The Hopping Mechanism of the Hydrated Excess Proton and Its Contribution to Proton Diffusion in Water

TL;DR

This study uses ab initio molecular dynamics to analyze proton transport in water, revealing that rattling dominates hopping events and supporting the special-pair dance model, with implications for understanding proton diffusion mechanisms.

Contribution

The paper provides a detailed analysis distinguishing rattling from true hopping in proton transport, introducing a filtering method to clarify the hopping mechanism in water.

Findings

Concerted hopping occurs in all simulations.

Most hopping events are due to proton rattling.

Filtering reduces apparent multiple hopping events.

Abstract

In this work a series of analyses are performed on ab initio molecular dynamics (AIMD) simulations of a hydrated excess proton in water to quantify the relative occurrence of concerted hopping events and <span>rattling</span> events, and thus to further elucidate the hopping mechanism of proton transport in water. Contrary to results reported in certain earlier papers, the new analysis finds that concerted hopping events do occur in all simulations, but that the majority of events are the product of proton rattling, where the excess proton will rattle between two or more waters. The results are consistent with the proposed <span>special-pair dance</span> model of the hydrated excess proton, wherein the acceptor water molecule for the proton transfer will quickly change (resonate between three equivalent special pairs) until a decisive proton hop occurs. To remove the misleading effect of simple rattling, a filter was applied to the trajectory such that hopping events that were followed by back hops to the original water are not counted. A steep reduction in the number of multiple hopping events is found when the filter is applied, suggesting that many multiple hopping events that occur in the unfiltered trajectory are largely the product of rattling, contrary to prior suggestions. Comparing the continuous correlation function of the filtered and unfiltered trajectories, we find agreement with experimental values for the proton hopping time and Eigen-Zundel interconversion time, respectively.