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

Abstract

A species observed in aqueous sulfidic solutions at high T and P has recently been identified as the anion radical S3−, 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 S3− 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 S3− 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 S3−.Our calculations indicate that the reaction:S6-2⇒2S3-in aqueous solution has a free energy change of +3kcal/mol at 298K and 1atm pressure but −13kcal/mol at 723K and 1atm pressure, consistent with the disappearance of disulfide species and the appearance of S3−at high T. Likewise, the free energy for the reaction:2H2S+SO4-2+H+⇒S3-+.75O2+2.5H2Odecreases from 44.1 to 19.0kcal/mol between 298 and 723K (again at 1atm). This is consistent with the decrease in concentrations of SH− and SO4−2 and the formation of S3− 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 34S–32S isotopic fractionations for S3− at 298K give δ values of around +4.3% relative to H2S, a value intermediate between that of S3 and S3−2.Calculated free energies for exchange reactions between S3− and SH− establish that S3− 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 <S–Cu–O while the (S3−)2Cu complex is planar and 4-coordinate at Cu.