Molecular bonds key to carbon's properties

By Associate Professor Allan Blackman

This article was orignally published in the Otago Daily Times on Tuesday 10 December 2002.

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Despite the vagaries of Auckland's weather and the strident protestations of regular Eastenders viewers, interest in the forthcoming America's Cup is steadily building.

In 1987, when New Zealand first contested the America's Cup in Fremantle, the "plastic fantastic" KZ7 was the first 12 m yacht to be constructed from fibreglass. The technological advances made over the intervening years mean that carbon fibre is now used extensively in the construction of America's Cup yachts.

Carbon can exist in a variety of forms, which have very different properties. Carbon fibres are stronger than steel, graphite is a soft lubricant, and diamond is the hardest substance known in nature. So how can this one element display such a diversity of properties? It all comes down to the ways in which the carbon atoms are arranged.

Different solid forms of the same element are called "allotropes". Until relatively recently, it was thought that the only allotropes of carbon were diamond and graphite, two forms of carbon with very different physical and chemical properties.

Diamond gains its remarkable strength from the fact that the carbon atoms are arranged in an infinite three-dimensional network, such that each atom is joined by strong bonds to four others situated at the corners of a tetrahedron (Figure 1).

In contrast, the carbon atoms in graphite are bonded together in flat hexagonal rings that join together to form infinite two-dimensional sheets in which each carbon atom is now bonded to only three others (Figure 2). While the bonding within these sheets is strong, the forces holding the sheets together are relatively weak — this allows the sheets to slide over each other easily and explains why graphite is such a good lubricant.

Carbon fibres consist primarily of graphite-like material which is specially prepared so that the sheets are oriented along the length of the fibre. Because of the very strong bonding within the sheets, this arrangement gives the material incredible strength and, on a weight basis, carbon fibres are stronger than steel. This combination of low density and high strength has seen carbon fibres used in the manufacture of everything from tennis racquets to aircraft.

In 1985, one UK and two US chemists trying to prepare some particularly exotic molecules found in interstellar space chanced upon a third allotrope of carbon. This serendipitous discovery earned them the 1996 Nobel Prize in chemistry (not to mention a knighthood for the Englishman).

Whereas both graphite and diamond are composed of essentially infinite two- and three-dimensional networks, this new allotrope was found to consist of discrete molecules containing exactly 60 carbon atoms. Each molecule was found to resemble a soccer ball, with alternating five- and six-membered rings of carbon atoms giving rise to an overall spherical shape. (Figure 3). In homage to R. Buckminster Fuller, the architect whose geodesic domes were based on a similar arrangement of five- and six-membered rings, the new allotrope was called Buckminsterfullerene.

In the years since this discovery, molecules containing 70, 76, 78 and 84 carbon atoms have been isolated and such molecules are now collectively known as Fullerenes. Sadly, Fuller did not live to see nature imitate his art, having died in 1983.

Of all these different allotropes of carbon, the stable form at room temperature and pressure is, perhaps surprisingly, graphite. This means that all diamonds are slowly and inexorably converting to graphite — in fact, diamonds are quite demonstrably not forever. However, I wouldn't rush out and sell your collection immediately — the process takes many millions of years, so they'll probably be a good investment for a few years yet.