Fullerenes are a class of closed, hollow carbon compounds that comprise only the third form of pure carbon ever discovered. The most remarkable of the fullerenes is the 60-carbon alkene buckminsterfullerene, also known as a Buckyball. This highly unusual molecule was named after the geodesic dome, a structure that exhibits a geometry that approximates a truncated icosohedral soccer ball-shape, invented by visionary engineer, author, and architect R. Buckminster Fuller. The roundest known molecule in the world, the Buckyball has carbon atoms at 60 chemically equivalent vertices that are connected by 32 faces, 12 of which are pentagonal and 20 hexagonal. Due to their unique structure, Buckyballs are remarkably rugged, being capable of surviving collisions with metals and other materials at speeds in excess of 20,000 miles per hour, a pace that would tear most organic molecules apart. Higher and lower order Buckyballs containing different numbers of carbon atoms deviate from the strict geodesic dome structure and are consequently not as stable, but are still composed primarily of pentagons and hexagons. Smaller fullerenes, often termed Buckybabies, are said to have a shape akin to that of asteroids, while larger fullerenes may appear similar in shape to a pentagon.
Edge of a C-60 Thin Film The discovery of fullerenes, which exhibit a number of notable characteristics that make the possibilities of their use seem almost limitless, was an important moment in the history of science. This achievement is generally attributed to Richard Smalley and Robert Curl of Rice University as well as Harold Kroto of the University of Sussex, whose experiments aimed at understanding the mechanisms by which long-chain carbon molecules are formed in interstellar space surreptitiously produced Buckyballs in the mid-1980s. In 1996, Smalley, Curl, and Kroto were awarded the Nobel Prize in Chemistry for their discovery. Yet, the discovery of fullerenes did not necessarily mean that there was a readily available way to study them. For a time, much of the work regarding fullerenes was chiefly theoretical since a practical means of producing significant amounts of the molecules was unknown until 1988, when astrophysicists Donald Huffman and Wolfgang Kratschmer realized that they had already discovered a way to produce them years before, though at that time they did not know the import of their actions. The Huffman/Kratschmer process soon made it possible to generate isolable quantities of fullerenes by causing an arc between two graphite rods to burn in a helium atmosphere and extracting the carbon condensate that resulted utilizing an organic solvent.
C-60 Thin Film In 1991, Science magazine dubbed the Buckyball "molecule of the year," professing it "the discovery most likely to shape the course of scientific research in the years ahead," a statement that, years later, does not appear unsubstantiated. Studies exploring the extraordinary characteristics of Buckyballs and potential uses for them are ongoing and the molecules may eventually wind their way into daily life as practical applications are developed. One of the most promising areas of Buckyball research is in the realm of materials science, many scientists believing that the extremely stable molecules could yield new and improved lubricants, protective coatings, and other materials. But, even more exciting to some are the possible materials that may be produced by combining the carbon framework of the Buckyball with different atoms. The process of knocking one or more carbon atoms out of the Buckyball structure and replacing it with metal atoms is known as doping, and the molecule in its altered form is often referred to as a dopeyball. The electrical and magnetic properties of dopeyballs have been the subject of intense study, which has already resulted in the discovery that potassium-doped Buckyballs are capable of superconducting at 18 K and those doped with rubidium superconduct at 30 K.
Edge of a C-60 Thin Film In addition to doping Buckyballs with other atoms, the hollow structure of the geodesic molecules makes it possible to trap atoms inside them like a molecular cage. This strange capability of Buckyballs has caught the attention of the medical community. Indeed, many researchers believe that eventually Buckyballs may be used to deliver medicines to specific tissues and cells, such as those that have been attacked by a certain bacteria, protecting the rest of the body from the toxic effects of potent pharmaceuticals. This same concept is currently being used to develop improved Medical Resonance Imaging (MRI) contrast agents and image enhancers that exploit the carbon cage of a Buckyball to shield patients from the radioactive materials inside. There are also many non-medical possibilities for atom-filled Buckyballs, which are termed endohedral metallofullerenes (EMFs) when the atoms trapped inside are metallic. For instance, EMFs are well on their way to being utilized in organic solar cells and may one day be crucial components of nanoelectronic devices, which many predict will eventually revolutionize the modern communications industry. Some EMFS have also shown potential for use as chemical catalysts that could be delivered to support surfaces in novel ways.
C-60 Thin Film What is perhaps most amazing about Buckyballs is that despite their circuitous discovery in the laboratory, they have been naturally present on Earth all along. The earliest evidence that Buckyballs occur in nature was discovered by Arizona State University researchers Semeon Tsipursky and Peter Buseck, who found that a sample of rare, carbon-rich rock called shungit, estimated to have been formed between 600 million and 4 billion years ago, contained both C60 and C70 fullerenes. Since then, the fascinating molecules have also been identified in meteorites, impact craters, and materials struck by lightning. This new information has led some scientists to speculate about the role that fullerenes may have played in the development of life on Earth. Indeed, gases are known to become readily trapped inside the hollow molecules, and one research group has already found evidence of a form of helium in Buckyballs taken from the Sudbury crater (which contains the greatest known concentration of fullerenes in the world) that likely originated not inside of our own solar system, but within a red giant star. Thus, many consider it to be theoretically possible that Buckyballs carried from their stellar origin both the carbon essential to life and the volatiles that helped produce the planetary conditions necessary for life to begin. ______________________________________...
