A unified orbital model of delocalised and localised currents in monocycles, from annulenes to azabora-heterocycles

Why are some (4n+2)π systems aromatic, and some not? The ipsocentric approach to the calculation of the current density induced in a molecule by an external magnetic field predicts a four-electron diatropic (aromatic) ring current for (4n+2)π carbocycles and a two-electron paratropic (antiaromatic) current for (4n)π carbocycles. With the inclusion of an electronegativity parameter, an ipsocentric frontier-orbital model also predicts the transition from delocalised currents in carbocycles to nitrogen-localised currents in alternating azabora-heterocycles, which rationalises the differences in (magnetic) aromaticity between these isoelectronic π-conjugated systems. Ab initio valence-bond calculations confirm the localisation predicted by the naïve model, and coupled-Hartree-Fock calculations give current-density maps that exhibit the predicted delocalised-to-localised/carbocycle-heterocycle transition. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.