As recently as 1984, carbon was thought to exist in only two solid forms (allotropes). There was graphite, in which the carbon atoms arranged themselves as layered sheets of hexagonally bonded atoms, and there was diamond, in which the carbon atoms formed octahedral structures in which each carbon atom had four nearest neighbors.
Then, in 1985, chemists R. E. Smalley, R. F. Curl, J. R. Heath, and S. O'Brien at Rice University, and H. W. Kroto of the University of Sussex in England observed that a hollow truncated icosahedron, similar in shape to a soccer ball, and consisting of 60 carbon atoms, tends to form spontaneously when carbon vapor condenses. In 1990, physicists D. R. Huffman and L. Lamb of the University of Arizona, working with W. Kratschmer and K. Fostiropoulos of the Max Planck Institute in Germany, discovered a way to make bulk quantities of this C-60 molecule, which investigations using high resolution electron microscopes have shown to have sizes of about one billionth of a meter.
As C-60 has the same structure as the geodesic dome developed by American engineer and philosopher R. Buckminster Fuller, these molecules were christened buckminsterfullerenes by the group at Rice University. The Swiss mathematician Leonhard Euler had proved that a geodesic structure must contain 12 pentagons to close into a spheroid, although the number of hexagons may vary. Later research by Smalley and his colleagues showed that there should exist an entire family of these geodesic-dome-shaped carbon clusters. Thus, C-60 has 20 hexagons; whereas its rugby-ball shaped cousin C-70 has 25. Research has since shown that laser vaporization of graphite produces clusters of carbon atoms whose sizes range from two to thousands of atoms. These molecules are now known as fullerenes. All the even numbered species between C-3 and C-600 are hollow fullerenes, but below C-32, the fullerene cage is too brittle to remain stable. Helical microtubules of graphitic carbon have also been found.
Although many examples are known of five-membered carbon rings attached to six-membered rings in stable organic compounds (for example, the nucleic acids adenine and guanine), only a few occur whose two five-membered rings share an edge. The smallest fullerene in which the pentagons need not share an edge is C-60; the next is C-70. C-72 and all larger fullerenes adopt structures in which the five-membered carbon rings are well separated, but the pentagons in these larger fullerenes occupy strained positions. This makes the carbon atoms at such sites particularly vulnerable to chemical attack. Thus, it turns out that the truncated icosahedral structure of C-60 distributes the strain of closure equally, producing a molecule of great strength and stability. This molecule will, however, react with certain free radicals.
When compressed to 70% of its initial volume, the fullerene is expected to become harder than diamond. After the pressure has been released, the molecule would be expected to take up its original volume. Experiments in which these molecules were thrown against steel surfaces at about 17,000 m/h (25,744 km/h) showed them to just bounce back.
Fullerenes are, in fact, the only pure, finite form of carbon. Diamond and graphite both form infinite networks of carbon atoms. Under normal circumstances, when a diamond is cut, the surfaces are instantly covered with hydrogen, which tie up the unattached surface bonds. The same is true of graphite. Because of their symmetry, fullerenes need no other atoms to satisfy their surface chemical bonding requirements.
The fullerene seems to have an incredible range of electrical properties. It is currently thought that it may alternately exist in insulating, conducting, semiconducting, or superconducting forms.
Fullerenes with metal atoms trapped inside the carbon cage have also been studied. These are referred to as endohedral metallofullerenes. Reports of uranium, lanthanum, and yttrium metallofullerenes have appeared in the literature. It has been exceptionally difficult to isolate pure samples of these shrink-wrapped metal atoms, however.
For reasons that are not yet fully understood, C-60 seems to be the inevitable result of condensing carbon slowly at high temperatures. At high temperatures when carbon is vaporized, most of the atoms initially coalesce into clusters of two to 15 atoms. Small clusters from chains, but clusters containing at least 10 atoms commonly form monocyclic rings. Although these rings are favored at low temperatures, at very high temperatures they break open to form linear chains of up to 25 carbon atoms. These carbon chains may then link together at high temperatures to form graphite sheets, which somehow manage to form the geodesic fullerenes. One theory has it that the carbon sheets, when heated sufficiently, close in on themselves to form fullerenes.
Kratschmer and his coworkers in Germany managed to prepare the first concentrated solution of fullerenes in 1990 by mixing a few drops of benzene with specially prepared carbon soot.
Scientists later demonstrated that fullerenes can be conveniently generated by setting up an electric arc between two graphite electrodes. In their method, the tips of the electrodes are screwed toward each other as fast as the graphite is evaporated to maintain a constant gap. The process has been found to work best in a helium atmosphere in which other gases such as hydrogen and water vapor have been eliminated.
Fullerenes have been reported to occur naturally in certain coals, as well as in structures produced by lightening known as fulgurites, and in the soot of many flames.
Fullerenes have so far failed to realize their commercial potential. This is partly for reasons of cost and partly because it has proven difficult to isolate large quantities of sought-after types. Fullerenes are actively being studied for the following applications: optical devices, hardening agents for carbides, chemical sensors, gas separation devices, thermal insulation, diamonds, batteries, catalysts, hydrogen storage media, polymers and polymer additives, and medical applications.
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