Buckminsterfullerene
Buckminsterfullerene
As recently as 1984, carbon was thought to exist in only two solid forms: graphite, in which the carbon atoms arranged themselves as layered sheets of hexagonally bonded atoms; and 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 C60 molecule, which investigations using high-resolution electron microscopes have shown to have sizes of about one-billionth of a meter.
As C60 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, C60 has 20 hexagons; whereas its rugby-ball-shaped cousin C70 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 C3 and C600 are hollow fullerenes, but below C32, 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 C60; the next is C70 .C72 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 C60 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 buckminsterfullerene 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 mph (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 buckminsterfullerene 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.
Production of fullerenes
For reasons that are not yet fully understood, C60 seems to be the inevitable result of carbon condensing slowly at high temperatures. At high temperatures when carbon is vaporized, most of the atoms initially coalesce into clusters of 2-15 atoms. Small clusters form 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 electrode tips 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.
KEY TERMS
Electric arc— A discharge of electricity through a gas.
Geodesic dome— A dome constructed of many light, straight structural elements in tension, arranged in a framework of triangles to reduce stress and weight.
Icosahedron— A 20-sided polyhedron.
Organic— Containing carbon atoms. Originally, the term was used to describe materials of living origin.
Polymer— A substance, usually organic, composed of very large molecular chains that consist of recurring structural units.
Uses
Fullerenes have so far failed to realize their anticipated commercial potential. This is partly for reasons of cost and partly because it has proven difficult to isolate large quantities of sought-after types. At the beginning of 1994, fullerenes were actively being studied for 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.
Large-scale applications for fullerenes are not expected to be found until manufacturing costs are close to those of aluminum (a few dollars per pound).
See also Platonic solids; Polyhedron.
Resources
BOOKS
Baggott, J. Perfect Symmetry. New York: Oxford University Press, 1994.
Fuller, Buckminster. Ideas and Integrities. Toronto: Collier BOOKS, 1963.
Stewart, Ian, and Martin Golubitsky. Fearful Symmetry: Is God a Geometer? Oxford: Blackwell, 1992.
PERIODICALS
Curl, Robert F., and Richard E. Smalley. “Fullerenes.” Scientific American (October 1991).
OTHER
Michigan State University. “Buckminsterfullerenes” <http://www.msu.edu/~hungerf9/bucky1.html> (accessed October 7, 2006).
Randall Frost
Buckminsterfullerene
Buckminsterfullerene
As recently as 1984, carbon was thought to exist in only two solid forms. 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 C60 molecule , which investigations using high resolution electron microscopes have shown to have sizes of about one billionth of a meter.
As C60 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, C60 has 20 hexagons; whereas its rugby-ball shaped cousin C70 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 C3and C600 are hollow fullerenes, but below C32, 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 C60; the next is C70. C72 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 C60 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 buckminsterfullerene 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 MPH (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 buckminsterfullerene 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.
Production of fullerenes
For reasons that are not yet fully understood, C60 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 2-15 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.
Uses
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. At the beginning of 1994, fullerenes were 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.
It has been predicted the first large-scale applications for fullerenes will not be found until manufacturing cost are close to those of aluminum (a few dollars per pound).
See also Platonic solids; Polyhedron.
Resources
books
Baggott, J. Perfect Symmetry. New York: Oxford University Press, 1994.
Fuller, Buckminster. Ideas and Integrities. Toronto: Collier Books, 1963.
Porter, Roy, and Marilyn Ogilvie, eds. The Biographical Dictionary of Scientists. Vol. 2, Oxford: Oxford University Press, 2000.
Stewart, Ian, and Martin Golubitsky. Fearful Symmetry: Is God a Geometer? Oxford: Blackwell, 1992.
periodicals
Curl, Robert F., and Richard E. Smalley. "Fullerenes." Scientific American (October 1991).
Randall Frost
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Electric arc
—A discharge of electricity through a gas.
- Geodesic dome
—A dome constructed of many light, straight structural elements in tension, arranged in a framework of triangles to reduce stress and weight.
- Icosahedron
—A 20–sided polyhedron.
- Organic
—Containing carbon atoms, when used in the conventional chemical sense. Originally, the term was used to describe materials of living origin.
- Polymer
—A substance, usually organic, composed of very large molecular chains that consist of recurring structural units.