| Buckminsterfullerene | |
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(C60-Ih)[5,6]fullerene |
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Other names
Buckyball; Fullerene-C60; [60]fullerene |
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| Identifiers | |
| CAS number | 99685-96-8 |
| PubChem | 123591 |
| ChemSpider | 110185 |
| ChEBI | CHEBI:33128 |
| Beilstein Reference | 5901022 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | C60 |
| Molar mass | 720.64 g mol−1 |
| Appearance | Dark needle-like crystals |
| Density | 1.65 g/cm3 |
| Melting point |
sublimates at ~600 °C[1] |
| Solubility in water | insoluble in water |
| Structure | |
| Crystal structure | Face-centered cubic, cF1924 |
| Space group | Fm3m, No. 225 |
| Lattice constant | a = 0.14154 nm |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Buckminsterfullerene (or buckyball) is a spherical fullerene molecule with the formula C60. It is a cage-like fused-ring structure (truncated (T = 3) icosahedron) which resembles an association football ball, made of twenty hexagons and twelve pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge.
It was first intentionally prepared in 1985 by Harold Kroto, James R. Heath, Sean O'Brien, Robert Curl and Richard Smalley at Rice University.[2] Kroto, Curl and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of buckminsterfullerene and the related class of molecules, the fullerenes. The name is a homage to Buckminster Fuller, as C60 resembles his trademark geodesic domes. Buckminsterfullerene was the first fullerene molecule discovered and it is also the most common in terms of natural occurrence, as it can be found in small quantities in soot.[3][4]
Buckminsterfullerene is the largest matter to have been shown to exhibit wave–particle duality.[5] Its discovery lead to the exploration of a new field of chemistry, involving the study of fullerenes.
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Buckminsterfullerene's name derives from the name of the noted futurist and inventor Buckminster Fuller. One of his designs of a geodesic dome structure bore a great resemblance to C60; as a result, the discovers of the allotrope named the newfound molecule after him. Today, many people refer to Buckminsterfullerene and Mr. Fuller's dome structure as buckyballs. [6] A common mistake in the application of the term "buckyball" is how some refer to the toy composed of numerous minuscule magnetic spheres. [7]
The serendipitous discovery of a third allotropic form of carbon in 1985, uncovered a fundamentally different structure of closed carbon cages, which become known as fullerenes. This new family of non-planar carbon compounds has generated immense interest within the scientific community in a short period of time, with thousands of papers published about fullerenes and fullerene-based materials in the 1990s.
Theoretical predictions of buckyball molecules appeared in the late 1960s – early 1970s,[8] but they went largely unnoticed. In the early 1970s, the chemistry of unsaturated carbon configurations was studied by a group at the University of Sussex, led by Harry Kroto and David Walton. In the 1980s a technique was developed by Richard Smalley and Bob Curl at Rice University, Texas to isolate these substances. They used laser vaporization of a suitable target to produce clusters of atoms. Kroto realized that by using a graphite target.[9]
C60 was discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley. Using laser evaporation of graphite they found Cn clusters (where n>20 and even) of which the most common were C60 and C70. For this discovery they were awarded the 1996 Nobel Prize in Chemistry. The discovery of buckyballs was serendipitous, as scientist aimed at producing carbon plasmas to replicate and characterize unidentified interstellar matter. Mass spectrometry analysis of the product indicated the formation of spheroidal carbon molecules.[8]
The experimental evidence, a strong peak at 720 atomic mass units, indicated that a carbon molecule with 60 carbon atoms was forming, but provided little structural information. The research group concluded after reactivity experiments, that the most likely structure was a spheroidal molecule. The idea was quickly rationalized as the basis of an icosahedral symmetry closed cage structure. Kroto mentioned geodesic dome structures of the noted futurist and inventor Buckminster Fuller that resulted in the name buckminsterfullerene.[8]
