A simple ring current model for describing in-plane aromaticity in pericyclic reactions

The ring current model allows the characterization of the in-plane aromaticity present in the transition structures of several pericyclic reactions. The differences between in-plane and pi aromaticity can be readily characterized by means of this model. In the presence of a magnetic field, in-plane aromaticity is manifested by a ring current moving along the molecular plane, thus yielding a maximum diamagnetic shielding at the ring point of the electron density. This shielding decays monotonically above and below the molecular plane. In contrast, pi aromaticity can be characterized by two ring currents moving at a certain distance above and below the molecular plane. This distance corresponds to the covalent radius of the atoms involved. Therefore, the diamagnetic shielding is not maximum at the ring point of the electron density.