© 2006 ACS
Fullerenes are closed-shell molecular cages made up of carbon atoms that are connected together to form five- and six-membered rings. Most fullerenes obey the isolated pentagon rule (IPR), which means that each pentagon is surrounded by five hexagons. In general, this arrangement is the most stable one because neighbouring pentagons increase the strain in the system and disrupt the ideal carbon–carbon bonding pattern.
Deviations from the IPR are sometimes found in the study of endohedral fullerenes — carbon cages with atoms trapped in their hollow interiors. The research groups of Alan Balch at the University of California, Davis, and Harry Dorn at Virginia Polytechnic Institute and State University have now isolated an egg-shaped fullerene comprising 84 carbon atoms that has a small cluster of one nitrogen and three terbium atoms trapped inside it1. Most significantly, X-ray crystallography shows that the cage contains a pair of pentagons fused along one edge — a violation of the IPR. There are 24 possible arrangements (isomers) of the atoms in a C84 fullerene cage that satisfy the IPR and over 50,000 that do not. It is quite remarkable, therefore, that only two of these structures were formed in significant quantities and the most abundant one flouts the IPR.
The work of Dorn and Balch not only has implications for the study of endohedral fullerenes and their properties, but also suggests that when considering carbon cages, non-IPR structures may be just as important as those that don't break the rules.
