Correlated Dynamics in Aqueous Proton Diffusion

TL;DR

This study uses ab initio simulations to reveal that proton hopping in water is correlated and not equally probable in all directions, challenging previous assumptions and suggesting a shorter Grotthuss mechanism timescale.

Contribution

The paper demonstrates that proton hopping in water exhibits correlated behavior, contradicting the assumption of equal probability in all directions, thus revising the understanding of proton diffusion dynamics.

Findings

Protons are more likely to revert to their previous location during hopping.

The Grotthuss mechanism occurs on a shorter timescale than previously estimated.

Proton diffusion involves correlated hopping rather than independent random steps.

Abstract

The aqueous proton displays an anomalously large diffusion coefficient that is up to 7 times that of similarly sized cations. There is general consensus that the proton achieves its high diffusion through the Grotthuss mechanism, whereby protons hop from one molecule to the next. A main assumption concerning the extraction of the timescale of the Grotthuss mechanism from experimental results has been that, on average, there is an equal probability for the proton to hop to any of its neighboring water molecules. Herein, we present ab initio simulations that show this assumption is not generally valid. Specifically, we observe that there is an increased probability for the proton to revert back to its previous location. These correlations indicate that the interpretation of the experimental results need to be re-examined and suggest that the timescale of the Grotthuss mechanism is significantly shorter than was previously thought.