Fullerene

As fullerenes ( singular: fullerene ) spherical molecules of carbon atoms are referred to ( with high symmetry, such as Ih symmetry of C60), which represent further modifications of the chemical element carbon ( next to diamond, graphite, carbon nanotubes and graphene ).

History

The first paper on fullerenes was already in 1970 by the Japanese chemist Eiji Osawa, the theoretically predicted their existence and calculated. These and following his publications he published in Japanese, which is why 15 years later published in the journal Nature on November 14, 1985 publication of the first explorer Robert F. Curl, Jr. (USA ), Sir Harold W. Kroto (England ) and Richard gained E. Smalley (USA) worldwide attention. These received for the 1996 Nobel Prize in Chemistry, while Osawa was disregarded.

Prior to these publications on fullerenes, there were some to " hollow molecules," for example, an article by David Jones in New Scientist in 1966, reprinted in the book " jitter gas and oblique water " (p. 27 f ), with bills to the stability of hollow molecules, the then largest known molecules only had dodecahedral form, that contained only 20 atoms.

2010 fullerenes were detected by infrared images of the Spitzer Space Telescope in the planetary nebula Tc 1. They are thus the largest proven molecules in extraterrestrial space.

Name

The best known and most stable representative of the fullerenes have the molecular formulas C60, C70, C76, C80, C82, C84, C86, C90 and C94. By far the best researched fullerene is C60, which was (in English also buckyball ) named in honor of the architect Richard Buckminster Fuller Buckminster fullerene, as it resembles the geodesic domes designed by him. It consists of 12 pentagons and 20 hexagons, which together form a truncated icosahedron ( Archimedean bodies). Since football has the same structure, it is also called soccer ball molecule ( footballen ).

Production

For the first time the C60 fullerene from the already mentioned research group ( Harold Walter Kroto, Richard Smalley Savior, Robert Floyd Curl, James R. Heath and SC O'Brien ) was established in 1985. In a helium atmosphere at 10 bar pressure a rotating graphite disk was irradiated by a pulsed laser. The laser light was in the test, a wavelength of 532 nm, was transmitted to the inside of an irradiation time of 5 ns, a power of 30 to 40 mJ. The carbon of the graphite linked to penta- and hexagonal ring structures after they were been driven out of the helium from the irradiation chamber, combined in a reaction chamber, among others, to fullerenes.

Due to the inefficiency of the process, a new procedure was developed in 1990 by Konstantinos Fostiropoulos, Wolfgang Krätschmer and Donald Huffman. Graphite under reduced pressure in an inert gas atmosphere (argon) or with a resistance heater in an arc vaporized in a helium atmosphere. The resulting soot contains up to 15 % fullerenes. This manufacturing process enabled only research on fullerenes on a large scale.

It is also possible the production using only rational syntheses, in which case the last step a flash vacuum pyrolysis takes place. However, the yield in this method is only about one percent, which is why it is significantly more expensive than the manufacture in an electric arc.

You can also extract a specially prepared carbon black sample with toluene in a Soxhlet extractor at 110 ° C. This produces carbon clusters such as C2, C4 and C6, which meet to form larger units again when cooled. Here C60 is the most common form, Next door you can C70 and higher fullerenes. In soot remaining after cooling, the fullerenes, as well as carbon nanotubes can be found. A first separation is however poorly with benzene, in which fullerenes dissolve readily, the remaining soot. By chromatography, for example on active carbon and / or silica gel, the fullerenes can be separated.

The fullerenes C60 and C70 are found naturally in Shungit and Fulgurit.

Properties

Fullerenes are brown-black powder of metallic luster. They dissolve in some organic solvents (eg, toluene ) under their characteristic color. Fullerenes can be at about 400 ° C sublimate.

Different ways to use as a catalyst, lubricant, for the production of artificial diamond, in medicine, as semiconductors and superconductors are subject of research.

