Ring-current aromaticity in open-shell systems

The ab initio ipsocentric approach to calculation and mapping of induced orbital current density is extended to open-shell R systems. ROHF/6-31G** calculations document the (magnetic) aromaticity of planar 4n pi triplet states of 4 pi/8 pi annulenes and isobenzofulvene, and quintet excited-state azulene. An orbital model for currents rationalises the generalised form of Baird's rules, by which 4n + 2 annulenes are aromatic (antiaromatic) in states of even (odd) total spin (vice versa for 4n pi annulenes). In contrast to geometric criteria, ringcurrents predict antiaromaticity for the doublet naphthalene radical anion. (c) 2007 Elsevier B.V. All rights reserved.