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

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+(H 2O)n, the Na+(H2O)n, the K+(H2O)n, the F-(H 2O)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 g1-O, g1-H, gO-O, g O-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-). Copyright © 1974 American Institute of Physics.