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In computer models of two fused pentalene molecules, carbon atoms can tunnel between equivalent structures as their π bonds shift. That change switches which rings are locally aromatic and which may be antiaromatic. The work predicts the process; it does not report an experimentally observed molecule in a superposition of both patterns.
What the study predicts
A 2025 computational study examines π-bond-shifting automerization in two related compounds—dinaphtho[2,1-a:1,2-f]pentalene and dinaphtho[1,2-a:2,1-f]pentalene—as well as substituted derivatives. Each is modeled as a symmetric, degenerate double-well system: two equivalent molecular forms occupy separate energy minima, with a barrier between them. The calculations predict that carbon tunnelling can carry the framework between those forms.
The local aromaticity pattern changes with the structure. Rings described as locally aromatic in one form become locally antiaromatic in the other, and vice versa. This is a prediction about these specific fused-ring systems—not evidence that aromaticity generally flips in ordinary molecules. The authors’ paper in Chemical Science presents the computational analysis.
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How tunnelling could change the molecular structure
Crossing the barrier
Classically, a particle needs enough energy to get over an energy barrier. Quantum tunnelling offers another possibility: a particle can pass through a finite barrier without first reaching its top. In the modeled molecules, the relevant motion is carbon tunnelling through a narrow barrier associated with the shift of π bonds between the two equivalent forms.
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The paper discusses rates for the fastest chemical tunnelling reactions on the order of 1013 s−1 as a limiting scale in chemical tunnelling calculations. That figure is not a measured switching rate for an experimentally prepared sample of these compounds.
Two possible descriptions
Whether the molecule is localized in one form or spread across both depends on its quantum coherence. The study distinguishes a decoherent regime from a coherent one:
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| Regime | Localization | Interpretation |
|---|---|---|
| Decoherent | Localized in one energy well at a time | The molecule can switch rapidly between the equivalent forms. |
| Coherent | Nuclear wavefunction delocalized across both wells | The two forms—and their different aromaticity patterns—could be described as a quantum superposition. |
What “Schrödinger’s aromaticity cat” means
The paper’s phrase “Schrödinger’s aromaticity cat” refers to the possible coherent regime, in which the nuclear wavefunction spans both equivalent structures. In that conditional picture, the molecule would not simply be localized in one form with one aromaticity pattern; the two patterns would be part of a superposed state.
The authors say this would require preparing the systems in a coherent regime. They do not report having prepared or observed such a state. The phrase is an analogy for the proposed quantum description, not a claim that an experiment has found a molecule simultaneously displaying both patterns.
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What is established—and what remains uncertain
The result is computational, not an experimental observation
The 2025 study models the tunnelling and aromaticity changes. Chemistry World reports that the predicted speed could make direct observation difficult, and that preparing and maintaining coherence would also be challenging. Low temperature and low pressure in the gas phase are discussed as a possible route, not a demonstrated experimental procedure. Chemistry World’s report includes comment from computational chemist Miquel Solà on the interpretation of the aromaticity analysis.
The “antiaromatic” label has a methodological caveat
Solà cautions that magnetic aromaticity indices can be influenced by strong currents in the pentalene core. Depending on the index used, neighboring rings described as antiaromatic could instead be classified as non-aromatic. The important observation is the change in the local aromaticity pattern between the two molecular forms; the exact antiaromatic assignment is less definitive.
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Applications are an outlook, not a result
The authors suggest that controllable aromaticity changes could eventually inform tunable π-conjugated systems or molecular quantum technologies. These are possible future directions, not demonstrated applications of the compounds in the study.
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Paper details and supporting files
Sindy Julieth Rodríguez-Sotelo, Juan Julian Santoyo-Flores, Katarzyna Młodzikowska-Pieńko, Renana Gershoni Poranne, and Sebastian Kozuch published “Aromaticity switching by quantum tunnelling” in Chemical Science, volume 16, pages 21386–21393. The paper was first published on 7 October 2025. The Royal Society of Chemistry record lists the publication details and DOI 10.1039/D5SC05717E.
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The authors state that molecular geometries and Gaussian output files are available through ioChem-BD. The supplementary information includes electronic-structure selection, tunnelling tables, aromaticity analysis, and example input files.
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