Sulfur Oxyacids and Radicals in the Gas Phase. A Variable-Time Neutralization−Photoexcitation−Reionization Mass Spectrometric and Ab Initio/RRKM Study

Abstract

Hydroxysulfinyl radical (1), hydrogensulfonyl radical (2), and dihydroxysulfane (6) were generated in the gas phase by collisional reduction of the corresponding cations and studied by the variable-time and photoexcitation methods of neutralization−reionization mass spectrometry and by ab initio and RRKM calculations. Radicals 1 and 2 were thermodynamically and kinetically stable. Two rotamers of 1, syn-1 and anti-1, were found computationally to be local energy minima. The computations suggested a complex potential energy surface for dissociations of 1. The minimum-energy reaction path was the rate-determining isomerization to 2 followed by fast loss of H• to form SO2. Direct H• loss from 1 was kinetically disfavored. Cleavage of the S−OH bond in 1 was highly endothermic and became kinetically significant at excitations >325 kJ mol-1. In contrast to ab initio/RRKM predictions, 1 formed by vertical reduction of hydroxysulfinyl cation (1+) dissociated mainly to OH• and SO, whereas loss of H• was less significant. Both dissociations showed microsecond kinetics as established by variable-time measurements. Photoexcitation of nondissociating 1 opened the H-loss channel, whereas collisional excitation did not change the branching ratio for the H• and OH• loss channels. The experimental results pointed to the formation of a large fraction of metastable and dissociative excited electronic states of 1 upon vertical electron transfer. Radical 2 was cogenerated with 1 by vertical reduction of a mixture of 1+ and 2+ produced by highly exothermic protonation of SO2 with H3+. Pronounced loss of H• from 2 occurred following collisional neutralization in accordance with RRKM predictions. Dihydroxysulfane (6) was stable following collisional neutralization of the cation-radical 6•+. The G2(MP2) potential energy surface predicted the isomerization to hydrogensulfinic acid (7) followed by loss of water to be the lowest-energy dissociation of 6. RRKM calculations showed the 6 → 7 isomerization to be the rate-determining step. Cation-radical 6•+ also eliminated water through unimolecular isomerization to a stable nonclassical isomer, OS•+···H2O (9). The thermochemistry of the neutral and ionic systems is discussed. The important role of excited electronic states in the formation of radicals by vertical electron transfer is emphasized.