New study uncovers key pathways in hydronium and hydroxide ion neutralization

A new study provides fresh insight into traditional acid-base chemistry by revealing that the mutual neutralization of isolated hydronium (H₃O⁺) and hydroxide (OH⁻) ions is driven by electron transfer rather than the proton transfer that is expected in bulk liquid water.

Using deuterated water ions and advanced 3D coincidence imaging of the neutral products, researchers found two electron-transfer mechanisms that produce hydroxyl radicals (OH), which are crucial in atmospheric chemistry. These findings reshape our understanding of fundamental reaction dynamics and help explain the surprising finding of high OH and H₂O₂ concentrations at water microdroplet surfaces.

This discovery is significant because OH radicals play a key role in air quality, climate science, and even biochemical processes in the human body. By uncovering unexpected chemical reaction pathways, the study could influence future research on planetary and interstellar medium chemistry, as well as pollution control and medical applications.

The new study was led by Prof. Daniel Strasser from the Institute of Chemistry at Hebrew University in collaboration with Dr. Richard Thomas from Stockholm University and with Prof. Henning Schmidt, the director of the DESIREE facility, and was published in Nature Chemistry.

The study has unveiled critical insights into one of the most fundamental chemical reactions: the mutual neutralization of hydronium (H₃O⁺) and hydroxide (OH⁻) ions. This reaction, essential to acid-base chemistry, is typically understood to yield two water molecules (H₂O). However, the new experimental evidence demonstrates that electron-transfer mechanisms, rather than a proton-transfer pathway, dominate this reaction in the isolated system, leading to efficient formation of hydroxyl radicals (OH).

“The electron-transfer mechanisms we’ve uncovered suggest several pathways for spontaneous OH formation at low temperature conditions, without a catalyst or an external energy source,” said Prof. Strasser. “Our work offers new insights not only into the quantum mechanism of the electron-transfer dynamics in acid-base chemistry, but also into broader processes like atmospheric chemistry, where OH radicals play an essential role.”

A joint team of researchers from Hebrew University of Jerusalem and Stockholm University recorded the neutral products of individual neutralization reactions at the unique DESIREE facility at Stockholm University. The experimental breakthrough was made possible by detailed analysis of patterns of coincident products from a single reaction at a time, impinging on a time and position sensitive detector.

This study follows the team’s previous research published in Science, where they first observed both the electron-transfer and proton-transfer products. In this latest work, they were able to record the distance at which an electron jumps from OH⁻ to H₃O⁺ and correlate it to the outcome of the reaction. Electron transfer at a short ~4Å distance was observed to result in OH + H₂O + H products, while transfer across a larger ~9Å distance was observed to produce two OH radicals and a molecular H₂ hydrogen.

“It is exciting to experimentally visualize the mechanisms that help explain the recently reported spontaneous formation of OH radicals (and subsequently hydrogen peroxide) on the surface of pure water microdroplets—an observation that may fundamentally change how we think about atmospheric chemistry,” says Dr. Thomas, who led the Stockholm team.

Significance of non-adiabatic dynamics

Non-adiabatic processes are omnipresent in chemistry. They play a key role in photochemistry, ionization, and recombination reactions, where electronic states rapidly transition through mechanisms such as conical intersections or intersystem crossings. Nevertheless, the theoretical modeling and prediction of non-adiabatic reactions is still a challenge for quantum chemistry.

“Providing detailed experimental evidence will enhance our ability to validate and fine-tune theoretical modeling,” says Prof. Henning Schmidt, the director of the DESIREE facility.

This study paves the way for further investigations into non-adiabatic reaction dynamics in other fundamental chemical systems. The findings have profound implications for our understanding of atmospheric chemistry, where OH radicals play a key role in oxidation processes, and for modeling chemical reactions in extreme environments such as interstellar space.

Additionally, the new insights into spontaneous H₂O₂ formation at water microdroplet surfaces could impact studies on atmospheric chemistry, environmental science, and even biomedical research.