A Multi-State Empirical Valence Bond Model for Weak Acid Dissociation in Aqueous Solution

Abstract

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 EVB 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.

Perspectives for hybrid ab initio/molecular mechanical simulations of solutions: from complex chemistry to proton-transfer reactions and interfaces

“…0.1-0.13 nm). The diffusion coefficients determined for an excess proton in water treated with the Garofalini model were found as 3.9 ×10 -9 m 2 s -1 , which again is well within the range determined by different MS-EVB models [110][111][112][113][114] being 2.9-8.3 ×10 -9 m 2 s -1 . Again, the deviation to the experimental value determined as 9.3 ×10 -9 m 2 s -1 [120,121] can be explained by inaccuracies in the model as well as the inability of classical MD simulations to include nuclear quantum effects.…”

“…However, in recent years the development of reactive FF techniques [105][106][107][108][109] has enjoyed increasing interest. In addition to the widely used multistate empirical valence bond (MS-EVB) approaches [110][111][112][113][114], a number of dissociate FF descriptions treating intra-and intermolecular contributions on the same footing have been developed, among which the model of Garofalini and co-workers [107,115,116] proved to be particularly useful for the application in the context of a QM/MM simulation.…”

Computer simulation of explicit proton translocation in cytochrome c oxidase: The D-pathway

“…Because of the delocalized nature of the excess proton, one can define a suitable coordinate to describe the position of the ''excess proton'' at each step, given by the ''center of excess charge'' (CEC) (33):…”

Aqueous bimolecular proton transfer in acid–base neutralization

“…As explained in historical overviews [17,[33][34][35][36][37] this work does not deal explicitly with transmission of protons (H + ) through water (H 2 O), as protons as elementary particles still had to await discovery. Nevertheless the explanation of diffusion of negative and positive (denoted as o or h, respectively) charged bodies between a chain of neighbouring water molecules by von Grotthuss is a rudimentary -but an important -foundation in the current understanding of the anomalously fast proton transmission as a topological defect along a hydrogen bonded chain of water molecules [26], which can occur not only in bulk water or other protic solvents [38][39][40][41][42][43][44][45][46] (we refer to the reviews [16,17] for a complete overview on this), but also in acid dissociation in water [47][48][49][50][51][52][53][54][55][56][57], in acid-base neutralization reactions [58][59][60][61][62][63][64][65][66][67][68], in fuel cells [69,70], and in protein ion channels in biological membranes …”