ION SOLVATION IN CLUSTERS

Solvent size effects on ion solvation in clusters of polar molecules are explored. Expressions for the adiabatic and the vertical solvation energies of an ion imbedded in a cluster are derived within the framework of the mean-spherical approximation (MSA). The short-range order in the finite system is assumed to be similar to that in the bulk liquid, allowing to use the bulk MSA pair correlation function. The solvent shell structure is manifested in the size dependence of the solvation energy for several (4-5) solvent shells. The hydration energies and the vertical electron photodetachment threshold energies of halide anions in water clusters were evaluated. The bulk values of these physical parameters for large anions (I- and Br-) are in good agreement with experimental data. The predicted size dependence of the vertical photodetachment threshold, I(v)(n), is in good agreement with experimental data for the small I(H2O)n- clusters (1 less-than-or-equal-to n less-than-or-equal-to 15). For intermediate sized clusters (12 less-than-or-equal-to n less-than-or-equal-to 50) the theory predicts plateaus and local minima in the absolute values of solvation energy and I(v) (n) due to the parallel solvent dipoles alignment in the distant solvation shells. For large clusters (n > 125) the finite size correction to the solvation energy scales inversely with the cluster size ( is-proportional-to n-1/3). This behavior, as well as the value of the slope, are in agreement with the prediction of the continuum dielectric (CD) model. The absolute values of the solvation energy and I(v)(n) predicted by MSA are significantly lower than within the CD model.