A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes. Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings; but they may also contain pentagonal (or sometimes heptagonal) rings.
The most common and most stable fullerene is buckminsterfullerene, a spheroidal molecule, resembling a soccer ball, consisting of 60 carbon atoms. Buckminsterfullerene is the most abundant cluster of carbon atoms found in carbon soot. It is also the smallest carbon molecule whose pentagonal faces are isolated from each other. Other fullerenes that have been produced in macroscopic amounts have 70, 76, 84, 90, and 96 carbon atoms, and much larger fullerenes have been found, such as those that contain 180, 190, 240, and 540 carbon atoms.
STRUCTURE OF FULLERENES (C60)
Some of the more stable members of the fullerene family. (a) C28. (b) C32. (c) C50. (d) C60. (e) C70.
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PREDICTION AND DISCOVERY
Fullerenes were first identified in 1985 as products of experiments in which graphite was vaporized using a laser, work for which R. F. Curl, Jr., R. E. Smally, and H. W. Kroto shared the 1996 Nobel Prize in Chemistry. Fullerenes have since been discovered in nature as a result of lightning strikes, in the residue produced by carbon arc lamps, in interstellar dust, and in The first fullerene to be discovered, and the family's namesake, was buckminsterfullerene C60, made in 1985 by Robert Curl, Harold Kroto and Richard Smalley. The name was an homage to Richard Buckminster Fuller, whose geodesic domes it resembles. Fullerenes have since been found to occur (if rarely) in nature.
The existence of C60 was predicted by Eiji Osawa of Toyohashi University of Technology in a Japanese magazine in 1970. He noticed that the structure of a corannulene molecule was a subset of a soccer-ball shape, and he made the hypothesis that a full ball shape could also exist. His idea was reported in Japanese magazines, but did not reach Europe or America.
Also in 1970, R.W.Henson (then of the Atomic Energy Research Establishment) proposed the structure and made a model of C60. The evidence for this new form of carbon was very weak and was not accepted, even by his colleagues. The results were never published but were acknowledged in the Carbon journal in 1999.
With mass spectrometry, discrete peaks were observed corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms. In 1985, Harold Kroto (then of the University of Sussex), James R. Heath, Sean O'Brien, Robert Curl and Richard Smalley, from Rice University, discovered C60, and shortly thereafter 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 the laboratory (e.g., in normal candle soot). By 1991, it was relatively easy to produce gram-sized samples of fullerene powder using the techniques of Donald Huffman and Wolfgang Krätschmer. Fullerene purification remains a challenge to chemists and to a large extent determines fullerene prices. 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. The first nanotubes were obtained in 1991.
Minute quantities of the fullerenes, in the form of C60, C70, C76, and C84 molecules, are produced in nature, hidden in soot and formed by lightning discharges in the atmosphere. Recently, fullerenes were found in a family of minerals known as Shungites in Karelia, Russia.
Buckminsterfullerene (C60) was named after Richard Buckminster Fuller, a noted architectural modeler 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 shortened name 'fullerene' was used to refer to the family of fullerenes. The suffix "ene" indicates that each C atom is covalently bonded to three others (instead of the maximum of four), a situation that classically would correspond to the existence of bonds involving two pairs of electrons ("double bonds").
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Since the discovery of fullerenes in 1985, structural variations on fullerenes have evolved well beyond the individual clusters themselves. Examples include:
buckyball clusters: smallest member is C20 (unsaturated version of dodecahedrane) and the most common is C60;
nanotubes: hollow tubes of very small dimensions, having single or multiple walls; potential applications in electronics industry;
megatubes: larger in diameter than nanotubes and prepared with walls of different thickness; potentially used for the transport of a variety of molecules of different sizes;
polymers: chain, two-dimensional and three-dimensional polymers are formed under high pressure high temperature conditions
nano"onions": spherical particles based on multiple carbon layers surrounding a buckyball core; proposed for lubricants;
linked "ball-and-chain" dimmers: two buckyballs linked by a carbon chain;
Buckminsterfullerene (IUPAC name (C60-Ih)[5,6]fullerene) is the smallest fullerene molecule in which no two pentagons share an edge (which can be destabilizing, as in 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 (T = 3) icosahedron, which resembles a soccer ball of the type made of twenty hexagons and twelve pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge.
The van der Waals diameter of a C60 molecule is about 1 nanometer (nm). The nucleus to nucleus diameter of a C60 molecule is about 0.71Â nm.
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). Its average bond length is 1.4 angstroms.
A new type of buckyball utilizing boron atoms instead of the usual carbon has been predicted and described in 2007. The B80 structure, with each atom forming 5 or 6 bonds, is predicted to be more stable than the C60 buckyball. One reason for this given by the researchers is that the B-80 is actually more like the original geodesic dome structure popularized by Buckminster Fuller which utilizes triangles rather than hexagons. However, this work has been subject to much criticism by quantum chemist as it was concluded that the predicted Ih symmetric structure was vibrationally unstable and the resulting cage undergoes a spontaneous symmetry break yielding a puckered cage with rare the symmetry (symmetry of a volleyball). The number of six atom rings in this molecule is 20 and number of five member rings is 12. There is an additional atom in the center of each six member ring, bonded to each atom surrounding it.
VARIATION OF BUCKYBALLS
Another fairly common buckminsterfullerene is C70, but fullerenes with 72, 76, 84 and even up to 100 carbon atoms are commonly obtained.
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). It follows from Euler's polyhedron formula, |V|-|E|+|F| = 2, (where |V|, |E|, |F| indicate the number of vertices, edges, and faces), that there are exactly 12 pentagons in a fullerene and |V|/2-10 hexagons.
