Fullerene
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 C
60 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.
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. C
60 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 (C
60) 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 (
IUPAC name
(C60-Ih)[5,6]fullerene) is the smallest fullerene in which no two pentagons share an edge (which is destabilizing â€" see
pentalene). It is also the most common in terms of natural occurrence, as it can often be found in soot.
The structure of C
60 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 C
60 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).
|
This animation of a rotating Carbon nanotube shows its 3D structure. |
Nanotubes are cylindrical fullerenes. These tubes of carbon are usually only a few nanometres wide, but they can range from less than a micrometre to several millimetres 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.
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 sp
2-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 sp
2-hybridized carbons into sp
3-hybridized ones.[
1] The change in hybridized orbitals causes the bond angles to decrease from about 120 degrees in the sp
2 orbitals to about 109.5 degrees in the sp
3 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
|
The C60 fullerene in crystalline form |
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 (C
60 / C
70) are listed below in order from highest solubility. The value in parentheses is the approximate saturated concentration.# 1,2,4-tri
chlorobenzene (20 mg/ml) #
carbon disulfide (12 mg/ml) #
toluene (3.2 mg/ml) - can also be used as a fullerene indicator, as fullerenes turn toluene purple#
benzene (1.8 mg/ml) #
chloroform (0.5 mg/ml) #
carbon tetrachloride (0.4 mg/ml) #
cyclohexane (0.054 mg/ml) # n-
hexane (0.046 mg/ml) #
tetrahydrofuran (0.037 mg/ml) #
acetonitrile (0.02 mg/ml) #
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, C
60 in water tends to pick up two more electrons and become an
anion. The nC
60 described below may be the result of C
60'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 C
60 can be suspended in water at low concentrations as large clusters often termed nC
60. These clusters are spherical clumps of C
60 between 250-350 nm in diameter. Thus, nC
60 represents a different chemical entity than solutions of C
60 in which the fullerenes exist as individual molecules. Recently, results presented at the ACS meeting in Anaheim, CA suggest that nC
60 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 C
60 (not nC
60) in previously peer-reviewed articles of C
60 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.
* Kim Allen
* Center for Nanoscale Science and Technology
* Dr. Smalley's brief autobiography
* Dr. Smalley's webpage
* Potential use of fullerenes in medicine
* Carbon Fullerene & Nanotube Models Vincent Herr, Houston, TX
* Fullerene Images for Web and Presentation
* Diffraction and Interference with Fullerenes: Wave-particle duality of C60, University of Vienna
* Fullerene-based architectures for quantum computing in Germany and in Great Britain at the QIP IRC
* Molview from bluerhinos.co.uk See Buckminsterfullerene (C60) in 3D
* Interactive 3D molecular visualization of fullerene (requires Macromedia Flash)
* Computational Chemistry Wiki
* A Spherical Revelation
* C60 3D-view and pdb-file
