The next crucial stage in the discovery of Buckminsterfullerene was when physicists Huffman and Kratschmer found a way of making the molecule in large quantities. It was known that anyone who had been heating up graphite electrically under similar conditions, were making the new substance all the time, and didn't know.
This lead to them producing a soot which contained around 10% of C60 and a small amount of C70.
This was detected using the UV absorption spectrum, which measures the amount of UV light absorbed at a particular frequency by the sample. By scanning across a wide range of the UV spectrum, the molecular masses of the products in the sample can be calculated. Here is a graph showing the UV absorption spectrum of the soot, from Huffman and Kratschmer's experiment.
It was found that when dissolved in benzene, C60 produced a red solution, which suggests the football-like structure, however, in order to be absolutely sure, a convincing NMR (Nuclear Magnetic Resonance) trace was needed to clarify this. This will indisputably prove the structure, as in the C60 structure suggested the carbon atoms can only be in one kind of chemical environment, bonded to three neighbouring carbons with the remaining electron delocalised over the whole structure. A sample of C60 using carbon 13 was manufactured (carbon 12 is not resonant) and only one peak was obtained which confirmed the structure. Here is the carbon 13 NMR spectra of C60 taken by Kroto.
NMR2 works as a nuclei in a strong magnetic field with an odd number of nucleons, which will resonate when electromagnetic radiation is in the radio waves region. This oscillation is affected by the masking of neighbouring electrons and can be detected, hence the electronic configuration around the atoms can be deduced. This proof was finally cast in concrete when the C60 molecules (deposited on the surface of the gold) were seen with a scanning electron microscope. Here is the football like structure of Buckminsterfullerene, noticing how it consists of exactly twelve pentagons and twenty hexagons, none of the pentagons are touching each other.
Before Buckminsterfullerene's were discovered, there were only two known allotropes of carbon, these being diamond and graphite. It is fundamentally different, as the other two forms have a giant molecular lattice and the fullerenes are the only form of molecular carbon known. The stability of fullerenes come from the giant delocalised electron system which, in the case of the C60, contains twelve pentagonal and twenty hexagonal rings, but no pentagonal will share a side (an effect known as the isolated pentagon rule).
Fullerenes with the general formula C60n2 can be built to form a family of 'buckyballs' of differing sizes.
Optical isomerism has also been discovered in fullerenes, with C76 having distinct right-hand and left-hand forms. The fullerenes have all been black solids so far but they produce distinctly coloured solutions in benzene dependant on the number of carbon atoms it posses. They have low densities (1.65 g/cm Vs 3.51 g/cm for diamond) and they are all good electrical insulators.
In solution C60 can also absorb light and form the excited *C60, which is darker than C60. It can act as an optical limiter, which will react quickly to incoming light, it may be used in protecting eyes against lasers. *C60 can also transfer energy to another molecule such as O2 to produce a single *O2, with 100% efficiency. This makes it toxic to humans, as *O2 damages tissues, however, if this can be controlled it can have medical applications, as *O2 can kill cancer cells.
The excess energy of *C60 can also be dissipated as heat or emitted as a photon, which makes it a fluorescent material.
The chemistry of the C60 molecule is much more similar to the chemistry of alkenes, despite of its large delocalised system, for example, it is easy to hydrogenate C60. Given the small size of its delocalised system, benzene is a lot more stable. C60 also has the interesting property of being an electron-accepting molecule (electrons are easily added), and ions can be formed for every valence up to C606-.
The crystal structure for solid C60 is face-centred-cubic, which basically means that the balls are packed together in the minimal amount of space. This configuration allows small metal ions (such as K and Ca) to reside in the gaps and donating an electron to the neighbouring C60 to form a salt. Such salts appear to have super-conducting capability (to conduct electricity with zero resistance at low temperatures), and therefore can be immensely useful if the critical temperature (below which the material becomes super-conductive) can be increased. Without resistance, electronic circuits can run much faster with no heat generated and electricity distribution can also suffer no losses en route.
The discovery of Buckminsterfullerene opens up a complete new area of organic chemistry, as it is possible to build large structures based on molecular carbon. For instance, 'carbon-onions', consisting of a smaller fullerene wrapped with a larger fullerene, and 'zeppelenes', also known as buckytubes, consisting of an elongated buckyball, can be built. Below are an electron micrograph of 'carbon onions', and the structure of an all-carbon zeppelene.
Zeppelenes are particularly useful because it may replace carbon-fibre as the next generation super-light strong material, due to its strong bonds between carbons. They may also act as 'nanotubes' in which miniature chemical reactions can be carried out. The mass production method of nanotubes has just been discovered, using conventional chemical preparation techniques without expensive equipment such as the laser. The discovery of the fullerenes, as many things are in science, was a complete accident, and relies on the careful observation of the scientists as well as funding for basic research of initially little or no commercial value. Basic research of this nature ought to be encouraged to promote our understanding of science and in search of potentially useful material.
References:
J.Baggott, 'The Accidental Discovery of Buckminsterfullerene'
J.Crane, 'Buckyballs Bounce into Action'
A.Coghlan, 'Buckytubes to be built to order'
