As the world prepares to go football crazy later this summer (come on England!), Christopher Ewels from the Institute of Materials in Nantes, France, has been contemplating (in Nano Letters) the world’s smallest football: buckminsterfullerene – C60.
Buckminsterfullerene, subject of the 1996 Nobel Prize in Chemistry, is a spherical cage of 60 carbon atoms – an Archimedean solid comprised of 12 pentagonal and 20 hexagonal faces which are stitched together to form a truncated icosahedron. C60 is unique in that it is the smallest fullerene that obeys the Isolated Pentagon Rule – i.e., each pentagon is completely surrounded by hexagons such that no two pentagons share an edge. Neighbouring pentagons in carbon networks are energetically unfavourable as they strain the system and disrupt the C=C bonding pattern. If you take a 4 panel section of C60 – two hexagons that share a common edge and the two pentagons that are linked by it – and rotate this grouping 90 degrees (the same effect as a Stone-Wales rotation), a less symmetric fullerene structure is obtained, which now has two pairs of edge-sharing pentagons. Calculations have shown that this isomer is a whopping 1.6 eV (~37 kcal/mol) less stable than Buckminsterfullerene!
Substitute one of the carbon atoms on each of the shared pentagon edges for nitrogen (i.e., C58N2), however, and it’s a whole new ball game. In this case, the isomer with paired pentagons is 0.54 eV (~12.5 kcal/mol) more stable than the icosahedral structure. This result suggests that there may be a whole new family of stable azafullerenes that contain a lot fewer than 60 atoms and that nitrogen substitution into nanotube and thin-film structures may have dramatic structural consequences.
It appears that azafullerenes are in a league of their own and it may just a matter of time before such structures are made and isolated – at which point this exercise in fantasy football will become a reality.
Stuart Cantrill (Associate Editor, Nature Nanotechnology)