On the Calculation of the Absolute Solvation Free Energy of Ionic Species: Application of the Extrapolation Method to the Hydroxide Ion in Aqueous Solution

Abstract

The absolute solvation free energy of the hydroxide ion in aqueous solution was calculated by Monte Carlo simulation and free energy perturbation. We have used the TIP3P model for water and the solute−solvent interaction was modeled as an effective two-body potential of charge−charge plus Lennard-Jones terms fitted to reproduce the interaction energy in the OH-(H2O)3 and OH-(H2O)4 ionic clusters. The electrostatic contribution to the solvation free energy was determined by using solvent boxes having 120, 160, 216, 350, and 512 water molecules, and the limit for N approaching infinity was obtained by an extrapolation procedure. The final solvation free energy obtained by considering the Lennard-Jones potential contribution, correcting for the cutoff surface potential, and including the surface potential of water cluster amounts to −108.0 kcal mol-1, in very good agreement with the experimental value of −105.0 kcal mol-1. This result shows that the extrapolation method coupled with the use of an effective two-body potential is a viable and accurate procedure for calculating the absolute solvation free energy of ionic species.