20 FULLERENE 26 FULLERENE 60 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..., roughly in proportion to n9 (sequence A007894 in OEIS). For instance, there are 1812 non-isomorphic fullerenes C60. Note that only one form of C60, 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.
STRUCTURE AND PROPERTIES
In the fullerene molecule an even number of carbon atoms are arrayed over the surface of a closed hollow cage. Each atom is trigonally linked to its three near neighbors by bonds that delineate a polyhedral network, consisting of 12 pentagons and n hexagons. (Such structures conform to Euler's theorem for polyhedrons in that n may be any number other than one including zero.) All 60 atoms in fullerene-60 are equivalent and lie on the surface of a sphere distributed with the symmetry of a truncated icosahedron. The 12 pentagons are isolated and interspersed symmetrically among 20 linked hexagons.
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In benzene solution, fullerene-60 is magenta and fullerene-70 red. Fullerene-60 forms translucent magenta face-centered cubic (fcc) crystals that sublime. The ionization energy is 7.61 eV and the electron affinity is 2.6-2.8 eV. The strongest absorption bands lie at 213, 257, and 329 nanometers. Studies with nuclear magnetic resonance spectroscopy yield a chemical shift of 142.7 parts per million; this result is commensurate with an aromatic system.
Solid C60 exhibits interesting dynamic behavior in that at room temperature the individual round molecules in the face-centered cubic crystals are rotating isotropically (that is, freely) at around 108 Hz. At around 260 K (8.3Â°F) there is a phase transition to a simple cubic (sc) lattice accompanied by an abrupt lattice contraction. Rotation is no longer free, and the individual molecules make rotational jumps between two favored (relative) orientational configurations-in the lower-energy one a double bond lies over a pentagon, and in the other it lies over a hexagon.
C 60 WITH ISOSURFACE OF GROUND STATE ELECTRON DENSITY
CHEMISTRY AND FORMATION
Fullerene-60 behaves as a soft electrophile, a molecule that readily accepts electrons during a primary reaction step. It can readily accept three electrons and perhaps even more. The molecule can be multiply hydrogenated, methylated, ammonated, and fluorinated. It forms exohedral complexes in which an atom (or group) is attached to the outside of the cage, as well as endohedral complexes in which an atom is trapped inside the cage.
The C60 molecule behaves as though it has only a single resonance form-one in which the 30 double bonds are localized in the bonds that interconnect the pentagons. This is a key factor, as addition to these double bonds is the most important reaction as far as the application of C60 in synthesis is concerned.
On exposure of C60 to certain alkali and alkaline earth metals, exohedrally doped crystalline materials are produced that exhibit superconductivity at relatively high temperatures (10 to 33 K or âˆ’440 to âˆ’400Â°F). The C60 molecule has a triply degenerate lowest unoccupied molecular orbital (LUMO), which in the superconducting materials is half filled, containing three electrons. Other ionic phases, such as MnC60 (n = 1, 2, 4, 6, where M is the intercalated metal atom), exist but are not superconducting-they appear to be metallic or semiconductor/insulators. See also Molecular orbital theory.
Perhaps the most important aspect of the fullerene discovery is that the molecule forms spontaneously. This fact has important implications for understanding the way in which extended carbon materials form, and in particular the mechanism of graphite growth and the synthesis of large polycyclic aromatic molecules. It has become clear that as far as pure carbon aggregates of around 60-1000 atoms are concerned, the most stable species are closed-cage fullerenes.
Nanotubes are cylindrical fullerenes. These tubes of carbon are usually only a few nanometres wide, but they can range from less than a micrometer to several millimeters in length. They often have closed ends, but can be open-ended as well. There are also cases in which the tube reduces in diameter before closing off. Their unique molecular structure results in extraordinary macroscopic properties, including high tensile strength, high electrical conductivity, high ductility, high resistance to heat, and relative chemical inactivity (as it is cylindrical and "planar" - that is, it has no "exposed" atoms that can be easily displaced). One proposed use of carbon nanotubes is in paper batteries, developed in 2007 by researchers at Rensselaer Polytechnic Institute. Another proposed use in the field of space technologies and science fiction is to produce high-tensile carbon cables required by a space elevator.
ROTATING CARBON NANOTUBE WITH 3D STRUCTURE
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. Popular Science has published articles about the possible uses of fullerenes in armor 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.
Researchers have been able to increase the reactivity of fullerenes by attaching active groups to their surfaces. 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, 72, 98, 128, etc.; i.e. twice a perfect square number; 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 trying to form a loose metallic bonding.
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 120Â° in the sp2 orbitals to about 109.5Â° 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. An unusual example is the egg shaped fullerene Tb3N@C84, which violates the isolated pentagon rule. Recent evidence for a meteor impact at the end of the Permian period was found by analyzing 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 aromatics, such as toluene, and others like carbon disulfide. Solutions of pure buckminsterfullerene have a deep purple color. Solutions of C70 are a reddish brown. The higher fullerenes C76 to C84 have a variety of colors. C76 has two optical forms, while other higher fullerenes have several structural isomers. Fullerenes are the only known allotrope of carbon that can be dissolved in common solvents at room temperature.
Some fullerene structures are not soluble because they have a small band gap between the ground and excited states. These include the small fullerenes C20, C36 and C50. The C72 structure is also in this class, but the endohedral version with a trapped lanthanide-group atom is soluble due to the interaction of the metal atom and the electronic states of the fullerene. Researchers had originally been puzzled by C72 being absent in fullerene plasma-generated soot extract, but found in endohedral samples. Small band gap fullerenes are highly reactive and bind to other fullerenes or to soot particles.
Solvents that are able to dissolve buckminsterfullerene (C60) are listed below in order from highest solubility. The value in parentheses is the approximate saturated concentration.
ISOHEDRAL FULLERENE C540