![]() ![]() ![]() In recent work we have calculated energetics for the formation of such oxidized dimer species which are in good agreement with the experimental data of Helz et al. ![]() They also presented visible-UV absorption spectra which showed a broad peak around 4.4 eV, consistent with the limited experimental data available on Sb(v) sulfides. Recently Helz and coworkers reported the results of a solubility study for stibnite, Sb 2S 3, and elemental S in equilibrium with alkaline sulfidic solutions, which could be best interpreted in terms of a number of dimeric species, including the mixed Sb(III,v) and the Sb(v,v) dimers, Sb 2S 5 2- and Sb 2S 6 2-, which were new species, not previously considered. The Sb-S distances determined by EXAFS were more consistent with those for model compounds with four-coordinate Sb(v) than for those with three-coordinate Sb(III) and the coordination numbers from the model fits to the data were close to 4. However, recent EXAFS studies have presented evidence for the presence of Sb(v) species in such solutions. By 1990 a consensus seemed to emerge that in alkaline sulfidic solutions Sb existed as Sb(III), based on numerous solubility studies and Raman studies. Typically Sb(III) compounds, which essentially have a 5 s 2 lone pair orbital, will be trigonal three-coordinate, while Sb(v) compounds, without the lone pair, will be tetrahedral four-coordinate. The main questions concern the oxidation state (III or v), the coordination number and the degree of oligomerization of the species. The speciation of Sb in sulfidic solutions has been studied for some time, but new results are still emerging. In neutral to alkaline sulfidic waters at low temperature, thio- species are believed to predominate. In hydrothermal solutions, As and Sb are often present in appreciable concentration, often occurring in association with Ag, Au and Hg, but the identities of the As and Sb species present are not well understood. Our results are in excellent agreement with available experimental data, and support the existence of a S 6 species. We have also calculated the aqueous phase energetics for the reaction of S 8 with SH - to produce the polysulfides, S nH -, n = 2–6. Our results indicate that for the As and Sb sulfides, the oxidation state, degree of protonation and degree of oligomerization can all be determined from the visible-UV absorption spectrum. The polysufides, on the other hand, show no simple systematic changes in UV spectra with chain length, n, or with protonation state. There is also a small reduction in absorption energy from monomeric to dimeric species, for both As and Sb III and v. As and Sb(v) species, both monomers and dimers, also absorb at characteristically higher energies than do the analogous As and Sb(III)species. ![]() We correctly reproduce the general trends observed experimentally, with absorption energies increasing from polysulfides to As, Sb sulfides to SH - to H 2S. We have used both Hartree-Fock based (CI Singles and Time-Dependent HF) and density functional based (TD B3LYP) techniques for the calculations of absorption energy and intensity and have used both explicit water molecules and a polarizable continuum to describe the effects of hydration. In general, small and/or highly charged anions are more difficult to describe computationally than larger, monovalent anions or neutral molecules. Many of these calculations are more difficult than those performed for the As(iii) bisulfides, since the As and Sb(v) species are more acidic and therefore exist as highly charged anions in neutral and basic solutions. We here extend these studies to As and Sb oxidation state III and v sulfides and to polysulfides S n 2-, n = 2–6, the bisulfide anion, SH -, hydrogen sulfide, H 2S and the sulfanes, S nH 2, n = 2–5. The calculated lowest energy transitions for these species were diagnostic of their protonation and oligomerization state. Recently we showed that visible-UV spectra in aqueous solution can be accurately calculated for arsenic (III) bisulfides, such as As(SH) 3, As(SH) 2S - and their oligomers. ![]()
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