STUDY OF THE AG-CHEMICAL SOLVATION MODEL( HYDRATION BY MEANS OF A SEMICONTINUUM QUANTUM)

The changes in the distance between the cation and the oxygen of the f irst water shell (M-Or) induced by the rest of the solvent and the hyd ration structure of Ag+ have been theoretically studied using a mixed discrete-continuum model of solvation. Ab initio calculations at the M P2 level for [Ag(H2O)(n)](+) clusters (n = 1, 2, 4, and 12, the last o ne 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 pseud opotentials. The bulk solvent was simulated by means of Nancy's group continuum solvation model. The clusters were placed in a cavity surrou nded by a continuum with the static dielectric permittivity of the wat er. Geometry optimization was performed in all cases. Calculations all ow the examination of the specific interaction effects on the first so lvation shell due to the hydrogen-bonded water molecules of the second shell as well as the long-range interactions of the bulk solvent, des cribed as a dielectric continuum. Likewise, the combination of both ef fects is studied by the explicit consideration of a Ag+ polyhydrate co ntaining 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 intera ctions lengthen it, leading to a mutual partial cancellation of the ef fects when the two types of interactions are jointly considered. Contr ibutions to the Ag+ hydration enthalpy have also been examined in term s of the semicontinuum model.