Transient hydrogen atom adducts to disulfides.: Formation and energetics

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 2N(+), respectively. Hypervalent radical 1H dissociated completely on the 4.6 mu s 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 S-S, S-C, and S-H 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 tol. A rear attack at the disulfide bond had a small activation energy, E-a = 9 kT mol(-1), while a side attack had E-a = 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.