Aromaticity Reversal Induced by Vibrations in Cyclo[16]carbon

Abstract

Aromaticity, most commonly defined as the ability to sustain a diatropic ring current, is typically regarded as an intrinsic property of a molecule. It is often correlated with electron delocalization, stability, and other properties. Small vari- ations in the molecular geometry usually result in small changes in aromaticity, in line with Hammond’s postulate: for exam- ple, introducing bond-length alternation in benzene and square cyclobutadiene gradually decreases the magnitude of their ring currents, making them less aromatic and less antiaromatic, respectively. A sign change in the ring current, corresponding to a reversal of aromaticity, typically requires a significant perturbation such as electronic excitation, addition or removal of two electrons, or a dramatic change in the molecular geometry. Here, we use multireference calculations to show how small changes in bond-length alternation induce a sudden reversal in the ring current of cyclo[16]carbon, C16. This reversal occurs when the two orthogonal π systems of C16 sustain opposing currents. These results are rationalized by a Hückel model which includes bond-length alternation combined with a minimal model accounting for orbital contributions to the ring current. Finally, we successfully describe the electronic structure of C16 with a divide-and-conquer approach suitable for execution on a quantum computer.