π-Electron Currents Induced in Polycyclic Benzenoid Hydrocarbons and Their Relationship to Clar Structures

When an external magnetic field is applied to polycyclic benzenoid hydrocarbon molecules, a π-electron current is induced in the π system. This current can be partitioned formally among the constituent benzene rings. Part of the π-electron current assigned to each benzene ring is called the ring current. The distribution of large diatropic ring currents in a polycyclic benzenoid π system was found to be in excellent agreement with that of sextet rings in the Clar structure. This supports the view that six-membered circuits are the main origin of aromatic stabilization. However, formal ring currents in nonbenzenoid hydrocarbons are not always related to their aromaticity.