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The fullerenes are a recently-discovered family of carbon allotropes named after Buckminster Fuller. They are molecules composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are sometimes called buckyballs, the C60 variant is often compared to the typical white and black soccer football, the Telstar (football) of 1970. Cylindrical fullerenes are called buckytubes. Fullerenes are similar in structure to graphite, which is composed of a sheet of linked hexagonal rings, but they contain pentagonal (or sometimes heptagonal) rings that prevent the sheet from being planar.

Prediction and discovery

In molecular beam experiments, discrete peaks were observed corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms. In 1985, Harold Kroto (of the University of Sussex), James Heath, Sean O'Brien, Robert Curl and Richard Smalley, from Rice University, discovered C60, and shortly after came to discover the fullerenes. Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of this class of compounds. C60 and other fullerenes were later noticed occurring outside of a laboratory environment (e.g. in normal candle soot). By 1991, it was relatively easy to produce grams of fullerene powder using the techniques of Donald Huffman and Wolfgang Krätschmer. Fullerene purification remains a challenge to chemists and determines fullerene prices to a large extent. So called endohedral fullerenes have ions or small molecules incorporated inside the cage atoms. Fullerene is an unusual reactant in many organic reactions such as the Bingel reaction discovered in 1993.

Naming

Buckminsterfullerene (C60) was named after Richard Buckminster Fuller, a noted architect who popularized the geodesic dome. Since buckminsterfullerenes have a similar shape to that sort of dome, the name was thought to be appropriate. As the discovery of the fullerene family came after buckminsterfullerene, the name was shortened to illustrate that the latter is a type of the former.

Buckminsterfullerene

Buckminsterfullerene (IUPAC name (C60-Ih)*fullerene) is the smallest fullerene in which no two pentagons share an edge (which is destabilizing — see pentalene). It is also the most common in terms of natural occurrence, as it can often be found in soot.

The structure of C60 is a truncated icosahedron, which resembles a round soccer ball of the type made of hexagons and pentagons, with a carbon atom at the corners of each hexagon and a bond along each edge.

The C60 molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered "double bonds" and are shorter than the 6:5 bonds (between a hexagon and a pentagon).

Carbon nanotubes

Probingtubes are cylindrical fullerenes. These tubes of carbon are usually only a few nanometres wide, but they can range from less than a micrometre to a full metre in length. Their unique molecular structure results in unique macroscopic properties, including high tensile strength, high electrical conductivity, high resistance to heat, and chemical inactivity.

Properties

For the past decade, the chemical and physical properties of fullerenes have been a hot topic in the field of research and development, and are likely to continue to be for a long time. In April 2003, fullerenes were under study for potential medicinal use: binding specific antibiotics to the structure to target resistant bacteria and even target certain cancer cells such as melanoma. The October 2005 issue of Chemistry and Biology contains an article describing the use of fullerenes as light-activated antimicrobial agents.

In the field of nanotechnology, heat resistance and superconductivity are some of the more heavily studied properties.

A common method used to produce fullerenes is to send a large current between two nearby graphite electrodes in an inert atmosphere. The resulting carbon plasma arc between the electrodes cools into sooty residue from which many fullerenes can be isolated.

Chemistry

Fullerenes are stable, but not totally unreactive. The sp2-hybridized carbon atoms, which are at their energy minimum in planar graphite, must be bent to form the closed sphere or tube, which produces angle strain. The characteristic reaction of fullerenes is electrophilic addition at 6,6-double bonds, which reduces angle strain by changing sp2-hybridized carbons into sp3-hybridized ones.* The change in hybridized orbitals causes the bond angles to decrease from about 120 degrees in the sp2 orbitals to about 109.5 degrees in the sp3 orbitals. This decrease in bond angles allows for the bonds to bend less when closing the sphere or tube, and thus, the molecule becomes more stable.

Other atoms can be trapped inside fullerenes to form inclusion compounds known as endohedral fullerenes. Recent evidence for a meteor impact at the end of the Permian period was found by analysing noble gases so preserved. Metallofullerene-based inoculates using the rhonditic steel process are beginning production as one of the first commercially-viable uses of buckyballs.

Solubility

Fullerenes are sparingly soluble in many solvents. Common solvents for the fullerenes include toluene and carbon disulfide. Solutions of pure Buckminsterfullerene have a deep purple color. Fullerenes are the only known allotrope of carbon that can be dissolved in common solvents at room temperature.

Solvents that are able to dissolve a fullerene extract mixture (C60 / C70) are listed below in order from highest solubility. The value in parentheses is the approximate saturated concentration.

