Quantum and simulation studies of X-(H2O)n systems

This paper reports on studies of the interactions of halide ions with water. The standard Hartree-Fock (HF) method was used to calculate the interaction between each of the four halide ions and the water monomer. The structural properties of the X--H2O systems (X=F, Cl, Br, I) are presented with a detailed comparison with experimental energies. A new ion-water parameterised potential, derived from quantum calculations, is proposed for the description of the X--H2O interactions in simulations. This potential was used in Monte Carlo (MC) studies of the gas-phase formation of X-(H2O)(n) clusters (n = 1,..., 10) and of the solvation of the ions in dilute aqueous solutions. Thermodynamic properties, such as enthalpies, Delta H-n-1,H-n, Gibbs free energies, Delta G(n-1,n), and entropies, Delta S-n-1,S-n, are presented for the gas-phase reactions: X-(H2O)(n) + H2O reversible arrow X-(H2O)(n). The results follow the general experimental trends, but overestimate their absolute values for the smaller clusters. The structural properties of the small clusters were found to be in good agreement with the results of quantum calculations. For small n, the so-called surface (S) structure was found to be predominant, while for larger n the interior (I) structure is preferred. The transition from an (S) structure to an (I) structure was found to occur for n between 4 and 6, depending on the ion. In solution, the energy of solvation and the structural properties of each ion are reported and compared with the experimental data available. The energy values were found to be in good agreement with estimates reported for the three larger ions, while for fluoride they are slightly overestimated. (C) 1999 Elsevier Science Ltd. All rights reserved.