Hydrated proton (yellow/green) with six water molecules (blue/grey): Quantum simulations of an extended Zundel complex provide new insight into how protons move through water.
Image Credit: © David Mendive-Tapia
Scientific Frontline: Extended "At a Glance" Summary: Quantum Dynamics of Proton Transport in Water
The Core Concept: Protons move through water not as single, drifting particles, but by rapidly "hopping" from one water molecule to the next. Recent full-dimensional quantum dynamics simulations reveal that this highly mobile state is governed by local asymmetries in the surrounding water molecules.
Key Distinction/Mechanism: Historically, hydrated protons were modeled using idealized symmetric (Zundel) or asymmetric (Eigen) cationic structures. The new simulations demonstrate a highly dynamic intermediate state, utilizing an artificial neural network trained on quantum-chemical data to capture the 51 interlocking vibrations of a proton shared among six water molecules without relying on traditional approximations.
Origin/History: The fundamental hopping motion, known as the Grotthuss mechanism, was first conceptualized in the nineteenth century. In July 2026, an international research team published findings in Nature Chemistry utilizing high-level machine learning and quantum simulations to elucidate the exact dynamics of this transport mechanism.
Major Frameworks/Components:
- Grotthuss Mechanism: The fundamental process by which a proton jumps between adjacent water molecules, which is responsible for water's acidity.
- Zundel and Eigen Cations: Theoretical structural extremes; the Zundel structure shares a proton equally between two water molecules, whereas the Eigen structure binds it to a single molecule, forming a hydronium core bonded to three others.
- Quantum Dynamics: The tracking of coupled motions and atomic forces with full quantum resolution to compute complete infrared spectra.
- Artificial Neural Networks: Machine-learning models trained on high-level quantum-chemical data to compute the precise atomic forces governing proton motion.
Branch of Science: Physical Chemistry, Theoretical Chemistry, Quantum Physics, and Computational Chemistry.
Future Application: Enhanced modeling of energy storage mechanisms in advanced batteries, such as hydrogen fuel cells, and an improved understanding of critical biological processes, including signal transmission within living cells.
Why It Matters: Elucidating the precise infrared fingerprint and molecular dynamics of hydrated protons resolves long-standing debates about the acidity of water. This fundamental insight into aqueous proton behavior creates a foundational model for advancing both biological research and clean energy technologies.
International Research Team Simulates Motions of a Hydrated Proton in Full Quantum Detail
Complex simulations—the most intricate of their kind to date—reveal how water governs the way protons move through it. They were carried out by an international research team led by scientists at Heidelberg University’s Institute for Physical Chemistry. Using their modeling, the researchers from Cambridge (UK), Bochum (Germany), Dijon (France), and Heidelberg (Germany) were able to trace in full quantum detail the movements of a proton shared among six water molecules. At its core, the work addresses how a proton moves through water: not as a single particle drifting along, but by “hopping” from one molecule to the next. The results were published in the journal Nature Chemistry on July 27, 2026.
This “hopping” motion has been known since the nineteenth century. When an acid dissolves in water, the released proton—a positively charged hydrogen ion—does not remain bound to a single water molecule. It is highly mobile and constantly jumps from one molecule to the next. This so-called Grotthuss mechanism, the basis of proton transport in water, is responsible, among other things, for the acidity of water and plays a crucial role in energy storage in batteries and signal transmission in living cells. Because these ultrafast proton motions are so complex, they are notoriously hard to elucidate, and despite intensive research, the fundamental dynamics of protons in water remain a matter of debate.
As Professor Oriol Vendrell of the Institute for Physical Chemistry at Heidelberg University explains, until now, hydrated protons have been represented using two idealized structures: the Zundel cations and Eigen cations. Each assumes a different number of water molecules to which the proton binds. In a Zundel structure, the proton is shared equally between two molecules; in an Eigen structure, it binds to a single molecule, forming a hydronium core that is in turn bonded to three additional water molecules. “However, recent studies using infrared spectroscopy reveal a state that is far more dynamic and lies between these two extremes,” explains Dr. David Mendive-Tapia, a postdoctoral researcher on Vendrell’s team.
For the current research, the scientists simulated an extended Zundel complex with six water molecules. They continuously modified the model system by removing molecules, causing it to transition from a symmetric Zundel structure to an asymmetric Eigen structure. Using these simulations, they succeeded in tracking, with full quantum resolution, the coupled motions of the hydrated proton together with its surrounding water molecules. These amount to 51 interlocking vibrations, and by following all of them at once, the research team was able to compute the complete infrared spectrum and reproduce the experimental measurements across the full range.
According to Professor Dominik Marx of Ruhr University Bochum, a decisive ingredient was an exceptionally accurate description of the forces between the atoms. Instead of the usual approximations, the researchers captured these forces with an artificial neural network trained in Bochum on high-level, quantum-chemical data. This machine-learning model allowed them to follow the proton’s quantum motion with unprecedented accuracy and without any adjustable parameters.
“Our simulations show that the configuration of the surrounding water molecules is the key factor determining how protons move in an aqueous solution. The infrared fingerprint of the hydrated proton, and ultimately its characteristic hopping, is governed above all by local asymmetries in its surroundings,” emphasizes Vendrell. According to the scientists, the latest research findings expand the current understanding of how water shapes the way protons move through it.
Funding: German Research Foundation and the Royal Society funded the research.
Published in journal: Nature Chemistry
Title: Deciphering the infrared spectrum of the hydrated proton using full-dimensional quantum dynamics
Authors: David Mendive-Tapia, Christoph Schran, Banshi Das, Fabien Gatti, Markus Schröder, Dominik Marx, and Oriol Vendrell
Source/Credit: Ruhr-Universität Bochum | Tullia Giersberg (Universität Heidelberg)
Edited by: Scientific Frontline
Reference Number: chm072726_01