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 C60Buckminsterfullerene (IUPAC name (C60-Ih)[5,6]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).
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.
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.
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.
Solubility 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.
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.
At first they were considered laboratory-created freaks. Then some of them turned up in outer space. Now they're being sent to ORNL from the frozen reaches of northern Russia. What's going on here?
RNL's Bob Hettich was on the case. He analyzed. He checked. He double checked. His conclusion?
"Buckyballs. Definitely buckyballs."
Buckyballs Video Clip (QuickTime, 2.3 minutes, 3 MB)
These enigmatic clusters of carbon atoms have been puzzling scientists since 1985 when they were discovered in a research laboratory among the by-products of laser-vaporized graphite. Their hollow spherical structure, reminiscent of the geodesic domes of eccentric architect Buckminster Fuller, earned them the names "buckyballs" and "fullerenes."
Qualities, such as their unique structure, heat resistance, and electrical conductivity, have fueled speculation about their possible applications in high-temperature lubricants, microfilters, more efficient semiconductors, and manufacturing processes.
To learn more about buckyballs and how they are formed, researchers began to look for naturally occurring fullerenes, particularly on the earth. The first evidence that fullerenes occur naturally on the earth came to light when Arizona State University researchers Semeon Tsipursky and Peter Buseck examined a sample of shiny black rock, known as shungite, from northeastern Russia. Shungite is a rare, carbon-rich variety of rock believed to have been formed between 600 million and 4 billion years ago, although how it was formed is debatable. Electron microscopy of the shungite samples revealed a pattern of white circles with black centers--similar to micrographs Tsipursky had seen of laboratory-produced fullerenes.
To confirm their suspicions, Buseck and Tsipursky sent a trace of powdered rock between two glass slides to Bob Hettich of ORNL's Chemical and Analytical Services Division for examination by mass spectroscopy, a technique that sorts molecules by weight and electric charge. Hettich had previously worked with Buseck to analyze samples from both meteorites and terrestrial rocks for evidence of fullerenes, but they had found none. The shungite sample was different, however; Hettich's analysis confirmed the presence of fullerenes in the rock.
"We wanted to make sure we were looking at only what was in the sample and not distorting it in any way," says Hettich. So, he conducted two separate analyses of the sample. In the initial analysis, he used a pulsed laser to vaporize and ionize the sample, preparing it for analysis by mass spectroscopy. Hettich also analyzed carbon samples known not to contain fullerenes to ensure that none were being created by the laser vaporization process itself. The initial analysis confirmed the presence of both C60 and C70, two common fullerenes, in the shungite sample.
To dispel any lingering doubt, Hettich repeated the analysis without a laser, this time using a 400°C stainless steel probe to vaporize the sample and introduce it into the mass spectrometer for ionization. This technique, known as thermal desorption, cannot create fullerenes in fullerene-free graphite material, yet it yielded identical results, confirming the presence of the two types of buckyballs in the sample.
When Buseck and Tsipursky told Hettich that the rock had come from Russia and not a meteorite, he was somewhat surprised. "In the laboratory," says Hettich, "fullerenes are created in an atmosphere of inert gases, like helium, because common diatomic gases, like nitrogen and oxygen inhibit fullerene growth. This is why fullerenes are not found in ordinary soot, like that in household fireplaces. It seemed more likely to find naturally occurring fullerenes in meteorites, where interaction with these gases would be less of a problem."
The discovery of fullerenes in the shungite sample has provided some hard information for buckyball hunters who have been working mostly on educated guesses and speculation. "We've been working with Peter Buseck for quite a while analyzing various samples, but until now we hadn't found any fullerenes," Hettich notes, "This discovery helps us redefine where to look." More recently, C60 and C70 have also been found in a sample of glassy rock from the mountains of Colorado. Known as a fulgurite, this type of rock structure is formed when lightning strikes the ground. Busek, Tsipursky, and Hettich speculated in a 1992 paper that lightning strikes could provide conditions that are favorable for the formation of buckyballs.
The shungite fullerenes are notable not only for their earthly origin, but also because they may have been formed as solids--most laboratory-created fullerenes are grown in the gas phase. "This is the first example of solid-phase fullerene growth," says Hettich, "It has raised a lot of questions about how the rock was formed, how old it is, and how its composition may have changed over time. Because the shungite sample may be volcanic in origin, you can imagine conditions, like those in a volcano, that would be hot enough to form fullerenes and, at the same time, have little or no oxygen or nitrogen present. But right now, no one is sure exactly how these fullerenes were produced."
