Microsolvation of thiosulfuric acid and its tautomeric anions [HSSO(3)](-) and [SSO(2)(OH)](-) studied by B3LYP-PCM and G3X(MP2) calculations.

The interaction of thiosulfuric acid and its monoanion with up to three water molecules has been studied by density functional and high-level ab initio calculations. More than 40 molecules and anions both as OH and SH tautomers were investigated. The structures in the gas phase as well as in a polarizable continuum were optimized at the B3LYP/6-31G(2df,p) level of theory, whereas G3X(MP2) single-point calculations were applied to obtain enthalpies and Gibbs energies of the gaseous species. In the gas phase, all monoanions of composition H,S(2),O(3).nH(2)O (n = 0-3) are predicted to be most stable as SH tautomers HSSO(3). The enthalpies of hydration are -51 +/- 5 kJ mol(-1) per water molecule. In a polarizable phase simulating the dielectric properties of water the OH forms SSO(2)(OH) of the hydrated and unhydrated monoanions are always most stable. Thiosulfuric acid and its mono- and dihydrates are most stable in the SH/OH form, both in the gas phase and in the polarizable continuum. However, the trihydrate H(2)S(2)O(3).3H(2)O prefers the OH/OH form in the polarizable phase, whereas in the gas phase the SH/OH tautomer represents the lowest minimum structure. The hydration enthalpy of the acid is slightly smaller than predicted for the monoanions. We predict that both thiosulfuric acid and its monoanion exist as equilibrium mixtures of the corresponding tautomers in aqueous solution. The acid decomposition of thiosulfate involves sulfur transfer reactions, but the formerly accepted reaction between two monoanions producing sulfite and HSSSO(3) ions is endothermic and endergonic both for the naked anions and their trihydrates. Therefore, we propose that the sulfur transfer takes place between HSSO(3) and H(2)S(2)O(3) producing HSSSO(3), SO(2), and H(2)O. This reaction is exothermic and exergonic both in the gas phase and in the polarizable phase. From HSSSO(3) the longer chain sulfane monosulfonate ions HS(n)SO(3) are formed by a series of sulfur transfer reactions, and these ions eventually split off homocyclic sulfur molecules S(n). The initial decomposition reaction is hindered by SO(2) which reacts with thiosulfate ions to the novel adduct O(2)SSSO(3) which seems to be also a key intermediate in the synthesis of trithionate from thiosulfate and SO(2). The implications of these results for the enzymatic thiosulfate metabolism by sulfur bacteria are discussed. The adduct H(2)S.SO(3), an isomer of thiosulfuric acid, has been studied in addition. Its conversion into H(2)S(2)O(3) is predicted to be exothermic, by -39.5 kJ mol(-1).