Quantum tunnelling drives aromaticity flip-flop

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.