C₆₀: Buckminsterfullerene
Sixty carbon atoms fold into a hollow soccer ball — a third form of pure carbon, a nanometre wide.
Aim a laser at ordinary pencil graphite, and the carbon does something no one expected: sixty atoms snap together into a hollow ball, shaped exactly like a soccer ball.
The big idea
Pure carbon was thought to come in just two forms: graphite (soft, grey, stacked sheets) and diamond (hard, clear, a rigid lattice). In 1985 a third turned up — a molecule of exactly sixty carbon atoms arranged on the surface of a tiny hollow sphere. Its pattern is the one on a soccer ball: twenty six-sided panels and twelve five-sided ones, with a carbon atom at every corner. It is about a nanometre wide — a hundred-thousandth the width of a human hair.
How it came about
Harry Kroto, a chemist from Sussex, was curious about long carbon molecules made by dying stars. He heard that Richard Smalley at Rice University in Texas had a machine that could blast any solid into a vapour and watch what the atoms formed. Over eleven days in September 1985, Kroto, Smalley, Robert Curl and two graduate students — James Heath and Sean O’Brien — vaporized graphite and kept seeing one stubborn signal: clusters of sixty carbons.
Why sixty, and why so steady? Late one night they cut pentagons and hexagons from paper and taped them together, trying to close a ball. Kroto remembered the geodesic domes of the architect Buckminster Fuller — and the soccer ball clicked into place. They named the molecule buckminsterfullerene; everyone soon just called it the buckyball.
Why it mattered
It revealed a whole new chemistry of carbon. If sixty atoms could close into a cage, so could seventy, and so could long tubes — the carbon nanotubes and, eventually, the single sheets of graphene that now run through modern electronics and materials science. A familiar element, the stuff of pencils and soot, turned out to have hidden architectures no one had imagined. The discovery won the 1996 Nobel Prize in Chemistry.
A way to picture it
Try to wrap a flat sheet of chicken wire — all six-sided holes — smoothly around a ball. You can’t; it stays flat or buckles. To make it curve and close, you have to slip in some five-sided holes. A soccer ball solves it with exactly twelve pentagons among the hexagons, and so does C₆₀. The twelve pentagons are what bend the flat carbon sheet into a closed cage.
Where it sits
A century earlier, Kekulé had drawn carbon into the flat six-sided ring of benzene (kekule-1865); the buckyball took those rings and curled them into the third dimension. It sits at the head of the nanocarbon family — buckyballs, nanotubes, graphene — that has shaped materials science ever since, and it began, fittingly, as a question about chemistry among the stars.
During experiments aimed at understanding the mechanisms by which long-chain carbon molecules are formed in interstellar space and circumstellar shells, graphite has been vaporized by laser irradiation, producing a remarkably stable cluster consisting of 60 carbon atoms.
Concerning the question of what kind of 60-carbon atom structure might give rise to a superstable species, we suggest a truncated icosahedron, a polygon with 60 vertices and 32 faces, 12 of which are pentagonal and 20 hexagonal. This object is commonly encountered as the football shown in Fig. 1.
The C60 molecule which results when a carbon atom is placed at each vertex of this structure has all valences satisfied by two single bonds and one double bond, has many resonance structures, and appears to be aromatic.
We are disturbed at the number of letters and syllables in the rather fanciful but highly appropriate name we have chosen in the title to refer to this C60 species.