Due to the bonding in the molecule, it can be extremely accommodate many radicals and bind ( scavenger). This should be partly responsible for the aging process of the skin. However, this effect of fullerenes is not scientifically proven.

The oral administration of C60 was dissolved in olive oil ( 0.8 mg / ml) in repeated doses has ( 1.7 mg / kg body weight) in rats, not only no chronic toxicity result, but nearly doubles the service life of the rats.

Nomenclature

For a long time refused the authority responsible for binding recommendations on the nomenclature of chemical compounds IUPAC to recognize the trivial names fullerene. Only in 2002 she changed her mind and is recommended since the use of fullerane, fullerene and Fulleroid. This means a considerable relief, because until then the correct, ie IUPAC -compliant name, such as desFullerens ( C60), the following would have been:

Structure and Stability

Many fullerenes have 12 pentagons, which are surrounded by a different number of hexagons. Due to the impossibility of a plane with regular pentagons ( and hexagons ) to completely cover, the spherical curvature results ( see figure at right ). The smallest fullerene is a dodecahedron, C20, and consists only of pentagonal carbon rings.

C60 has approximately the diameter of 700 pm, thus 7 × 10-10 m. The van der Waals diameter is, however, about 1000 pm, so a nanometer or 1 × 10-9 m. The mass of the C60 fullerene is about 720 u, C60 also has icosahedral symmetry. Fullerenes containing more than 60 carbon atoms generally have less symmetry about C70 is approximately an ellipsoid with D5h symmetry.

The stability of a fullerene is greatest when

• The pentagons are not contiguous, but only surrounded by hexagons are ( Fünfeckregel, English:. isolated pentagon rule, IPR),
• The aromatic character is pronounced ( see aromaticity, although in this case the so-called spherical aromaticity must be considered ), and
• It (60, 70, 76, etc.) having a magic number of carbon atoms.

Fullerenes are closely related to the graph, a modification of the carbon in which carbon atoms form a monomolecular layer having a hexagonal structure.

Reactions of C60

Fullerenes offer three starting points for chemical modifications. By addition reactions at the double bonds is obtained exohedral adducts. The replacement of carbon atoms of the cage shell by, for example, the nitrogen atoms are known as substitution C59N elles doping. Finally, such cage structures still offer the option of atoms or compounds into the cavity contribute. Compounds of this type are called endohedral complexes. For identification of endohedral complexes X @ Cn the spelling has prevailed in the literature, in which X is an atom or cluster inside a fullerene of n carbon atoms.

C60 has a cavity with a diameter of 700 pm, to be incorporated into the metal and non- metal atoms. One example is the inclusion compound of the helium, which is referred to with the notation correctly He @ C60. He @ C60 is formed when graphite is vaporized in a helium atmosphere.

Furthermore, C60 can enter the aromatics but also alkenes typical reactions such as hydrogenation, halogenation, ozonolysis and Birch reduction. However, usually there will be no full implementation of all double bonds; only with fluorine, the composition can be achieved C60F60.

Further interesting compounds are the ionic alkali metal fullerides: C60 can be reduced with sodium and potassium. This gives compounds of the composition MC60, M2C60 and M3C60 (M = Na, K). KC60 crystallizes in the sodium chloride structure. In K3C60, the C603 is - anion and forms a cubic close packing of spheres, with the K cations occupy all the available tetrahedral and octahedral voids in the crystal structure. K3C60 is a superconductor.

In the group of Anton Zeilinger at the University of Vienna (see web link) the interference of C60 molecules was observed on the grid. This postulated by Louis de Broglie matter waves were also shown for relatively macroscopic objects.

In the group of Jochen Mattay at the University of Bielefeld extensive studies on the functionalization of fullerenes have been made to aza- Heterofullerenes.

Natural Occurrence

Fullerenes are found in certain crystals in nature. Most commonly found in Shungit, whose name derives from its location near the Shunga region of Karelia in Russia.