Sulfur oxyacids and radicals in the gas phase.: A variable-time neutralization-photoexcitation-reionization mass spectrometric and ab initio RRKM study

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-OK 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 H-3(+). 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.