OLIGOMERIZATION IN AS(III) SULFIDE SOLUTIONS - THEORETICAL CONSTRAINTS AND SPECTROSCOPIC EVIDENCE

Bond distances, vibrational frequencies, gas-phase energetics, and pro ton affinities for various thioarsenite molecules and ions are predict ed from molecular orbital theory and used to interpret EXAFS and Raman spectra of dissolved thioarsenites in undersaturated, alkaline 1 M Na HS solutions. From MO predictions, Raman peaks at 325 and 412 cm(-1) a re assigned to AsS(SH)(2)(-) and a peak at 382 cm(-1) to AsS2(SH)(2-). At alkaline pH, As-S distances in dissolved thioarsenites are 2.21-2. 23 Angstrom and no statistically significant As-As interactions are re corded, consistent with predominance of the monomers, AsS(SH)(2)(-) an d AsS2(SH)(2-). Estimated proton affinities suggest that thioarsenites with a negative charge greater than 2 are unstable in water. In seemi ng contradiction to this spectroscopic evidence, a new analysis of pub lished solubility studies reinforces previous inferences that the trim er, As3S4(SH)(2)(-), is the predominant thioarsenite in systems satura ted with As2S3. Previously proposed dimeric species of the form, HxAs2 S4x-2, are rejected based on predicted thermodynamic properties. Dimer plus tetramer combinations also are rejected. Estimated free energies for AsS(OH)(SH)(-) and AsS(SH)(2)(-) are presented. We reconcile the spectroscopic and solubility evidence by showing that in undersaturate d solutions monomers can become thermodynamically favored over oligome rs. This pattern should be looked for in other sulfide systems as well . Sulfidic natural waters are in many cases undersaturated with respec t to As2S3 phases, so monomeric thioarsenites could be more important in nature than the trimers that have been characterized in saturated s olutions. EXAFS spectra show that amorphous As2S3 resembles orpiment i n the first shell around As, but that higher shells are disordered. Di sorder may be caused by occasional realgar-like, As-As bonds, consiste nt with the observation that amorphous As2S3 is slightly S deficient.