Features of Ar solvation shells in neutral and ionic clustering: the competitive role of two-body and many-body interactions.

The semiempirical methodology, introduced to describe noncovalent intermolecular interactions in atom/ion-molecule systems, is here extended to investigate a prototype cluster, formed by benzene (Bz) and closed-shell ions (Na(+) and/or Cl(-)), surrounded by neutral species (Ar), forming solvation shells. The involved multidimensional potential energy surface (PES) is assumed to depend on a critical balancing of some effective interaction components. In particular, for the Ar solvated Bz-Na(+)-Cl(-) system, the nonelectrostatic component of the total interaction has been formulated as a combination of two-, three-, and four-body contributions, each one represented by a proper function, with the four-body and part of the three-body terms arising from nonadditive induction effects. The proposed formulation, in which the induction is included both implicitly and explicitly, ensures the accurate description of all dissociation channels, leading to simpler clusters and/or pure solvent. Some properties of the solvent, represented by an ensemble of 500 Ar atoms, have been analyzed by performing molecular dynamics simulations at several temperatures. The obtained results have been found to be consistent with experimental observations. In order to investigate propensities, similarities, and differences in the competing clusters, the Ar solvation shells of Bz, Bz-Na(+), Bz-Cl(-) and Bz-Na(+)-Cl(-) have been characterized. The inspection of the solvation shell of Bz allows one to distinguish between groups of Ar atoms occupying positions on and out of the plane defined by the aromatic ring. Regarding the solvation shells of Bz-Na(+) and Bz-Cl(-), it has been observed that they are strongly affected by the most stable structures of the unsolvated systems. However, Bz-Na(+) shows more compact solvation shells than Bz-Cl(-). Finally, important asymmetries, basically promoted by the additional many-body induction effects on the solvent atoms, have been observed in the solvation shells of Bz-Na(+)-Cl(-).