Ring currents in large [4n + 2]-annulenes

Abstract

Reported computational results for large [4n + 2]-annulenes indicate a falling off of aromaticity in D_3h geometries but its retention in D_6h geometries, as diagnosed on both energetic and magnetic criteria. Ipsocentric pseudo-π mapping of the current density induced by a perpendicular external magnetic field shows that ring currents follow this trend. Diatropic ring currents are quenched in D_3h geometries but survive in D_6h geometries of [4n + 2]-annulenes (4n + 2 = 30, 42, 54, 66). Ipsocentric orbital contributions explain this distinction in terms of the translational/diatropic, rotational/paratropic selection rules for current in monocycles, coupled with an account of differential angular-momentum mixing in D_3h and D_6h symmetries. The orbital model rationalises the differences between D_6h and D_3h geometries, accounts for the decay of aromaticity with ring size for D_3h [4n + 2]-annulenes, and predicts trends for anti-aromatic [4n]-annulenes in the two symmetry groups.