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Calculations predict that carbon atoms in two fused pentalene systems can tunnel between equivalent molecular structures, switching which rings are locally aromatic and antiaromatic. The result is a computational prediction—not an experimentally observed molecular flip. A more striking possibility, a molecule in a coherent superposition of both structures, is conditional on preparing a state the study did not report achieving.
What the study predicts
In a 2025 Chemical Science paper, Sindy Julieth Rodríguez-Sotelo, Juan Julian Santoyo-Flores, Katarzyna Młodzikowska-Pieńko, Renana Gershoni Poranne, and Sebastian Kozuch modelled π-bond-shifting automerization in dinaphtho[2,1-a:1,2-f]pentalene, dinaphtho[1,2-a:2,1-f]pentalene, and substituted derivatives. The molecules are treated as symmetric, degenerate double-well systems: two equivalent structures occupy separate energy minima, with a barrier between them. The authors’ calculations predict that carbon tunnelling through this barrier can connect the two structures.
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The local pattern of aromaticity changes as the π bonds shift. Rings that are locally aromatic in one structure are predicted to become locally antiaromatic in the other, and vice versa. This is not a claim that aromaticity in ordinary molecules generally flips, nor does it describe a measured switching event in a prepared sample.
How tunnelling could change the rings
Classically, a particle without enough energy to climb over a barrier remains on its side. Quantum tunnelling allows a particle to cross a finite barrier even without that classical energy. In this case, the model concerns carbon nuclei moving through the narrow barrier associated with π-bond-shifting automerization.
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The paper discusses tunnelling rates for the fastest chemical 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 regimes: rapid switching or a coherent state
The distinction between the two regimes matters because rapid tunnelling does not by itself establish that a molecule is simultaneously in both structures. In the decoherent picture, the molecule is localized in one well at a time and can switch between wells. In the coherent picture, its nuclear wavefunction is delocalized across both wells.
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| Regime | Localization | Interpretation | Experimental challenge |
|---|---|---|---|
| Decoherent | Localized in one well at a time | Rapid switching between equivalent structures, with the associated local aromaticity pattern changing | Detecting a very fast process |
| Coherent | Delocalized across both wells | A superposition of the two structures and their different local aromaticity patterns | Preparing and maintaining coherence, then detecting the state |
The authors call the proposed coherent possibility a “Schrödinger’s aromaticity cat.” Their abstract says: “If these systems can be prepared in a coherent regime, the quantum superposition between the original states would delocalise their nuclear wavefunctions in a state that we describe as a ‘Schrödinger’s aromaticity cat.’” The conditional wording is important: the paper describes what could follow from a coherent state; it does not report preparing or observing one.
Why “antiaromatic” needs a qualification
The aromaticity label for neighboring rings is not beyond methodological debate. In Chemistry World, computational chemist Miquel Solà of the University of Girona cautioned that magnetic aromaticity indices can be affected by strong currents in the pentalene core. Other indices could characterize the neighboring rings as non-aromatic rather than antiaromatic. Solà considered the change in aromaticity between the two structures the key observation. Chemistry World’s report also notes that the predicted speed could make experimental observation difficult.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What has—and has not—been established
- Established by the paper: a computational analysis of tunnelling during automerization in specified dinaphthopentalene systems and substituted derivatives, with a predicted change in local aromaticity pattern.
- Not established in the cited sources: an experimental observation of the switching, or preparation and observation of the proposed coherent superposition.
- Possible route, not a demonstrated procedure: the report discusses low temperature and low pressure in the gas phase as conditions that might help prepare coherence; it does not establish a successful experimental protocol.
- Outlook rather than application: the authors suggest possible future relevance to tunable π-conjugated systems and molecular quantum technologies, but do not demonstrate those uses.
The paper appeared in Chemical Science, volume 16, pages 21386–21393, DOI 10.1039/D5SC05717E; it was first published on 7 October 2025. The Royal Society of Chemistry record lists its publication details. The authors state that molecular geometries and Gaussian output files are available through ioChem-BD; supplementary information includes electronic-structure selection, tunnelling tables, aromaticity analysis, and example input files.
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