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A study of acetic acid and 1-methylimidazole found that the acidic hydrogen in their strong hydrogen bond is not well described as belonging entirely to either molecule. Instead, infrared and proton NMR measurements, supported by first-principles simulations, led the researchers to interpret it as quantum mechanically delocalized between the acid and base.
What the researchers found
The 2022 study examined a nonaqueous mixture of acetic acid and 1-methylimidazole. In the complex, the acid’s hydrogen forms an unusually strong hydrogen bond with the base. The study’s interpretation is that this hydrogen is shared across the two partners rather than being wholly localized on the acid or fully transferred to the base.
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Here, “quantum” describes the behavior of the light hydrogen nucleus and the electrons in a short, strong bond. Their quantum effects influence the hydrogen’s possible positions and energies. “Delocalized” is more accurate than imagining a tiny proton classically darting back and forth between molecules: the claim concerns a shared quantum state, not a directly observed trajectory.
How the evidence supports that interpretation
| Method | What it contributes |
|---|---|
| Infrared spectroscopy | Measures vibrational absorption features. For this particular acetic acid–1-methylimidazole system, the paper reports a broad band centered around 2700 cm⁻¹. |
| Proton NMR spectroscopy | Provides evidence about the hydrogen’s chemical environment in the complex. |
| First-principles simulations | Test how well calculations that account for quantum effects reproduce the observed behavior. |
The authors interpret these different lines of evidence together. The broad infrared feature is not a universal signature of every hydrogen bond, and the proton was not visually observed moving between the molecules. The case for hydrogen sharing comes from spectroscopy considered alongside simulations. The study in Chemical Science describes the experiment and analysis.
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What this means for acid–base chemistry
Brønsted–Lowry theory remains useful: it describes acids as proton donors and bases as proton acceptors. The result does not overturn that framework. It shows that, for this particular nonaqueous weak acid–base complex, a simple picture in which the proton is either localized on the acid or completely transferred to the base is insufficient without accounting for quantum effects.
In a 2022 Chemistry World report, study researcher Daniel Kuroda described how the project began: “It was luck,” admits Daniel Kuroda of Louisiana State University, one of the principal researchers involved in the study. The report says the team was examining liquid structure and noticed an acid–base mixture with conductivity close to sulfuric acid but no ionisation. Computational and spectroscopic expert Carlos Baiz explained the role of quantum effects: “Quantum mechanical effects make a big difference, especially when we think about energy barriers,” and, “The proton itself is quantum mechanical, and it can be thought of in terms of resonance structures that coexist.” These are explanatory comments in the report, not numerical results from the paper. Read the Chemistry World report.
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A related result uses a different acid
A 2025 paper reports a strong hydrogen bond and quantum-delocalized hydrogen in a supramolecular complex made from perfluoro-tert-butanol and 1-methylimidazole. It shares the broader theme of quantum effects in weak acid–base complexes, but it is a different system from the 2022 acetic acid study. The 2025 paper in the Journal of the American Chemical Society describes that separate result.
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