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A third member of the family of hollow carbon molecules known as ‘fullerenes’
has been identified by chemists in California. The first two were highly
symmetrical, shaped like a soccer ball and a rugby ball, but the new molecule,
which contains 76 carbon atoms, is slightly twisted. In this week’s issue
of Nature, the team that discovered the molecule calls it ‘one of the most
fascinating molecular architectures ever selected by nature’.

The simplest fullerene, carbon-60 or ‘buckministerfullerene’, is one
of the most symmetrical molecules known. All 60 of its carbon atoms occupy
equivalent positions, which means the molecule’s nuclear magnetic resonance
(NMR) spectrum has a single peak. The spectrum of the more complex carbon-70
has five peaks.

Robert Whetten and his colleagues at the University of California at
Los Angeles have found that carbon-76 has 19 peaks of equal intensity in
its NMR spectrum. This tells them that there are 19 ways in which a carbon
atom in carbon-76 can be attached to its neighbours, and that four atoms
in each molecule adopt each of these arrangements.

According to the chemists, only one structure has the required symmetry
properties: an array of edge-sharing pentagons and hexagons which spiral
around the molecule. The symmetry properties of carbon-76 are ‘extremely
interesting, unprecedented, and largely unexpected’, say Whetten and his
colleagues.

The complex structure of the molecule makes it ‘chiral’, in other words
the molecule, like a human hand, cannot be superimposed on its own mirror
image. Pairs of such molecules have interesting properties; for instance,
right-handed and left-handed versions typically rotate the polarisation
of light in opposite directions.

Fullerenes became available in reasonable quantity only late last year,
and chemists are still perfecting techniques to purify them. Whetten and
his colleagues were the first to isolate enough carbon-76 to study its structure.

New discoveries are likely to follow when other fullerenes are purified.
Robert Johnson of the IBM Almaden Research Laboratory in San Jose, California,
is working on carbon-84, which has 25 lines in its NMR spectrum. According
to at least one theorist, it may have a structure similar to carbon-76.