Lifetimes of Excess Protons in Water Using a Dissociative Water Potential

Abstract

Molecular dynamics simulations using a dissociative water potential were applied to study transport of excess protons in water and determine the applicability of this potential to describe such behavior. While originally developed for gas-phase molecules and bulk liquid water, the potential is transferrable to nanoconfinement and interface scenarios. Applied here, it shows proton behavior consistent with ab initio calculations and empirical models specifically designed to describe proton transport. Both Eigen and Zundel complexes are observed in the simulations showing the Eigen-Zundel-Eigen-type mechanism. In addition to reproducing the short-time rattling of the excess proton between the two oxygens of Zundel complexes, a picosecond-scale lifetime was also found. These longer-lived H3O(+) ions are caused by the rapid conversion of the local solvation structure around the transferring proton from a Zundel-like form to an Eigen-like form following the transfer, effectively severing the path along which the proton can rattle. The migration of H(+) over long times (>100 ps) deviates from the conventional short-time multiexponentially decaying lifetime autocorrelation model and follows the t(-3/2) power-law behavior. The potential function employed here matches many of the features of proton transport observed in ab initio molecular dynamics simulations as well as the highly developed empirical valence bond models, yet is computationally very efficient, enabling longer time and larger systems to be studied.

Potentials of mean force in acidic proton transfer reactions in constrained geometries

“…The first maximum, centered around 2.5Å in all cases, should be attributed to the first solvation shell of the proton. The location of this maximum is in good agreement with the findings of Bankura and Chandra [41] and also with the RDF reported in the work of Lockwood and Garofalini [42] where the authors show that, within the framework of their potential model and at ambient conditions, the first maximum in (O * -O) is located at slightly smaller distances that the maximum of oxygen-oxygen RDF when all oxygens are considered. This is called "inward shift" and indicates that the first solvation shell of the proton is significantly smaller than that of oxygens in pure water.…”

“…The comparison of these results to those of Lockwood and Garofalini indicates that the size of the free energy barriers is in overall good agreement: in the present work the obtained value is of about ∼ 4k B T = 2.3 kcal/mol (at 300 K) for the unconstrained setup, that is of the same order of magnitude of the values reported in Ref. [42] (between 0.8 and 1.2 kcal/mol).…”

“…This fact has been recently related to three-body interactions by Wiedemair et al [40]. Further, this feature was also previously observed in our simulations [43] and it suggests the equivalence of the present MS-EVB simulations with the MD calculations with the dissipative potential [42] regarding the meaning of local proton structures. In our case, this first maximum of the RDF is essentially invariable, regardless of T or d. This fact gives an indication of the strength of the proton to create a first solvation shell which remains virtually invariable at all conditions.…”

“…First of all, this feature is located at distances closer than those reported elsewhere [41,42] including the observation (see [43]) of the compression of the local coordination of the proton (the "inward shift" by Lockwood and Garofalini [42]). Second, at the LDA state the maximum is centered at 4.2Å in all cases but moves to 3.5Å when d = 0.7 nm.…”

“…Finally, Car-Parrinello simulations by Bankura and Chandra [41] indicated that in water inside narrow two-dimensional environments the lone proton is normally solvated as the Eigen cation and that PT rates are much lower than in the quasi-one-dimensional case described above. However, few works have focussed on the free energy barriers of PT and their relationship to the local structure of the proton [42]. Although there is plenty of information on microscopic properties of lone protons in constrained water, the influence of a variety of factors on them, from variations in thermal energy to hydrophobicity effects makes the problem very difficult to handle.…”