THE SPECIATION OF ANTIMONY IN SULFIDIC SOLUTIONS - A THEORETICAL-STUDY

To assist in identifying the Sb sulfide species present in alkaline sulfide solutions, we have used ab initio quantum mechanical methods to calculate the structures, stabilities, and vibrational spectra of a number of monomeric and oligomeric Sb(III) sulfides. In agreement with the interpretation of Wood (1989), we assign a prominent feature observed at 369 cm-1 in the Raman spectrum of sulfidic Sb solutions to Sb-S stretching vibrations in a monomeric complex, although our calculations are most consistent with its assignment to the SbS2(SH)-2 complex, rather than the fully deprotonated complex SbS3(-3). A shoulder observed at 380 cm-1 is best assigned to SbS(SH)2-. Raman features observed at 314 and 350 cm-1 are assigned to Sb-S (H) symmetric stretching vibrations of the dimeric species Sb2S2(SH)2, which is calculated to be thermodynamically stable, with respect to both the monomer Sb(SH)3 and the trimer Sb3S3(SH)3. The mixed-ligand complex Sb2S2(OH)2 is calculated to become stable compared to Sb2S2(SH)2 at high temperatures, in agreement with experimental solubility data. The Sb sulfide monomers are found to H-bond to water through their -SH or -S groups, but with only small changes in the Sb-S distances and Sb-S stretching frequencies. Accurate gas-phase proton affinities and estimated solution proton affinities are presented for the anionic species in solution, and the estimated energetics are consistent with the presence of SbS2(SH)-2, SbS(SH)2(-1), and Sb2S2(SH)2 suggested by the Raman data.