Heidelberg simulations tie proton hopping in water to local molecular asymmetry

Heidelberg University researchers, working with colleagues including at Ruhr University Bochum, used full-dimensional quantum simulations to show that proton hopping in water is governed mainly by local asymmetries in surrounding water molecules rather than fixed idealized structures, phys.org reported of a Nature Chemistry paper. Proton transport in water, known as the Grotthuss mechanism, matters for acidity, batteries, and biological signal transmission. Hydrated protons have long been described with Zundel cations, where a proton is shared by two water molecules, and Eigen cations, where a hydronium core bonds to three further waters. First author Dr. David Mendive-Tapia said recent infrared work points to a more dynamic state between those extremes. The team simulated an extended Zundel complex with six water molecules and tracked 51 interlocking vibrations with full quantum resolution, reproducing the experimental infrared spectrum. Forces between atoms came from an artificial neural network trained in Bochum on high-level quantum chemical data, without adjustable parameters. Dr. Oriol Vendrell said the infrared fingerprint of the hydrated proton and its hopping are governed above all by local asymmetries in its surroundings. The paper's DOI is 10.1038/s41557-026-02209-3.