Study of the Ag+ hydration by means of a semicontinuum quantum-chemical solvation model

The changes in the distance between the cation and the oxygen of the first water shell (M-Or) induced by the rest of the solvent and the hydration structure of Ag+ have been theoretically studied using a mixed discrete-continuum model of solvation. Ab initio calculations at the MP2 level for [Ag(H2O)(n)](+) clusters (n = 1, 2, 4, and 12, the last one formed by two water shells (4 + 8)) in gas phase and solution were carried out with DZ+polarization basis sets and Stevens et al.'s pseudopotentials. The bulk solvent was simulated by means of Nancy's group continuum solvation model. The clusters were placed in a cavity surrounded by a continuum with the static dielectric permittivity of the water. Geometry optimization was performed in all cases. Calculations allow the examination of the specific interaction effects on the first solvation shell due to the hydrogen-bonded water molecules of the second shell as well as the long-range interactions of the bulk solvent, described as a dielectric continuum. Likewise, the combination of both effects is studied by the explicit consideration of a Ag+ polyhydrate containing two hydration shells, [Ag(H2O)(12)](+), immersed in a cavity. Opposite effects on the Ag-Or distance were observed by the specific and long-range (continuum) solvent interactions. Specific interactions, mainly hydrogen bonding, shorten the bond, whereas long-range interactions lengthen it, leading to a mutual partial cancellation of the effects when the two types of interactions are jointly considered. Contributions to the Ag+ hydration enthalpy have also been examined in terms of the semicontinuum model.