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 interacti on between each of the four halide ions and the water monomer. The structur al properties of the X--H2O systems (X=F, Cl, Br, I) are presented with a d etailed comparison with experimental energies. A new ion-water parameterise d potential, derived from quantum calculations, is proposed for the descrip tion 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 solution s. Thermodynamic properties, such as enthalpies, Delta H-n-1,H-n, Gibbs fre e energies, Delta G(n-1,n), and entropies, Delta S-n-1,S-n, are presented f or the gas-phase reactions: X-(H2O)(n) + H2O reversible arrow X-(H2O)(n). T he results follow the general experimental trends, but overestimate their a bsolute values for the smaller clusters. The structural properties of the s mall clusters were found to be in good agreement with the results of quantu m calculations. For small n, the so-called surface (S) structure was found to be predominant, while for larger n the interior (I) structure is preferr ed. The transition from an (S) structure to an (I) structure was found to o ccur for n between 4 and 6, depending on the ion. In solution, the energy o f solvation and the structural properties of each ion are reported and comp ared with the experimental data available. The energy values were found to be in good agreement with estimates reported for the three larger ions, whi le for fluoride they are slightly overestimated. (C) 1999 Elsevier Science Ltd. All rights reserved.