I. Morao et Fp. Cossio, A simple ring current model for describing in-plane aromaticity in pericyclic reactions, J ORG CHEM, 64(6), 1999, pp. 1868-1874
The ring current model allows the characterization of the in-plane aromatic
ity present in the transition structures of several pericyclic reactions. T
he differences between in-plane and pi aromaticity can be readily character
ized 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 plan
e, thus yielding a maximum diamagnetic shielding at the ring point of the e
lectron density. This shielding decays monotonically above and below the mo
lecular 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. Th
erefore, the diamagnetic shielding is not maximum at the ring point of the
electron density.