  1. 1,2,4-trichlorobenzene (20 mg/ml)
  2. carbon disulfide (12 mg/ml)
  3. toluene (3.2 mg/ml) - can also be used as a fullerene indicator, as fullerenes turn toluene purple
  4. benzene (1.8 mg/ml)
  5. chloroform (0.5 mg/ml)
  6. carbon tetrachloride (0.4 mg/ml)
  7. cyclohexane (0.054 mg/ml)
  8. n-hexane (0.046 mg/ml)
  9. tetrahydrofuran (0.037 mg/ml)
  10. acetonitrile (0.02 mg/ml)
  11. methanol (0.0009 mg/ml)

Diffraction

In 1999, researchers from the University of Vienna demonstrated that the wave-particle duality applied to macro-molecules such as fullerene.

Quantum mechanics

Researchers have been able to increase the reactivity by attaching active groups to the surfaces of fullerenes. Buckminsterfullerene does not exhibit "superaromaticity": that is, the electrons in the hexagonal rings do not delocalize over the whole molecule.

A spherical fullerene of n carbon atoms has n pi-bonding electrons. These should try to delocalize over the whole molecule. The quantum mechanics of such an arrangement should be like one shell only of the well-known quantum mechanical structure of a single atom, with a stable filled shell for n = 2, 8, 18, 32, 50, 98, 128, etc, i.e. twice a perfect square; but this series does not include 60. As a result, C60 in water tends to pick up two more electrons and become an anion. The nC60 described below may be the result of C60's trying to form a metallic bonding type loose combination.

Safety issues

Although buckyballs have been thought in theory to be relatively inert, a presentation given to the American Chemical Society in March 2004 and described in an article in New Scientist on April 3 2004, suggests the molecule is injurious to organisms. An experiment by Eva Oberdörster at Southern Methodist University, which introduced fullerenes into water at concentrations of 0.5 parts per million, found that largemouth bass suffered a 17-fold increase in cellular damage in the brain tissue after 48 hours. The damage was of the type lipid peroxidation, which is known to impair the functioning of cell membranes. There were also inflammatory changes in the liver and activation of genes related to the making of repair enzymes. At the time of presentation, the SMU work had not been peer reviewed.

Pristine C60 can be suspended in water at low concentrations as large clusters often termed nC60. These clusters are spherical clumps of C60 between 250-350 nm in diameter. Thus, nC60 represents a different chemical entity than solutions of C60 in which the fullerenes exist as individual molecules. Recently, results presented at the ACS meeting in Anaheim, CA suggest that nC60 is moderately toxic to water fleas and juvenile largemouth bass at concentrations in water of around 800 ppb. The first study of its kind on marine life, these preliminary results quickly spread across the scientific community. However, the overwhelming evidence of the essential non-toxicity of C60 (not nC60) in previously peer-reviewed articles of C60 and many of its derivatives indicates that these compounds are likely to have little (if any) toxicity, especially at the very low concentration at which it is used (~1-10 µM).

A new study published in December 2005 in Biophysical Journal raises a red flag regarding the safety of buckyballs when dissolved in water. It reports the results of a detailed computer simulation that finds buckyballs bind to the spirals in DNA molecules in an aqueous environment, causing the DNA to deform, potentially interfering with its biological functions and possibly causing long-term negative side effects in people and other living organisms.

Mathematics behind fullerenes

In mathematical terms, the structure of a fullerene is a trivalent convex polyhedron with pentagonal and hexagonal faces. In graph theory, the term fullerene refers to any 3-regular, planar graph with all faces of size 5 or 6 (including the external face). Using Euler's polyhedron formula, |V|-|E|+|F| = 2, (where |V|, |E|, |F| indicate the number of vertices, edges, and faces), one can easily prove that there are exactly 12 pentagons in a fullerene.

The smallest fullerene is the dodecahedron--the unique C20. There are no fullerenes with 22 vertices. The number of fullerenes C2n grows with increasing n = 12,13,14... For instance, there are 1812 non-isomorphic fullerenes C60. Note that only one of the C60's, the buckminsterfullerene alias truncated icosahedron, has no pair of adjacent pentagons (the smallest such fullerene). To further illustrate the growth, there are 214,127,713 non-isomorphic fullerenes C200, 15,655,672 of which have no adjacent pentagons.

Popular culture

Examples of fullerenes in popular culture are numerous. In fact, fullerenes appeared in fiction well before science started to take serious interest in them.

See fullerenes' entries at fictional applications of real materials.

  • In New Scientist there used to be a weekly column called Daedalus written by David Jones, which contained humourous descriptions of unlikely technologies. In 1966 the columnist included a description of the C60 and other forms of graphite. This was meant as pure entertainment.
  • Also in the New Scientist magazine, a free book was enclosed entitled, "100 Things to Do Before You Die", one of which was to kick a buckyball.
  • The buckyball is the state molecule of Texas.*

See also

Further reading

References

External links

Chemical elements | Nanotechnology | Carbon forms | Buckminster Fuller | science fiction themes

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This article is licensed under the GNU Free Documentation License. It uses material from the "Fullerene".

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