Some such structures may contain aromatic π-sextets, namely groups of six π-electrons localized in a benzene-like moiety and separated by adjacent rings through C–C bonds.
[1][2] In 1984, Glidewell and Lloyd provided an extension of Clar's rule to polycyclic aromatic hydrocarbons containing rings of any size.
Following rule 4 above, anthracene is better described by a superposition of these three equivalent structures, and an arrow is drawn to indicate the presence of a migrating π-sextet.
Following the same line of reasoning, one can find migrating π-sextets in other molecules of the acene series, such as tetracene, pentacene, and hexacene.
The dependence of the color and reactivity of some small polycyclic aromatic hydrocarbons on the number of π-sextets in their structures was reported by Clar himself in his seminal contribution.
[5] Clar's rule has also been supported by experimental results about the distribution of π-electrons in polycyclic aromatic hydrocarbons,[7] valence bond calculations,[8] and nucleus-independent chemical shift studies.
[10] Aromatic π-sextets play an important part in the determination of the ground state of open shell biradical-type structures.,[4] Clar's rule can rationalize the observed decrease in the bandgap of holey graphenes with increasing size.