Aromaticity of ring carbo-mers of [N]annulenes and [N]cycloalkanes
Maps of current density induced by a perpendicular external magnetic field are calculated at the ipsocentric CTOCD-DZ/6-31G**//B3PW91/6-31G** level for ring carbo-mers of [N]-annulenes (closed-shell singlet states of C3NHqN, N = 3 to 7, with q = −1, 0, +1, 0, −1, respectively, and also the triplet ground state for N = 4) and of [N]-cycloalkanes (C3NH2N, N = 3, 4, 5). Strong four-electron diatropic ring currents indicate conventional π aromaticity for all the singlet and triplet carbo-[N]annulenes studied, with the exception of C12H4, where instead the strong two-electron paratropic ring current is the signature of π antiaromaticity. The carbo-[N]cycloalkanes (also known as [N]pericyclynes) show only localized π currents, consistent with non-aromaticity. There is no indication of a ‘homo-aromatic’ ring current attributable to the in-plane π orbitals of the inserted C2 units in any of the maps. Consequences for the interpretation of ELF (electron localisation function) populations are discussed.