Calculation of the properties of the S3- radical anion and its complexes with Cu+ in aqueous solution

A species observed in aqueous sulfidic solutions at high T and P has recently been identified as the anion radical S-3(-), based on the Raman spectrum obtained in a hydrothermal diamond-anvil cell (Pokrovski and Dubrovinsky, 2011, Science, 331, 1052-1054). Such a species had not been expected to occur in such environments, although S-3(-) as an component of lapis lazuli, other solids and even albite melt has been well studied (Winther et al., 1998; Reinen and Lindner, 1999; Arieli et al., 2004; Shnitko et al., 2008; Bacci et al., 2009). We have calculated the structures, energetics, vibrational and UV-visible spectra of S-3(-) and several other similar species and confirm the species identification of Pokrovski and Dubrovinsky, although we are still somewhat concerned about the apparent lack of a third peak which we calculate to be present in the Raman spectrum of S-3(-). Our calculations indicate that the reaction: S-6(-2) double right arrow 2S(3)(-) in aqueous solution has a free energy change of +3 kcal/mol at 298 K and 1 atm pressure but -13 kcal/mol at 723 K and 1 atm pressure, consistent with the disappearance of disulfide species and the appearance of S-3(-) at high T. Likewise, the free energy for the reaction: 2H(2)S + SO4-2 + H+ double right arrow S-3(-) + .75O(2) + 2.5H(2)O decreases from 44.1 to 19.0 kcal/mol between 298 and 723 K (again at 1 atm). This is consistent with the decrease in concentrations of SH- and SO4-2 and the formation of S-3(-) observed by Pokrovski and Dubrovinski over this temperature range. The corresponding log K values are in semiquantitative agreement with those found by Pokrovski and Dubrovinsky. The main contribution to these changes in reaction free energy with temperature come from the VRT (vibrational-rotational-translational) contribution to the gas-phase free energy, while the hydration free energy difference changes little. Calculation of S-34-S-32 isotopic fractionations for S-3(-) at 298 K give delta values of around +4.3% relative to H2S, a value intermediate between that of S-3 and S-3(-2). Calculated free energies for exchange reactions between S-3(-) and SH- establish that S-3(-) forms complexes with Cu+ which are similar in stability to its complexes with SH-. The S3Cu(OH2) complex shows two coordination at Cu and a nearly linear