Transient Hydrogen Atom Adducts to Disulfides. Formation and Energetics

Abstract

Hydrogen atom adducts to the disulfide bonds in dimethyl disulfide (1) and 1,2-dithiolan (2) were generated transiently by collisional electron transfer to gaseous cations CH3SS(H)CH3+ (1H+) and 2H+, respectively. Hypervalent radical 1H dissociated completely on the 4.6 μs time scale to 1 + H•, CH3SH + CH3S•, and CH3SSH + CH3•. The sulfur-containing dissociation products were detected and identified by neutralization−reionization mass spectrometry. Radical 1H was found to be weakly bound; the SS, SC, and SH 0 K bond dissociation energies were calculated by a modified G2++(MP2) method as 8, 52, and 96 kJ mol-1, respectively. The energy to drive these dissociations was supplied by Franck−Condon effects on vertical neutralization of 1H+ (161−230 kJ mol-1). Two stereochemically distinct pathways were found for the exothermic hydrogen atom addition to 1. A rear attack at the disulfide bond had a small activation energy, Ea = 9 kJ mol-1, while a side attack had Ea = 33 kJ mol-1. Collisional neutralization of 2H+ provided a fraction of nondissociating 2H at equilibrium with open-ring isomers, HS−CH2CH2CH2−S• (syn-5 and anti-5) and HSS−CH2CH2CH2• (syn-6 and anti-6). Hydrogen atom addition to the disulfide bond in 2 was calculated to be 118 kJ mol-1 exothermic and proceeded via a rear attack mechanism without an energy barrier. Stereoelectronic effects in hydrogen atom capture by disulfide bonds are discussed.