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 i n its ionized state by inserting a chloride counter ion into the protonated water solution. Both equilibrium and dynamical properties differ only slig htly from the previously studied isolated excess proton in water. The free- energy profile as a function of the separation between the excess charge an d chlorine atom reveal minimal barrier for the anion-excess charge separati on. To model the weak acid, protonated imidazole, the MS-EVE model was exte nded 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 wea k acid deprotonation show that several solvation shells around the weak aci d molecule need to be included in the EVE model to correctly describe the s tabilization of the solvated species. Structurally, one water molecule is c oordinated to the proton donor in the protonated acid case, while two water molecules coordination is likely when the acid is deprotonated. (C) 2000 E lsevier Science B.V. All rights reserved.