Study of the structure of molecular complexes - XIV. Coordination numbers for selected ion pairs in water

Abstract

A cluster of 200 molecules of water containing one of the LiF, LiCl, NaF, NaCl, KF or KCl ion pairs has been studied at the temperature T= 298°K using Monte Carlo techniques. The anion-cation internuclear separations considered in this work for any of the above pairs are 6.0 Å, 8.0 Å and 10.0 Å. The water-water potential is obtained from quantum-mechanical Hartree-Fock type computations corrected by inclusion of dispersion forces; the ion-water potentials have been obtained from Hartree-Fock type computations on the single ion-water complex. The computed radii for the first hydration shell are 2.7±0.1 Å, 3.4±0.3 Å, 4.0±0.3 Å, 3.0±0.5 Å, and 3.9±0.4 Å, for Li+, Na+, K+, F- and Cl-, respectively. The computed coordination numbers are 5.4±0.7,6.0±1.1, 7.2±1.2,4.5±0.6 and 5.1±0.8 for the same ions, respectively. The range of the coordination number obtained from compressibility, enthalpy, NMR spectroscopy and other experimental methods is much larger than the error ranges above given. Therefore the Monte Carlo simulation provides reliable information on the cluster shape, cluster structure and on the coordination numbers and hydration shell radii for the cations and anions, when both are present in a water cluster. © 1976 Springer-Verlag.