Fullerenes:
Chemical, Physical,
and Biological Relevance
An Overview
by Justin Fuller
Earlham College, Richmond, IN 47374
Image Courtesy of Buckyballs
History
In 1985 Professors Robert E. Curl and Richard E. Smalley of Rice University and Harold W. Kroto of the University of Sussex proposed the familiar structure of the then recently discovered group of molecular allotropes of Carbon known affectionately as fullerenes. Eleven years later, in 1996, they were awarded the Nobel Prize in Chemistry for their accomplishments. (Fryhle et. al, 1998) The first practical synthesis of fullerenes was described by W. Krätschemer of the Max Planck Institute and D. Huffman of the University of Arizona in 1990. These molecules are named after the great architect, Buckminster Fuller, for his work on developing structures with geodesic domes. Geodesic domes are truncated icosahedrons similar in shape to a that of a soccer ball.
From the left: Robert
F. Curl, Harold W. Kroto, and Richard E. Smalley, Nobel Prize recipients in
1996 for their work on fullerenes.
Image courtesy of Fryhle et. al, 1998.
Structure
Fullerene is a general term acknowledging the family of hollow-cage carbon clusters made up of an even number of triply coordinated Carbons arranged with twelve pentagonal rings and a varying numbers of hexagonal rings. (Mintmire, 1996) The Buckminsterfullerene, or buckyball, is the name designated for the the specific C60 cluster in the truncated icosahedral form. The Carbons of the buckyball that are arranged in the hexagonal rings, remain in a flat, planar form relative to the Carbons of the pentagonal rings which are rounded slightly, giving the overall molecule its curvature. Upon further inspection of the icosahedral form, it is observed that the pentagonal carbon rings do not bond to one another. In other words, the pentagonal rings are always completely surrounded by the hexagonal rings. This phenomenon is known as the Isolated Pentagon Rule (IPR) and it is due to the steric strain that can be formed by adjacent cylcopentanes. Buckminsterfullerene is the smallest fullerene in which IPR is observed. IPR is also observed in C70. (Mintmire, 1996)
Synthesis: Old and New Methods
Fullerenes are generally synthesized by blasting graphite with high-power lasers. This was the common method for many years. However, by producing fullerenes in this method it is difficult to isolate useful amounts of the less-studied, larger fullerenes for analysis. 99% of the products formed using this method are the more commonly studied C60 and C70. (Sample,2001) Lawrence Scott of Boston College and his Colleagues at the University of Warwick in the U.K. have developed a new method for synthesizing fullerenes. They start by taking several carbon compounds in ring form and then reacting them in such a way so that the molecules are interconnected and flat. This large, flat molecule, is essentially a non-shperical fullerene. Now the molecule can be curled into the familiar spherical form by reacting it with laser light radiation. This new method of fullerene synthesis has been performed successfully on a flattened C60 molecule and leads Scott and Colleagues to believe that the method can be used to synthesize larger fullerenes. "Because the size and shape of the flat molecule can be controlled, they could be used to make large quantities of bigger [fullerenes]" says Scott. (Sample, 2001)
Organic Compounds in Space?
Recent evidence supports the idea that fullerenes may occur naturally in space. What lead to this discovery? A crater-- approximately 100 micrometers in diameter-- found on NASA's Long-Duration Exposure Facility, formed when a chondritic micrometeoroid impacted upon its surface, leaving a dent full of carbon residues in the aluminum panel. Using Laser ionization mass spectroscopy and Raman spectroscopy, Space Scientist Filippo Radicati di Brozolo and his Colleagues at the Charles Evans and Associates Institute analyzed the Carbon residues and found evidence of fullerenes. Further studies indicated that the fullerenes may have been produced as a result of the impact itself, thus providing a viable mechanism for fullerene synthesis in space. They conducted tests to determine if fullerenes could survive such a high velocity impact into aluminum and found that the compound could, indicating the durability and strength of fullerenes. This knowledge becomes important in understanding recent discoveries regarding the mass extinction that occurred at the end of the Permian Era. (Lipkin, 1994)
Mass Extinction and Fullerenes,
What's the Connection?
What destroyed the dinosaurs and many other life forms at the end of the Cretaceous Period? Scientists have hypothesized that a large meteorite may have been the cause for this mass extinction. But before the extinction of the dinosaurs there was another mass extinction on Earth.
| This occurred at the end of the Permian Era (resulting in the beginning of the Triassic Period) and was much larger than the one that occurred at the end of the Cretaceous. Upon inspection of sediments from areas of China and Japan associated with the Permian-Triassic boundary (PT boundary), Geochemists Luann Becker of the University of Washington and Robert J. Pareda of the University of Rochester and their colleagues found what they believe to be the strongest chemical evidence, to date, supporting the impact scenario. |
 Image Courtesy of C&E News |
After analysis of their collected samples they have reported findings of fullerenes that encapsulate atoms of Helium and Argon. Upon further analysis of the isotopic composition of these atoms, it was concluded that these fullerenes are of extra terrestrial origin. These gas-filled fullerenes (as seen here) were hypothesized to originate from stars or collapsing gas clouds. The PT body that transported these fullerenes to Earth has been estimated at being 9 +/- 3 km in diameter, based off of the Helium-3 measurements. |
Conversely, it has been hypothesized that these fullerene traces could have been formed in the Earth's atmosphere. This theory is problematic because compounds formed in the Earth's atmosphere would exhibit an atmospheric signature, which these gas-filled fullerenes do not have. (Dagani, 2001)
The Future of Fullerenes
Because of much research on the subject fullerenes, the field of fullerene chemistry has lead to many fascinating discoveries. Fullerenes are known to have a high electron affinity and readily accept electrons from alkali metals which give the compound a metallic phase. Known as a "Buckide Salt" they can be used in new and interesting ways by Chemists. For example, the Buckide Salt K3C60 is simply a buckyball with a Potassium atom in the middle. However, when cooled below 18K, it can become a superconductor. (Fryhle et. al, 1998) The basic design of the buckyball has been used in the making of nanotubes, which are used in Atomic Force Microscopy a method by which biologists can analyze DNA and Proteins. (Fryhle et. Al, 1998) They have been found to occur naturally in space and also have given us a window into Earth's history. (Dagani et. Al, 2001) With more and more research being conducted on the nature of fullerenes, who knows what else these molecules will be able to tell us?
Literature Cited