The Many-Body Expansion for Aqueous Systems Revisited: IV. Stabilization of Halide-Anion Pairs in Small Water Clusters.

We report the structures, energetics, many-body effects, and vibrational spectra of water clusters stabilizing pairs of halide-anions, X^-(H₂O)_kY^- (k = 2-6, X/Y = F, Cl, Br, I) as well as their stability in the gas phase relative to fragmentation. We find that these metastable cluster structures mimicking the solvent-separated ion pair (SSIP) configurations in aqueous solutions are less stable relative to fragmentation into smaller ionic aqueous clusters containing a single halide anion. The many-body expansion (MBE) at these geometries was found to converge at the 4-body term, which is, however, significant, amounting to >20% of the total binding energy in several instances. The binding motif of these ion pair aqueous clusters starts as networks in which the water molecules form a "bridge" between the two halide-anions. As the cluster grows, these structures become destabilized by a more repulsive 3-body term for distances R(O-O) < 3.45 Å with respect to networks in which water molecules move outside the bridge, solvating the other side of the anion.