A multi-state empirical valence bond model for weak acid dissociation in aqueous solution

The development and application of a multistate empirical valence bond (MS-EVB) model for a weak acid dissociation and subsequent proton transport in aqueous solution is described. The weak acid dissociation step is modeled by the inclusion of an additional EVE state describing the case when proton is bound to the acid's conjugate base. The model was parametrized for the imidazolium cation deprotonation. Classical molecular dynamics simulation methodology was used to study both equilibrium and dynamic properties of this system. Free energy profiles of the deprotonation reaction, studied using a novel center of excess charge reaction coordinate, reveal the need to include several solvation shells around the weak acid in order to stabilize the hydronium species formed upon the weak acid deprotonation. The solvent atomic density plots examined at selected points along the proton transfer coordinate display a relatively large reorganization of the solvent around the weak acid molecule, caused by the shift in the weak acid molecule atomic point charges caused by the deprotonation. Finally, since the concentration of the weak acid in the system under study is low, its presence has only mimimal effect on the solvent diffusion and on the transfer dynamics of the excess proton in the water solution after the weak acid dissociation step.