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Elias Lectures Chemistry of Carbon Fullerenes Final 30Th The inorganic chemistry of Carbon Carbon Nano tubes Graphite intercalated compounds Graphene Fullerenes In 1985, Harold Kroto (Sussex), Robert Curl and Richard Smalley, (Rice University,) discovered C 60 , 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 H. Kroto R. Smalley molecules. C 60 and other fullerenes were later noticed occurring outside the laboratory (for example, in normal candle-soot).. Fullerenes An idea from outer space Kroto's special interest in red giant stars rich in carbon led to the discovery of the fullerenes. For years, he had had the idea that long- chained molecules of carbon could form near such giant stars. To mimic this special environment in a laboratory, Curl suggested contact with Smalley who had built an apparatus which could evaporate and analyze almost any material with a laser beam. During the crucial week in Houston in 1985 the Nobel laureates, together with their younger co- workers J. R. Heath and J. C. O'Brien, starting from graphite, managed to produce clusters of carbon consisting mainly of 60 or 70 carbon atoms. These clusters proved to be stable and more interesting than long-chained molecules of carbon. Two questions immediately arose. How are these clusters built? Does a new form of carbon exist besides the two well-known The read-out from the mass spectrometer shows forms graphite and diamond? how the peaks corresponding to C 60 and C70 become more distinct when the experimental conditions are optimized. C60 is soccer-ball-shaped or Ih with 12 pentagons and 20 hexagons. According to Euler's theorem these 12 pentagons are required for closure of the carbon network consisting of n hexagons and C 60 is the first stable fullerene because it is the smallest possible to obey this rule (higher ones C 180, 540). In this structure none of the pentagons make contact with each other. Both C 60 and its relative C 70 obey this so-called isolated pentagon rule (IPR) . Non-IPR fullerenes have thus far only been isolated as endohedral fullerenes such as Tb 3N@C 84 The double bonds in fullerene are not all the same. Two groups can be identified: 30 so- called [6,6] double bonds connect two hexagons and 60 [5,6] bonds connect a hexagon and a pentagon. Of the two the [6,6] bonds are shorter with more double-bond character and therefore a hexagon is often represented as a cyclohexatriene and a pentagon as a pentalene or [5]radialene. In other words, although the carbon atoms in fullerene are all conjugated the superstructure is not a super aromatic compound. The X-ray diffraction bond length values are 135.5 pm for the [6,6] bond and 146.7 pm for the [5,6] bond. Preparation and purification of C60 and C70 Kratschmer’s methodology for preparation of fullerenes is the one widely used currently which has been modified by various workers to increase yields. It was Kroto again who separated C60 and C70 for the first time in pure forms. Graphite electrodes are evaporated in an atmosphere of ~ 100 torr of Helium (Kratschmer) or 50 -100 torr of Argon (Kroto) in a glass vessel (modified RB flask, Power from a transformer) . The soot formed is scrapped and dispersed in benzene whereupon a wine red solution is obtained. This is filtered from the insoluble solids and concentrated . This mixture of C60 and C70 is then run on an alumina column using hexane as eluant. The magneta colored C60 elutes out first followed by the port wine colored C70 . In a typical case the ratio of C60 to C70 will be 5:1. Solid pure C60 will be mustard colored [UV 596 , 604 , 625 nm] and C70 will be red [600, 617, 644 nm]. (laser vaporization of graphite and graphite doped with other elements and compounds such as lanthanide metals, boron nitride has also been used for synthesis of fullerenes ) How to draw a C60? step 1 step 2 step 3 step 4 step 5 ( no double bonds in pentagons) The structure of C60 can be specifically described as having 12 pentagons and 20 hexagons with the pentagons sharing no common edge and the hexagons sharing edges with another hexagon or a pentagon. All the carbons are tricoordinate, pyramidal and all pentagons and hexagons are planar. Two types of C- C bonds are present in the molecule with differing bond lengths, 1.388 Å for a 6,6 bond (common for two hexagons) and 1.432 Å for a 5,6 bond (common for a hexagon and a pentagon ). In contrast to C60 were all carbon atoms are identical, C70 has five different types of carbon atoms depending on the carbon environment. These are easily 13 differentiated by C NMR. C76 , another fullerene whose X ray structure has been solved belongs to D2 point group, is chiral and occurs as a racemic mixture. Stability of C 60 ( a ) ( b) (c) The unusual stability of C60 compared to other higher / lower fullerenes has been