GAS-PHASE CHEMISTRY AND THERMOCHEMISTRY OF THE HYDROXYSULFIDE ANION, HOS-

The chemistry and thermochemistry of the hydroxysulfide anion, HOS-, has been studied in a tandem flowing afterglow-selected ion flow tube (FA-SIFT). The reactions of HOS- with O2, CO, CO2, CS2, SO2, H2S, (CH3)3CSH, CH3SH, CH3Cl, and (CH3)2CHCl have been examined and compared to the corresponding reactions of HOO-. The gas-phase basicity of HOS- has been established via the bracketing technique [DELTAG(acid)0(HOSH) = 347 +/- 3 kcal mol-1, DELTAH(acid)0(HOSH) = 354 +/- 3.5 kcal mol-1]. Ab initio calculations have been carried out on HOS-, HSO-, HOSH, and H2SO which demonstrate that HOS- and HOSH are the more stable isomers. At the MP4SDTQ(FC)/6-311++G**//MP2(Full)/6-311++G** level of theory, these calculations predict DELTAH(acid)0(HOSH) = 356.7 kcal mol-1, in agreement with the experimentally determined value. Measurement of the forward and reverse rate constants for the reaction generating HOS- yields DELTAH(f)0 (HOS-) = -38.7 +/- 2 kcal mol-1, which when combined with the experimental electron affinity of HOS, determines DELTAH(f)0(HOS) = -0.5 +/- 2 kcal mol-1; this value is in good agreement with a recent ab initio determination. The gas-phase basicity of HSS-, the major product ion in the reaction of HOS- and CS2, has also been determined via the bracketing technique [DELTAG(acid)0(HSSH) = 339 +/- 3 kcal mol-1, DELTAH(acid)0(HSSH) = 346 +/- 3.5 kcal mol-1]. Combination of our gas-phase acidity values for HOSH and HSSH as well as for CH3SSH reported in a previous paper, with known electron affinities for the corresponding radicals, allows determination of the S-H bond dissociation energies (D298): HOSH (79 +/- 3.5 kcal mol-1), HSSH (76 +/- 3.5 kcal mol-1), and CH3SSH (79 +/- 3.5 kcal mol-1).