ZundEig: The Structure of the Proton in Liquid Water From Unsupervised Learning

TL;DR

This paper uses unsupervised learning to analyze the structure of excess protons in liquid water, revealing a single charged defect and neutral clusters, challenging traditional Eigen and Zundel models.

Contribution

It introduces an agnostic, data-driven approach to characterize proton environments, showing the charged defect as an ionic topological defect rather than distinct states.

Findings

Only a single charged cluster observed, not multiple states.

Charged defect acts as an ionic topological defect in water.

Proposes the ZundEig framework combining charged and neutral defects.

Abstract

The structure of the excess proton in liquid water has been the subject of lively debate from both experimental and theoretical fronts for the last century. Fluctuations of the proton are typically interpreted in terms of limiting states referred to as the Eigen and Zundel species. Here we put these ideas under the microscope taking advantage of recent advances in unsupervised learning that use local atomic descriptors to characterize environments of acidic water combined with advanced clustering techniques. Our agnostic approach leads to the observation of only a single charged cluster and two neutral ones. We demonstrate that the charged cluster involving the excess proton, is best seen as an ionic topological defect in water's hydrogen bond network forming a single local minimum on the global free-energy landscape. This charged defect is a highly fluxional moiety where the idealized Eigen and Zundel species are neither limiting configurations nor distinct thermodynamic states. Instead, the ionic defect enhances the presence of neutral water defects through strong interactions with the network. We dub the combination of the charged and neutral defect clusters as ZundEig demonstrating that the fluctuations between these local environments provide a general framework for rationalizing more descriptive notions of the proton in the existing literature.