A multi-state empirical valence bond model for acid-base chemistry in aqueous solution

Classical molecular dynamics simulations in conjunction with a multi-state empirical valence bond (MS-EVB) model are used to study proton transport in strong and weak acid aqueous solutions. The strong acid, HCl, is modeled in its ionized state by inserting a chloride counter ion into the protonated water solution. Both equilibrium and dynamical properties differ only slightly from the previously studied isolated excess proton in water. The free-energy profile as a function of the separation between the excess charge and chlorine atom reveal minimal barrier for the anion-excess charge separation. To model the weak acid, protonated imidazole, the MS-EVE model was extended to include the protonated form of the acid in the EVE description, so that the dissociation step can be studied. Free energy profiles for the weak acid deprotonation show that several solvation shells around the weak acid molecule need to be included in the EVE model to correctly describe the stabilization of the solvated species. Structurally, one water molecule is coordinated to the proton donor in the protonated acid case, while two water molecules coordination is likely when the acid is deprotonated. (C) 2000 Elsevier Science B.V. All rights reserved.