Double Aromaticity and Ring Currents in All-Carbon Rings

Double aromaticity of neutral, planar rings of carbon atoms is demonstrated through visualisation of the induced ring currents, mapped at the ipsocentric B3LYP/6-31G(d)//B3LYP/6-31G(d) level for species C6 to C30, with onset of delocalised current in the in-plane π system at C10/C11. Both in-plane and conventional out-of-plane π systems have diatropic/paratropic current in accordance with the Huckel rule, with 4 m+2 occupation of the out-of-plane π system taking precedence, as predicted by simple nesting of Frost–Musulin diagrams. The current-density maps show characteristic double-doughnut and double-track topographies for out-of-plane and in-plane ring currents, respectively, both governed by a common framework of angular momentum rules.