CALCULATION OF THE ENERGETICS FOR THE OXIDATION OF SB(III) SULFIDES BY ELEMENTAL S AND POLYSULFIDES IN AQUEOUS SOLUTION

Abstract

Recent experimental studies have reported the existence of two new Sb sulfide species, Sb2S52− and Sb2S62−, in alkaline sulfidic solutions in equilibrium with stibnite, Sb2S3, and orthorhombic S. These species contain Sb(V), which has also recently been identified in similar solutions using EXAFS by other researchers. This represents a significant change from the consensus a decade ago that sulfidic solutions of Sb contained only Sb(III) species. I have calculated from first principles of quantum mechanics the energetics for the oxidation of the Sb(III) sulfide dimer Sb2S42− to the mixed Sb(III,V) dimer Sb2S52− and then to the all Sb(V) dimer, Sb2S62−. Gas-phase reaction energies have been evaluated using polarized valence double zeta effective core potential basis sets and Moller-Plesset second order treatments of electron correlation. All translational, rotational and vibrational contributions to the gas-phase reaction free energy have been calculated. Hydration energies have been obtained using the COSMO version of the self-consistent reaction field polarizable continuum method. Negative free energy changes are calculated for the oxidation of the dianion of the III,III dimer to the III,V dimer by both small polysulfides, like S4H−, and elemental S, modeled as S8. For the further oxidation of the III,V dimer to the V,V dimer the reaction free energies are calculated to be close to zero. The partially protonated Sb III,III dimer monoanion HSb2S4− can also be oxidized, but the reaction is not so favorable as for the dianion. Comparison of the calculated aqueous deprotonation energies of H2Sb2S4, H2Sb2S5 and H2Sb2S6 and their dianions with values calculated for various oxyacids indicates that the III,V and V,V dimers will have pKa2 values