++G(2d,p) and perturbational MD2(FULL)/6-31++G(2 d,p) calculations found a weakly bound structure, (CH3)(2)SOH., 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)(2)S-OH. bonding energy to be 40 kJ mol(-1) at 298 K. The s tandard heat of formation of (CH3)(2)SOH. was assessed from dissociation an d isodesmic reactions as -45 +/- 5 kJ mol(-1). No other local minima corres ponding to C2H7OS radicals were found at the present level of theory that c ould be derived from DMS or dimethyl sulfoxide (DMSO). A very weak complex, CH3S(H)-(OCH3)-O-., was found that was bound by mere 4 kJ mol(-1) against dissociation to CH3SH and . OCH3. Vertical electron capture by (CH3)(2)SOHC is predicted to form (CH3)(2)SOH. with a highly non-relaxed geometry corre sponding to a vibrational excitation of 138 kJ mol(-1) above the local mini mum and 88 kJ mol(-1) above the dissociation threshold to DMS and OH.. Unim olecular dissociation of (CH3)(2)SOH. to methanesulfenic acid (CH3SOH) and CH3. faces an energy barrier that diminishes at shorter S-O distances. The dipole-allowed electronic excitation in (CH3)(2)SOH. was calculated with CI S/6-311++G(2df,p) to have lambda(max) = 248 nm in the gas phase. The result ing B state represents a charge-transfer complex of (CH3)(2)S+. and OH-. Th e present computational results allowed us to explain the existing controve rsy between the experimental results obtained by gas-phase flow kinetics, r adiolysis in aqueous solution, and neutralization-reionization mass spectro metry.