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Unravelling non-adiabatic pathways in the mutual neutralization of hydronium and hydroxide

Nature Chemistry volume 17, pages 541–546 (2025)Cite this article

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Abstract

The mutual neutralization of hydronium and hydroxide ions is a fundamental chemical reaction. Yet, there is very limited direct experimental evidence about its intrinsically non-adiabatic mechanism. Chemistry textbooks describe the products of mutual neutralization in bulk water as two water molecules; however, this reaction has been suggested as a possible mechanism for the recently reported spontaneous formation of OH radicals at the surface of water microdroplets. Here, following three-dimensional-imaging of the coincident neutral products of reactions of isolated D3O+ and OD−, we can reveal the non-adiabatic pathways for OD radical formation. Two competing pathways lead to distinct D2O + OD + D and 2OD + D2 product channels, while the proton-transfer mechanism is substantially suppressed due to a kinetic isotope effect. Analysis of the three-body momentum correlations revealed that the D2O + OD + D channel is formed by electron transfer at a short distance of ~4 Å with the formation of the intermediate unstable neutral D3O ground state, while 2OD + D2 products are obtained following electron transfer at a distance of ~10 Å via an excited state of the neutral D3O.

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Fig. 1: Schematic potentials involved in isolated H3O+ + OH− MN via an electron-transfer mechanism.
Fig. 2: Schematic representation of the experimental set-up.
Fig. 3: Three-body product assignment and total KER analysis.
Fig. 4: Dalitz plots of three-body momentum correlations in the two product channels.

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Acknowledgements

This work was performed at the Swedish National Infrastructure, DESIREE (Swedish Research Council contract nos. 2017-00621, 2021-00155 and 2023-00170). H.Z. and H.T.S. thank the Swedish Research Council for individual project grants (contract nos. 2020-03437 and 2022-02822). R.D.T., H.T.S. and H.Z. acknowledge the Project Grant ‘Probing charge- and mass-transfer reactions on the atomic level’ (2018.0028) from the Knut and Alice Wallenberg Foundation. R.D.T. acknowledges support by the Air Force Office of Scientific Research (award no. FA8655-24-1-7004). This publication is based upon work from COST Action CA18212—Molecular Dynamics in the GAS phase (MD-GAS), supported by COST (European Cooperation in Science and Technology). D.S. and A.B. acknowledge support from ISF grant 674/21 and the Minerva Center for Making Bonds by Fragmentation.

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Authors and Affiliations

  1. Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel

    Alon Bogot & Daniel Strasser

  2. Department of Physics, Stockholm University, Stockholm, Sweden

    Mathias Poline, MingChao Ji, Arnaud Dochain, Stefan Rosén, Henning Zettergren, Henning T. Schmidt & Richard D. Thomas

  3. Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, Louvain-la-Neuve, Belgium

    Arnaud Dochain

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  1. Alon Bogot
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Contributions

All authors contributed to the reported investigations and participated in the experimental measurements. D.S., H.T.S., H.Z. and R.D.T. acquired the funding and supervised the research. A.B. and D.S. wrote the original draft of the paper and A.B., M.P., A.D., H.Z., H.T.S., R.D.T. and D.S. reviewed and edited the paper.

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Correspondence to Daniel Strasser.

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Bogot, A., Poline, M., Ji, M. et al. Unravelling non-adiabatic pathways in the mutual neutralization of hydronium and hydroxide. Nat. Chem. 17, 541–546 (2025). https://doi.org/10.1038/s41557-025-01771-6

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