Current-density maps as probes of aromaticity: global and Clar pi ring currents in totally resonant polycyclic...

Calculation and visualisation of induced current density are important aids to the study of both aromaticity and observable molecular magnetic response properties. The ipsocentric method offers an accurate and economical approach to calculation of induced current density, and a physical interpretation in terms of occupied-orbital contributions. In monocyclic systems, these contributions allow rationalisation of the existence, sense, and strength of ring current using simple symmetry and node-counting arguments. Here we show maps computed with the model pseudo-pi version of the ipsocentric theory for large polycyclic aromatic hydrocarbons (PAH) of the Clar type. Maps are presented for D(6h)-symmetric systems with up to 438 carbon centres; a full ab initio calculation for an isomer of C114H30 confirms that the success of the pseudo-pi method for small PAH carries over to these larger systems. The computed maps follow a generic overall pattern that can be understood with a natural extension of the orbital arguments to a 'band theory' of totally resonant PAH. All show pi current densities that combine a Clar-sextet structure of localised benzenoid diatropic ring currents (contributed by just four HOMO electrons) with a global perimeter diatropic ring current contributed by the remaining pi electrons. Both currents are explained in the ipsocentric model: the localised currents arise from excitations from the HOMO; the perimeter current from excitations from the rest of the HOMO-band.