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

Abstract

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.