Aromatic and antiaromatic structures are the chalk and cheese of cyclic chemistry. Aromatics, such as benzene, are stable and serene; antiaromatics, like pentalene, are fractious and flighty. Yet computational chemists in Israel have identified molecules in which the character of cyclic rings can switch rapidly between these two personas, or even adopt both at once.

The researchers at Ben-Gurion University of the Negev and Technion-Israel Institute of Technology made the find while studying quantum tunnelling, a phenomenon by which particles can pass through a barrier they lack the energy to surmount. The mechanism appears in many automerisation reactions, where a molecule rearranges to an equivalent form of itself, tunnelling from one well in the potential energy landscape to another. For example, π-bond shifting in antiaromatic molecules such as cyclobutadiene and pentalene proceeds extremely rapidly, even at very low temperatures where thermal activation energy is scant.

Sebastian Kozuch was about to call time on his postdocs studying yet more cases of tunnelling in antiaromatics, when they drew his attention to a molecule with unexpected behaviour. ‘I saw that there was one of the rings that switches aromaticity in the middle of the reaction, and that was particularly interesting,’ he recounts.

The molecule was dinaphtho-[2,1-a : 1,2-f]pentalene, which consists of a pentalene core fused to a double-ringed structure on either side. As part of their computations, the researchers analysed nucleus-independent chemical shifts to determine local aromaticity and antiaromaticity within the molecule. What they found was an asymmetrical electronic structure, where one ring adjoining the core showed aromaticity while the other was forced to be antiaromatic. An isomer, dinaphtho[1,2-a : 2,1-f]pentalene, had a similar configuration.

The team realised that this asymmetry could allow for automerisation between the mirror images, which, unlike previous examples such as plain pentalene, would see the character of individual rings flip between aromatic and antiaromatic. Quantum tunnelling simulations not only confirmed this hypothesis, but predicted flip-flopping so fast that experiments would struggle to see it.

‘For carbon tunnelling, it’s enormously fast,’ says Kozuch, who attributes the speed to the energy barrier’s remarkable slimness. ‘The atoms are hardly moving, and in that small moment they are breaking and forming bonds. The fact that it’s so narrow is very hard to achieve; it’s very specific of this reaction and a couple more reactions.’

A molecular Schrödinger’s cat

The spookiness doesn’t end there. In addition to flitting back and forth between potential energy wells – the so-called decoherent regime – quantum tunnelling can also leave a system’s wavefunction with a foot in each well. Such quantum superposition is colourfully contemplated in Schrödinger’s famous thought experiment, which asked whether a cat in a box could be simultaneously alive and dead. Instead of a cat, Kozuch and his co-workers have a molecule – one with rings that can be aromatic and antiaromatic at the same time.

Preparing the molecule in this superposed state would be an experimental challenge. Just as opening Schrödinger’s box cements the cat’s fate, the slightest disturbance could make the molecule decohere. But Kozuch believes it could be achieved in the gas phase at low pressure and temperature.

Miquel Solà, a computational chemist at the University of Girona, Spain, is impressed by how the work brings together the concepts of quantum tunnelling and aromaticity. However, he questions whether the system transitions to a truly antiaromatic state.

‘I would have suggested to use other indices apart from the magnetic ones to discuss the aromaticity of the rings,’ he says, noting that intense currents in the pentalene core could influence what is measured in its neighbours. ‘Maybe other indices will indicate that this [state] maybe is non-aromatic, not antiaromatic. But it’s just a minor point, because at the end you [still] observe a change of aromaticity going from one form to the other, which I think is what is really important.’