PNNL

Abstract

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 Pairs (SSIP) in an aqueous solution 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 is several instances. The binding motif of these ion pair aqueous clusters starts off as networks in which the water molecules “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)