The versatility of fullerene molecules has led to a great deal of research exploring their properties. One potentially useful property is its large capacity internal spaces that atoms of different elements may be placed inside the molecular cage formed by the carbon atoms, producing a shrink-wrapped version of these elements.[10]
Beam-experiments conducted between 1985 and 1990 provided more evidence for the stability of C60 as well as supporting the closed cage structural theory and predicting some of the bulk properties such a molecule would have. Around this time, intense theoretical group theory activity also predicted that C60 should have only four IR active vibrational bands, on account of its icosahedral symmetry.[11]
In 1989, the Heidelberg/Tuscon group, led by physicists Wolfgang Krätschmer and Donald Huffman, had observed unusual optical absorptions in thin carbon films produced by arc-processed graphite rods. Among other features, the IR spectra showed four discrete bands in close agreement to those proposed for C60. A paper published by the group in 1990 followed on from their thin film experiments, and detailed the extraction of a benzene soluble material from the arc-processed graphite. This extract had crystal and X-ray analysis consistent with arrays of spherical C60 molecules, approximately 0.7 nm in diameter.[11]
In 1990, W. Krätchmer and D. R. Huffman's developed a simple and efficient method of producing fullerenes in gram and even kilogram amounts which boosted the fullerene research. In this technique, carbon soot is produced from two high-purity graphite electrodes by igniting an arc discharge between them in an inert atmosphere (helium gas). Alternatively, soot is produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons. Fullerenes are extracted from the soot using a multistep procedure. First, the soot is dissolved in appropriate organic solvents. This step yields a solution containing up to 75% of C60, as well as other fullerenes. These fractions are separated using chromatography.[12]
The structure of a buckminsterfullerene is a truncated icosahedron with 60 vertices and 32 faces (20 hexagons and 12 pentagons where no pentagons share a vertex) 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 1.01 nanometers (nm). The nucleus to nucleus diameter of a C60 molecule is about 0.71 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 0.14 nm. Each carbon atom in the structure is bonded covalently with 3 others.[13]
The C60 molecule is extremely stable, being able to withstand high temperatures and pressures. The exposed surface of the structure is able react with other species while maintaining the spherical geometry.[14] The hollow structure is also able to entrap atoms and small molecules, which do not react with the fullerene molecule.
C60 can undergo six reversible, one-electron reductions to C606−, whereas oxidation is irreversible. The first reduction requires is ~1.0 V (Fc/Fc+), indicating that C60 is an electron acceptor. C60 has a tendency of avoiding having double bonds within the pentagonal rings which makes electron delocalization poor, and results in the fact that C60 is not "superaromatic". C60 behaves very much like an electron deficient alkene and readily reacts with electron rich species.[11]
A carbon atom in the C60 molecule can be substituted by a nitrogen or boron atom yielding a C59N or C59B respectively.[15]
| Centered by | Vertex | Edge 5-6 |
Edge 6-6 |
Face Hexagon |
Face Pentagon |
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| Image | |||||
| Projective symmetry |
[2] | [2] | [2] | [6] | [10] |
| Solvent | S |
|---|---|
| 1-chloronaphthalene | 51 |
| 1-methylnaphthalene | 33 |
| 1,2-dichlorobenzene | 24 |
| 1,2,4-trimethylbenzene | 18 |
| tetrahydronaphthalene | 16 |
| carbon disulfide | 8 |
| 1,2,3-tribromopropane | 8 |
| xylene | 5 |
| bromoform | 5 |
| cumene | 4 |
| toluene | 3 |
| benzene | 1.5 |
| carbon tetrachloride | 0.447 |
| chloroform | 0.25 |
| n-hexane | 0.046 |
| cyclohexane | 0.035 |
| tetrahydrofuran | 0.006 |
| acetonitrile | 0.004 |
| methanol | 0.00004 |
| water | 1.3×10−11 |
| pentane | 0.004 |
| octane | 0.025 |
| isooctane | 0.026 |
| decane | 0.070 |
| dodecane | 0.091 |
| tetradecane | 0.126 |
| dioxane | 0.0041 |
| mesitylene | 0.997 |
| dichloromethane | 0.254 |
