Double aromaticity and ring currents in open-shell systems

Current–density maps have been computed at the spin-unrestricted ipsocentric UHF-CTOCD-DZ/6-31G∗//UB3LYP/6-31G∗ level for singlet, triplet and quintet states of C16 and give a direct visualisation of the double character of the aromaticity in these systems. A qualitative interpretation, based on combining symmetry/spin selection rules for the Frontier orbitals with a nested version of the simple Frost–Musulin mnemonic diagram, accounts for the main features of the maps. The notion of double aromaticity, Baird’s electron-counting rule and the annulene analogy are all shown to extend to open-shell systems.