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A 2022 study found that in a complex made from acetic acid and 1-methylimidazole, the acidic hydrogen is best described as shared, or quantum mechanically delocalized, between the acid and base—not simply fixed on one side or fully transferred to the other. Infrared and proton NMR measurements, supported by first-principles simulations, underpin that interpretation.
What does it mean for a hydrogen bond to share a proton?
Acetic acid can act as a Brønsted–Lowry acid by donating a hydrogen ion, while 1-methylimidazole can act as a base by accepting one. In a simple picture, the hydrogen belongs to the acid before transfer and to the base afterward. The 2022 study describes a strong hydrogen bond between these molecules in which neither fully localized picture is adequate: the acidic hydrogen is delocalized across the two partners.
“Quantum” here refers to the behavior of the light hydrogen nucleus and the electrons in a short, strong bond. At that scale, the hydrogen’s position and energy are described by quantum mechanics; its state can extend across the bond rather than correspond to a single classical location. This does not mean a tiny proton can be pictured as literally darting back and forth along a predictable path.
As computational and spectroscopic expert Carlos Baiz told Chemistry World in 2022, “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 interview explanations help describe the interpretation; they are not numerical measurements from the paper.
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How did the researchers support the interpretation?
The study combined complementary evidence rather than claiming to observe a proton directly. Its infrared spectrum showed an unusual broad absorption band centered around 2700 cm⁻¹ for the acetic acid–1-methylimidazole system. The researchers interpreted that feature alongside proton NMR measurements and first-principles simulations to support the presence of the complex and hydrogen sharing. The band is a reported feature of this particular system, not a signature to assume for every hydrogen bond.
| Evidence | What it contributes |
|---|---|
| Infrared spectroscopy | Measures vibrational absorption; the study reports a broad band near 2700 cm⁻¹ for this mixture. |
| Proton NMR spectroscopy | Provides information about the hydrogen’s chemical environment in the complex. |
| First-principles simulations | Test whether a quantum-mechanical account can reproduce and explain the observed behavior. |
The convergence of these methods strengthens the proposed account, but it is evidence interpreted through measurements and modeling—not a photograph or direct visual observation of a proton.
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Does this overturn the usual acid–base theory?
No. Brønsted–Lowry theory remains a useful way to describe acids as proton donors and bases as proton acceptors. The finding shows that, for this nonaqueous weak acid–base complex, a simple picture in which the proton is wholly localized on one molecule or wholly transferred to the other leaves out important quantum effects. It does not disprove acid–base theory generally.
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How does the later related study differ?
A 2025 paper reports strong hydrogen bonding and quantum-delocalized hydrogen in a supramolecular complex involving perfluoro-tert-butanol and 1-methylimidazole. It is a related example of the broader theme, but it uses a different acid and is not the 2022 acetic acid study.
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Sources
- Zhang et al., “Quantum mechanical effects in acid–base chemistry,” Chemical Science (2022)
- Victoria Atkinson, “Quantum nature of hydrogen bonds observed in acid–base complex,” Chemistry World (2022-06-07)
- “Quantum Mechanical Behavior of Hydrogen Bonds Enables Supramolecular Structure in a Weak Acid–Base Monoprotic Complex,” Journal of the American Chemical Society (2025)
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