explained. In the structure of C60 , all the twelve pentagons are isolated from each other. Again only in C60 we can see double bonds arranged in such a way that they are located only in six membered rings and none in five membered rings. This is favored as there will be less strain on the already strained five membered rings as a result of such an arrangement. The need to avoid double bonds in pentagons largely governs the stability of fullerenes as the five membered rings with five planar hexagons around are already strained and unsaturation is going to increase the strain further. C60 has only arrangement (a) in its structure while C70 and C84 has five and six of the (b) arrangements in their structures respectively. Concept of aromaticity on C60 Initially C60 was predicted to be extremely stable and aromatic. Even more than 12,500 resonance structures were proposed. The terms superaromaticity and three dimensional aromaticity were freely used to describe its structure. However the presence of strained five membered rings adjacent to benzenoid rings were overlooked. Only one structure exists for C60 which avoids having any double bonds in pentagons. The two important consequences of this finding was that a) the delocalization of electrons in C60 is poor and so it is more reactive than expected. b) C60 is not so much an aromatic compound and the double bonds in it are isolated. This is clearly indicated by the η2 coordination shown by C60 when reacted with Vaska’s complex Chemistry of fullerenes: Different directions Twenty-nine years on from the discovery of C 60 , the outstanding properties and potential applications of the synthetic carbon allotropes — fullerenes, nanotubes and graphene — overwhelmingly illustrate their unique scientific and technological importance. a, Fullerene salts; b, exohedral adducts/derivatives; c, open-cage fullerenes; d, quasi-fullerenes; e, heterofullerenes; f, endohedral fullerenes. Endohedral and open cage Molecular Surgery! The molecular surgery technique involves a series of carefully controlled chemical reactions to open up the fullerene cage and then insert the guest molecule into the fullerene cage through high temperature and pressure condition. It then follows up with another series of reaction to reseal the orifice while the molecule is inside. This is the technique used to synthesize all variant of dihydrogen endofullerenes Exohedral Fullerenes ηηη2 – Complexes of fullerenes OC CO OC CO OC Ru CO Ru CO OC Ru CO ηηη5- Complexes of Fullerenes Bucky metallocenes Lewis acidity of Fullerene: Reaction with an N Heterocyclic carbene Different dimensionalities of carbon materials Graphene is a 2D building material for different dimensionalities of carbon materials, can be wrapped into fullerenes, rolled into nanotubes or stacked into graphite Carbon Nanotubes The first observation of the multiwalled carbon nanotubes was credited to Iijima. In 1993 Iijima and Donald Bethune found single walled nanotubes known as buckytubes. This helped the scientific community make more sense out of not only the potential for nanotube research, but the use and existence of fullerene. Nanotubes discovered in the soot of arc discharge at NEC, by Japanese researcher Sumio Iijima Nano 10 -9 10 -6 10 -3 10 0 10 3 10 6 10 9 m • Size – 10 -9 m (1 nanometer) • Border to quantum mechanics What are Carbon nanotubes? •Carbon nanotubes (CNTs ) are allotropes of carbon. These cylindrical carbon molecules have interesting properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science, as well as potential SWNT uses in architectural fields. •They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Their final usage, however, may be limited by their potential toxicity. MWNT Nanotube Three types based on style of roll formation ZIG ZAG , ARM CHAIR , CHIRAL Nanotube High aspect ratio: Length: length >1000 typical few μm diameter → quasi 1D solid The shortest carbon nanotube is the organic compound cycloparaphenylene, having 8 phenyl rings connected Diameter: through para positions which was synthesized in early as low as 1 nm 2009 The longest carbon nanotubes grown so far are over 550 mm long was reported in 2013 Electrical Properties Electrical conductance depending on helicity • If the nanotube structure is armchair then the electrical properties are metallic • If the nanotube structure is chiral then the electrical properties can be either semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor • In theory, metallic nanotubes can carry an electrical current density of 4×10 9 A/cm 2 which is more than 1,000 times greater than metals such as copper Mechanical Properties Strong Like Steel Light Like Aluminum Elastic Like Plastic Strength Properties • Carbon nanotubes have the strongest tensile strength of any material known.
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