Fullerenes are sparingly soluble in many aromatic solvents such as toluene and others like carbon disulfide, but not in water. Solutions of pure C60 have a deep purple color which transforms into brown upon drying off the solvent. The reason for this color change is the relatively narrow energy width of the band of molecular levels responsible for green light absorption by individual C60 molecules. Thus individual molecules transmit some blue and red light light resulting in a purple color. Upon drying, intermolecular interaction results in the overlap and broadening of the energy bands, thereby eliminating the blue light transmittance and causing the purple to brown color change.[19]
Solubility of C60 in some solvents shows unusual behavior due to existence of solvate phases (analogues of crystallohydrates). For example, solubility of C60 in benzene solution shows maximum at about 313 K. Crystallization from benzene solution at temperatures below maximum results in formation of triclinic solid solvate with four benzene molecules C60·4C6H6 which is rather unstable in air. Out of solution, this structure decomposes into usual fcc C60 in a few minutes. At temperatures above solubility maximum the solvate is not stable even when immersed in saturated solution and melts with formation of fcc C60. Crystallization at temperatures above the solubility maximum results in formation of pure fcc C60. Millimeter-sized crystals of C60 and C70 can be grown from solution both for solvates and for pure fullerenes.[20][21]
In a solid, buckminsterfullerene molecules normally stick together via the van der Waals forces; however, exposure to light or oxygen can result in their dimerization and polymerization. At low temperatures they are arranged in a simple cubic structure and locked against rotation. Upon heating, they start rotating at about −20 °C that results in a first-order phase transition to a face-centered cubic (fcc) structure and a small, yet abrupt increase in the lattice constant from 0.1411 to 0.14154 nm.[22]
C60 solid is as soft as graphite, but when compressed to less than 70% of its volume it transforms into a superhard form of diamond (see aggregated diamond nanorod). C60 films and solution have strong non-linear optical properties, particularly, their optical absorption increases with the light intensity (saturable absorption).
C60 forms a brownish solid with an optical absorption threshold at ~1.6 eV.[23] It is an n-type semiconductor with a low activation energy of 0.1–0.3 eV; this conductivity is attributed to intrinsic or oxygen-related defects.[24] The unit cell of solid C60 contains voids at 4 octahedral and 12 tetrahedral sites. They are large enough (0.6 and 0.2 nm respectively) to accommodate impurity atoms. When electron-donating elements, such as alkali or other metals, are doped into these voids, C60 converts from a semiconductor into a conductor or even superconductor.[22][25]
In 1991, Haddon et al.[26] found that intercalation of alkali-metal atoms in solid C60 leads to metallic behavior.[27] In 1991, it was revealed that potassium-doped C60 becomes superconducting at 18 K.[28] This was the highest transition temperature for a molecular superconductor. Since then, superconductivity has been reported in fullerene doped with various other alkali metals.[29][30] It has been shown that the superconducting transition temperature in alkaline-metal-doped fullerene increases with the unit-cell volume V.[31][32] As caesium forms the largest alkali ion, caesium-doped fullerene is an important material in this family. Recently, superconductivity at 38 K has been reported in bulk Cs3C60,[33] but only under applied pressure. The highest superconducting transition temperature of 33 K at ambient pressure is reported for Cs2RbC60.[34]
The increase of transition temperature with the unit-cell volume had been believed to be evidence for the BCS mechanism of C60 solid superconductivity, because inter C60 separation can be related to an increase in the density of states on the Fermi level, N(εF). Therefore, there have been many efforts to increase the interfullerene separation, in particular, intercalating neutral molecules into the A3C60 lattice to increase the interfullerene spacing while the valence of C60 is kept unchanged. However, this ammoniation technique has revealed a new aspect of fullerene intercalation compounds: the Mott transition and the correlation between the orientation/orbital order of C60 molecules and the magnetic structure.[35]
