Study of the structure of molecular complexes. VIII. Small clusters of water molecules surrounding Li+, Na+, K+, F−, and Cl− ions

Abstract

The two-body Hartree-Fock potential for the ion-water interaction and the two-body Hartree-Fock potential for the water-water interaction have been used in the pairwise additivity approximation to study the Li+(H2O)n, the Na+(H2O)n, the K+(H2O)n, the F−(H2O)n, and the Cl−(H2O)n complexes, where n =2,3,4, ⋯ ,10. The complex configurations have been constrained to have either symmetrical geometries around the central ion or to be free to assume the lowest energy configuration. For n smaller than 5 (depending on the specific ion in consideration), the symmetrical configuration is the lowest energy configuration. For higher values of n, some of the water molecules tend to form a second shell of solvated water around the ion. The configurational optimalization was carried out only at T =0°K; but for a small cluster containing only four molecules of water, calculations have been performed at T =298°K. From the study at 298°K we have computed the correlation functions gI–O, gI–H, gO–O, gO–H, and gH–H (where the subscript I is a shorthand notation for ``ion''). Correlation functions are reported for the cluster F−(H2O)n at T =298°K with n =27. By comparing the results obtained at T =0°K and with n =10, with those obtained at T =298°K and n =4 and finally with the results obtained at T =298°K and n =27, we feel confident that the conclusions (given below) will remain valid for n >27 and at t ≠0°K. The coordination numbers for the ion-water clusters are computed (approximatively) to be about 4 for Li+, between 5 and 6 for Na+, between 5 and 7 for K+, between 4 and 6 for F, and between 6 and 7 for Cl−. In the first solvation layer, the average ion oxygen distances are 1.9–2.0 Å for Li+, 2.3–2.4 Å for Na+, 2.8–2.9 Å for K+, 2.7–2.8 Å for F−, and 3.4–3.5 Å for Cl−. The validity of the pairwise additivity approximation has been tested for the small clusters ion (H2O)n with n =2,3 (and 4 and 5 for Li+ and F−).