Carbon is mostly (98.89%) the isotope 12C with 13C, less common, at 1.11%. 14C is a radioactive isotope used as a marker in radiocarbon dating certain historical remains. Carbon-14, which is renewed within living organisms, decays with a half-life of 5730 years, and accurate estimates of age can be obtained up to 5,000 years.
Two well known forms of carbon are diamonds and graphite. In graphite the interlocking hexagonal rings of carbon are arranged in layers. It is a much softer material than diamond, and is used, for example in lead pencils. In 1767, the British scientist and preacher Joseph Priestley (1733-1804) discovered that graphite can conduct electricity. The different structures of carbon are referred to as allotropes.
The discovery of a third allotropic form in 1985, uncovered a fundamentally different structure of closed carbon cages, known as fullerenes. In the early 1970''s, the properties of unsaturated carbon configurations was studied by a group at the University of Sussex, led by Harry Kroto and David Walton. They developed methods for synthesising long chain polyynes, whose vibration-rotation dynamics were studied by microwave spectroscopy. They then used these observations for molecular radioastronomy. From 1975-78 they studied the long-chained polyynylcyanides, HC5N, HC7N and HC9N. These molecules, produced by red giant stars, were found in the cloud material of the interstellar medium.
In the 1980''s a technique was developed by Richard Smalley and Bob Curl at Rice University, Texas. They used laser vaporisation of a suitable target to produce clusters of atoms. Kroto realised that by using a graphite target, that the cluster apparatus would be ideal to probe the formation of carbon chains, and organised a joint collaboration between his group at Sussex and the one at Rice.
The Sussex/Rice experiment took place in September 1985. The technique probed the carbon plasma produced by the laser vaporisation by mass spectrometry. The experiments confirmed that large carbon chain/clusters were being formed. During the experiments it was noted that the peak for the C60 molecule (and to a lesser extent C70) behaved unusually and formed under all conditions as well as exhibiting great stability. The experimental evidence, a strong peak at 720 amu (atomic mass units), indicated that a carbon molecule with sixty carbon atoms was forming, but provided little structural information. The research group concluded after reactivity experiments, that the most likely structure was a spheroidal molecule. Kroto mentioned Fuller''s geodesic dome structures, which contained pentagons as well as hexagons. The idea was quickly rationalised as the basis of an icosohedral symmetry closed cage structure. The geodesic and electronic bonding factors in the structure accounted for the stability of the molecule, and it was named after Buckminster Fuller.
Fullerenes are closed cage structures. Each carbon atom is bonded to three others. Hexagonal rings are present but pentagonal rings are required for the cage to close. There is evidence for species as small as C20, as well as stable peaks for the cluster ions C2n (where 2n>32). Fullerenes which are stable or abundant enough to exist in macroscopic quantities have been studied using a wide range of physical and spectroscopic methods.
C60 and C70 have similar properties, with six reversible, one electron reductions to C606- and C706- having been observed, whereas oxidation is irreversible. The first reduction for both fullerenes is ~1.0 V (Fc/Fc), indicating they have electron accepting properties. C76 exhibits both electron donor/acceptor properties. C60 has a tendency of avoiding having double bonds within the pentagonal rings which makes electron de-localisation poor, and results in the fact that C60 is not "superaromatic". C60 behaves very much like an electron deficient alkene and readily reacts with electron rich species.