The C60 molecules compose a solid of weakly bound molecules. The fullerites are therefore molecular solids, in which the molecular properties still survive. The discrete levels of a free C60 molecule are only weakly broadened in the solid, which leads to a set of essentially nonoverlapping bands with a narrow width of about 0.5 eV.[27] For an undoped C60 solid, the 5-fold hu band is the HOMO level, and the 3-fold t1u band is the empty LUMO level, and this system is a band insulator. But when the C60 solid is doped with metal atoms, the metal atoms give electrons to the t1u band or the upper 3-fold t1g band.[36] This partial electron occupation of the band may lead to metallic behavior. However, A4C60 is an insulator, although the t1u band is only partially filled and it should be a metal according to band theory.[37] This unpredicted behavior may be explained by the Jahn-Teller effect, where spontaneous deformations of high-symmetry molecules induce the splitting of degenerate levels to gain the electronic energy. The Jahn-Teller type electron-phonon interaction is strong enough in C60 solids to destroy the band picture for particular valence states.[35]
A narrow band or strongly correlated electronic system and degenerated ground states are important points to understand in explaining superconductivity in fullerene solids. When the inter-electron repulsion U is greater than the bandwidth, an insulating localized electron ground state is produced in the simple Mott-Hubbard model. This explains the absence of superconductivity at ambient pressure in caesium-doped C60 solids.[33] Electron-correlation-driven localization of the t1u electrons exceeds the critical value, leading to the Mott insulator. The application of high pressure decreases the interfullerene spacing, therefore caesium-doped C60 solids turn to metallic and superconducting.
A fully developed theory of C60 solids superconductivity is still lacking, but it has been widely accepted that strong electronic correlations and the Jahn-Teller electron-phonon coupling[38] produce local electron-pairings that show a high transition temperature close to the insulator-metal transition.[39]
Hydrated fullerene C60HyFn is a stable, highly hydrophilic, supra-molecular complex consisting of С60 fullerene molecule enclosed into the first hydrated shell that contains 24 water molecules: C60@(H2O)24. This hydrated shell is formed as a result of donor-acceptor interaction between lone-electron pairs of oxygen, water molecules and electron-acceptor centers on the fullerene surface. Meanwhile, the water molecules which are oriented close to the fullerene surface are interconnected by a three-dimensional network of hydrogen bonds. The size of C60HyFn is 1.6–1.8 nm. The maximal concentration of С60 in the form of C60HyFn achieved by 2010 is 4 mg/mL.[40] [41][42][43]
C60 molecules can encage and transport atoms and molecules (e.g. radioactive labels) through the human body. For instance, lanthanum carbide (LaC2), which reacts very strongly with water vapor and oxygen and rapidly degrades in air, has been successfully protected inside C60 molecules for more than six months.[10]
In the medical field, elements such as helium (that can be detected in minute quantities) can be used as chemical tracers in impregnated buckyballs. Buckminsterfullerene could also inhibit the AIDS virus. The C60 molecule could block the active site in a key enzyme in the human immunodeficiency virus known as HIV-1 protease; this could inhibit reproduction of the HIV virus in immune cells. Experiments suggest that C60 incorporated with the alkali metals can possess catalytic properties resembling those of platinum.[10]
The C60 molecule can also bind large numbers of hydrogen atoms (up to one hydrogen for each carbon) without disrupting the structure. This property suggests that C60 may be a better storage medium for hydrogen than metal hydrides (currently regarded as the best material for that purpose), and hence a key factor in the development of a new class battery or even non-polluting automobiles based on fuel cells, lighter and more efficient than lead-acid batteries.[10]
The optical absorption properties of C60 match solar spectrum that favors C60-based films for photovoltaic applications. Conversion efficiencies up to 5.7% have been reported in C60-polymer cells.[44]
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