"This kind of discovery raises more questions than it answers," says Hettich, "but that's not necessarily a bad thing."--Jim Pearce
Sizing Up Fullerenes--"SANS Doute" "Sans doute!" a confident Frenchman might say--"without a doubt!" But in the brand new world of fullerenes, this sort of certainty is sometimes in short supply. Much of the uncertainty surrounding these newly discovered carbon clusters stems from their size--you could line up 25 million C60 molecules on a ruler before passing the inch mark.
So, although tools like mass spectrometers can be used to distinguish heavier fullerenes from lighter ones--separating C120 from C180, for instance--researchers still have trouble answering some of the most basic questions about them. How big are they? Are they shaped like spheres, dumbbells, or what? How and where do other atoms bond to their inner and outer surfaces?
Using a time-tested analysis technique of small-angle neutron scattering, appropriately labeled SANS, a team of researchers from ORNL's Biology, Chemical Technology, Health Sciences Research, and Solid State divisions is working to dispel some of the mystery surrounding fullerenes, including how they interact and bond with other elements and with each other.
The preferred method of studying the structure of most materials is crystallography. This technique enables researchers to pinpoint the location of every atom in a sample. "Even though C60 has been crystallized, this is not always possible with other materials," says Stephen Henderson of ORNL's Biology Division. "Other techniques, like SANS, are more accessible, though they give less structural information." SANS requires only that the material be dissolved, rather than crystallized; then scattered neutrons are counted for several hours and the data are analyzed.
The SANS research facility, located at ORNL's High Flux Isotope Reactor, is operated by George Wignall of the Solid State Division. There, dissolved fullerene samples are placed in the path of a neutron beam. As the beam passes through the sample, neutrons are deflected, or scattered, by carbon molecules in the solvent. This scattering is recorded by a detector, providing a two-dimensional pattern, or "signature," for the material, which can then be analyzed to determine the size and shape of the dissolved molecules.
"The greatest significance of using SANS to analyze fullerenes is its ability to discern shapes," says Bob Haufler, a postdoctoral fellow in the Health Sciences Research Division (HSRD). "This is clearly fertile ground for new chemistry. I think it will be especially helpful in situations where atoms of hydrogen or metals are attached to the inside of the fullerenes." "It's also interesting to see how the fullerenes interact with the solvents," says Kathleen Affholter of the Solid State Division, "to see if polymers are forming, for example."
The SANS facility "sees" objects in its neutron beam by keeping track of the neutrons the objects scatter. This scattering varies with the square of an object's volume, so when its diameter decreases by half, it scatters only one-quarter as many neutrons. As a result, the smallest fullerenes are near the lower limit of what the SANS can see--a factor in the past reluctance of researchers to use SANS in this type of research.
Even though the outcome was in doubt, Wignall encouraged Affholter and others to pursue the project because the potential scientific payoff was so high. "If it hadn't been for George," Affholter says, "the project wouldn't have started in the first place." When Affholter introduced the idea of using the SANS facility for fullerene studies to Bob Compton of HSRD, he said his group was making some C120 and C180 molecules and they didn't know what they looked like--whether they were dumbbell-shaped or just bigger versions of the more or less round C60 and C70 balls. "We decided to look at the C60 and C70 balls first," says Affholter, "because they were available and their sizes and shapes were already known."
Because of the relatively small sizes of these fullerenes, the researchers sought to optimize several factors in the experiments. First, the distance the neutron beam traveled through the fullerene solution was increased from a typical 2 mm to 20 mm, increasing the likelihood of interactions between the fullerenes and the neutron beam. "As a result," says Henderson, "we got incredibly good statistics after an hour or so. Often, for solution work, differences are hard to see even after 10 hours of counting."
Second, the fullerenes were dissolved in a solvent that is relatively transparent to neutrons to maximize the contrast between the two. "A visual analogy," says Henderson, "would be observing blue balls in a transparent solvent, rather than in a blue solvent." Fortunately, the solvent that provided the best contrast also dissolved C60 and C70 most effectively, again putting more molecules in the path of the beam.
The team hopes to expand its work to include further explorations of the basic chemistry of buckyballs, including imaging fullerenes that have been combined with other elements, such as hydrogen, fluorine, and various metals. They expect to be able to determine how many and where these "piggyback" atoms are attached to the inner and outer surfaces of the fullerenes. They also expect to be able to produce and analyze larger fullerenes.
"It is difficult to get this kind of information from other techniques," Henderson says. "We also expect to be able to see whether these additional atoms have expanded the structure of the fullerenes. The actual mechanics and chemistry of adding other atoms to these molecules helps us understand how they react and combine with other elements. It could be that these materials--hydrogenated fullerenes, for instance--are better starting points for making other products."
"This project was an excellent example of cooperation among four research divisions," says Affholter. "Everybody had something to add to the project. Everybody talked and pulled together to make it work." The success of this group bodes well for the future of the informal collaboration. "We've gotten together to determine what we want to do next," Affholter says. "We like doing this kind of work, and if we don't do it, people at other labs will."
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