Polyradicals

If you are into large aromatic hydrocarbons, here is a new paper for you: "The Polyradical Character of Triangular non-Kekulé Structures, Zethrenes, p-Quinodimethane Linked Bisphenalenyl and the Clar Goblet in Comparison: An Extended Multireference Study," which just appeared in J. Phys. Chem. A. In this work we studied different bonding patterns for polycyclic hydrocarbons and how they lead to the formation of radical character. Some of the systems studied are based on phenalenyl:

In these cases the polyradical character derives from Ovchinnikov's rule. Simply speaking, in all these cases there are more "starred" atoms than "unstarred" atoms. Therefore, some of the "starred" atoms are always left out of the bonds and become radical centers.

For the other systems we studied, Clar's sextet rule comes into play. In an attempt to maximize the number of Clar sextets, the molecule creates empty valences somewhere else, leading to openshell character.