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 Mo/ller–Plesset (MP2) level were performed using extended basis sets. Full structural optimization was conducted at the MP2 level for n=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−(H2O)n (n=2–5), while a ‘‘transition’’ from surface to interior structure may be exhibited for I−(H2O)6.