Solvation properties of microhydrated sulfate anion clusters: insights from ab initio calculations

Sulfate-water clusters play an important role in environmental and industrial processes, yet open questions remain on their physical and chemical properties. We investigated the smallest hydrated sulfate anion clusters believed to have a full solvation shell, with 12 or 13 water molecules. We used ab initio molecular dynamics and electronic structure calculations based on density functional theory, with semilocal and hybrid functionals. At both levels of theory we found that configurations with the anion at the surface of the cluster are energetically favored compared to fully solvated ones, which are instead metastable. We show that infrared spectra of the anion with different solvation shells have similar vibrational signatures, indicating that a mixture of surface and internally solvated geometries are likely to be present in the experimental samples at low temperature. In addition, the computed electronic density of states of surface and internally solvated clusters are hardly distinguishable at finite temperature, with the highest occupied molecular orbital belonging to the anion in all cases. The equilibrium structure determined for SO(4)(2-)·(H(2)O)(13) differs from that previously reported; we find that the addition of one water molecule to a 12-water cluster modifies its hydration shell and that water-water bonds are preferred over water-anion bonds.