A first principles study on the solvation and structure of SO4 2- (H2O)n, n=6-12

The structures of hydrated sulfate clusters, SO(4) (2-)(H(2)O)(n) with n=6-12, are obtained by density functional theory calculations. For SO(4) (2-)(H(2)O)(12), two structures with symmetric distribution of H(2)O molecules around the sulfate group are favored in energy. The structures for the smaller clusters, SO(4) (2-)(H(2)O)(n) with n=6-11, are obtained by taking away one H(2)O molecule successively from the two symmetric SO(4) (2-)(H(2)O)(12) isomers. The hydrogen bonding between the sulfate O atoms and H(2)O molecules are strong. So are the hydrogen bonds among H(2)O molecules, which are facilitated by the structure of the polyatomic sulfate group. The solvation energy is quite large (often exceeding 15 kcal/mol). The patterns for structural and energy changes as the cluster size increases are very different from the well studied hydrated halide ions, although the competition between solute-solvent and solvent-solvent interactions is again an important factor. Ab initio molecular dynamics simulations also show "crowding" effects in the first solvation of SO(4) (2-)(H(2)O)(12) at raised temperature.