A first principles study on the solvation and structure of SO42-(H2O)n, n=6-12

The structures of hydrated sulfate clusters, SO42-(H2O)(n) with n=6-12, are obtained by density functional theory calculations. For SO42-(H2O)(12), two structures with symmetric distribution of H2O molecules around the sulfate group are favored in energy. The structures for the smaller clusters, SO42-(H2O)(n) with n=6-11, are obtained by taking away one H2O molecule successively from the two symmetric SO42-(H2O)(12) isomers. The hydrogen bonding between the sulfate O atoms and H2O molecules are strong. So are the hydrogen bonds among H2O 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 SO42-(H2O)(12) at raised temperature. (C) 2004 American Institute of Physics.