We studied hydrated calcium oxalate and its ions at the restricted Hartree-Fock RHF/6-31G* level of theory. Performing a configurational search seems to improve the fit of the HF/6-31G* level to experimental data. The first solvation shell of calcium oxalate contains 13 water molecules, while the first solvation shell of oxalate ion is formed by 14 water molecules. The first solvation shell of Ca(II) is formed by six water molecules, while the second shell contains five. At 298.15 K, we estimate the asymptotic limits (infinite dilution) of the total standard enthalpies of hydration for Ca(II), oxalate ion and calcium oxalate as -480.78, -302.78 and -312.73 kcal mol(-1), resp. The dissociation of hydrated calcium oxalate is an endothermic process with an asymptotic limit of +470.84 kcal mol(-1).

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.

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where n ≤ 6 ; m ¼ n n > 6 ; m ¼ 6For the process shown in Scheme II, where n = 1-16. The choice of values for m is based on Woon and Dunning [58], who have shown that water preferentially solvates the cation over the anion when adding water molecules, so the first 6 water molecules were used to solvate the Ca(II).The online version of the original article can be found at http://dx.doi.