Microsolvation and hydration enthalpies of CaS2O3(H2O)n (n = 0–19) and S2O32−(H2O)n (n = 0–16): an ab initio study

We studied microhydrated calcium thiosulfate and its ions at the restricted Hartree–Fock RHF/6-31G* level of theory. A semiempirical molecular dynamics search of progressively more hydrated species provided lowest-energy configurations that were then fully optimized and characterized as energy minima at the RHF/6-31G* level of theory. The first solvation shell of calcium thiosulfate contains 18 water molecules, while the first solvation shell of thiosulfate ion consists of 15 water molecules. QTAIM calculations show hydrogen bonding to sulfur. At 298.15 K, we estimate the total standard enthalpies of hydration for thiosulfate ion and calcium thiosulfate at infinite dilution as −301 kcal mol−1 and −335 kcal mol−1, respectively. The dissociation of hydrated calcium thiosulfate at infinite dilution is predicted to be an endothermic process with an enthalpy of 262 kcal mol−1. Based on some experimental data, the predominant form of calcium thiosulfate in solution is predicted to be the contact ion pair.