Energy-structure relationships for microscopic solvation of anions in water clusters

In this paper we present a quantum-mechanical study of anions in water clusters, X^- (H₂O)_n (X=Cl, Br, I, and n=1-6). Molecular orbital calculations at the self-consistent field (SCF) level and at the second-order Møller-Plesset (MP2) level were performed using extended basis sets. Full structural optimization was conducted at the MP2 level for R=1 and at the SCF level for n=2-6. The energies and charge distribution of X ^- (H₂O) were calculated at the MP2 level, while the energies of the X^-(H₂O)_n (n=2-6) clusters were calculated at the MP2 level using the SCF optimized geometry. Calculations of total and sequential enthalpies of hydration and for the vertical ionization potentials were conducted for X-(H₂O), the hydrogen bonded and linear isomers of X^-(H₂O)₂, the pyramidal structure of X^-(H₂O)³, and the interior and surface isomers of X^-(H₂O)_n, n=4-6. The calculated hydration enthalpies account well for their experimental size dependence for n=1-6. However, the isomer specificity of the hydration enthalpies is reflected by a small energy difference (δ=1-5 kcal mol^-1) between the surface and interior isomers at a fixed n, precluding the assignment of structural isomers on the basis of ground-state energetics. The cluster size dependence and isomer specificity of the calculated vertical ionization potentials in conjunction with experimental data provide a diagnostic tool for the structural assignment of isomers and for the distinction between surface and interior structures. The central prediction emerging from the structure-energetic relations based on cluster size dependence and isomer specificity of vertical ionization potentials, is the prevalence of surface structures for Cl^-(H₂O)_n (n=2-6), Br ^-(H₂O)_n (n=2-6), and I^-(H ₂O)_n (n=2-5), while a "transition" from surface to interior structure may be exhibited for I^-(H₂O) ₆.