A fullerene is an allotrope of carbon whose molecules consist of carbon atoms connected to form a closed cage. The molecules may have hollow sphere- and ellipsoid-like forms, tubes, or other shapes. The carbons are usually arranged in 5- and 6-membered rings. The family is named after buckminsterfullerene (C60), which in turn is named after Buckminster Fuller. C60 is also the first discovered and best characterized fullerene. C60 has a hollow sphere-like form, but other fullerenes are known with ellipsoid-like shapes.
The closed fullerenes, especially C60, are also informally called buckyballs for their resemblance to the standard ball of association football. IUPAC defines fullerenes as "polyhedral closed cages made up entirely of n three-coordinate carbon atoms and having 12 pentagonal and (n/2-10) hexagonal faces, where n ≥ 20."
History
Predictions and limited observations The icosahedral cage was mentioned in 1965 as a possible topological structure. Eiji Osawa predicted the existence of in 1970. He noticed that the structure of a corannulene molecule was a subset of the shape of a football, and hypothesised that a full ball shape could also exist. Japanese scientific journals reported his idea, but neither it nor any translations of it reached Europe or the Americas.
Also in 1970, R. W. Henson (former member of the UK Atomic Energy Research Establishment) proposed the structure and made a model of it. Unfortunately, the evidence for that new form of carbon was very weak at the time, so the proposal was met with skepticism, and was never published. It was acknowledged only in 1999.
In 1973, independently from Henson, D. A. Bochvar and E. G. Galpern made a quantum-chemical analysis of the stability of and calculated its electronic structure. The paper was published in 1973, but the scientific community did not give much importance to this theoretical prediction.
Around 1980, Sumio Iijima identified the molecule of from an electron microscope image of carbon black, where it formed the core of a particle with the structure of a "bucky onion".
Also in the 1980s at MIT, Mildred Dresselhaus and Morinobu Endo, collaborating with T. Venkatesan, directed studies blasting graphite with lasers, producing carbon clusters of atoms, which would be later identified as "fullerenes."
Fullerenes had been predicted for some time, but only after their accidental synthesis in 1985 were they detected in nature and outer space.Stars reveal carbon 'spaceballs', BBC, 22 July 2010. The discovery of fullerenes greatly expanded the number of known allotropes of carbon, which had previously been limited to graphite, diamond, and amorphous carbon such as soot and charcoal. They have been the subject of intense research, both for their chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology.
Discovery of In 1985, Harold Kroto of the University of Sussex, working with James R. Heath, Sean O'Brien, Robert Curl and Richard Smalley from Rice University, discovered fullerenes in the sooty residue created by vaporising carbon in a helium atmosphere. In the mass spectrum of the product, discrete peaks appeared corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms, namely and . The team identified their structure as the now familiar "buckyballs".
The name "buckminsterfullerene" was eventually chosen for by the discoverers as an homage to American architect Buckminster Fuller for the vague similarity of the structure to the geodesic domes which he popularized; which, if they were extended to a full sphere, would also have the icosahedral symmetry group.Buckminsterfullerene, . Sussex Fullerene Group. chm.bris.ac.uk The "ene" ending was chosen to indicate that the carbons are unsaturated, being connected to only three other atoms instead of the normal four. The shortened name "fullerene" eventually came to be applied to the whole family.
Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of this class of molecules.
Further developments Kroto and the Rice team already discovered other fullerenes besides C60, and the list was much expanded in the following years. Carbon nanotubes were first discovered and synthesized in 1991.
After their discovery, minute quantities of fullerenes were found to be produced in sooty flames, and by lightning discharges in the atmosphere. In 1992, fullerenes were found in a family of mineraloids known as shungites in Karelia, Russia.
The production techniques were improved by many scientists, including Donald Huffman, Wolfgang Krätschmer, Lowell D. Lamb, and Konstantinos Fostiropoulos. Thanks to their efforts, by 1990 it was relatively easy to produce gram-sized samples of fullerene powder. Fullerene purification remains a challenge to chemists and to a large extent determines fullerene prices.
In 2010, the spectral signatures of C60 and C70 were observed by NASA's Spitzer infrared telescope in a cloud of cosmic dust surrounding a star 6500 light years away. Kroto commented: "This most exciting breakthrough provides convincing evidence that the buckyball has, as I long suspected, existed since time immemorial in the dark recesses of our galaxy." According to astronomer Letizia Stanghellini, "It's possible that buckyballs from outer space provided seeds for life on Earth." In 2019, ionized C60 molecules were detected with the Hubble Space Telescope in the space between those stars.
Buckyballs
Inventory Below is a table of main closed carbon fullerenes synthesized and characterized so far. Fullerenes with fewer than 60 carbon atoms have been called "lower fullerenes", and those with more than 70 atoms "higher fullerenes". Formula Num.Isom.Mol.Symm. Cryst.Symm. Space group No Pearson symbola (nm)b (nm)c (nm)β°Z ρ(g/cm3)1 Ih 1 Ih 1 D5h 1 D6h 1 D3h 2 D2* Monoclinic P21 4 mP2 1.102 1.1081.768108.102 1.48 Cubic Fmm 225 cF4 1.5475 1.5475 1.5475904 1.645 D2v 7 9 , C2v, C3v Monoclinic P21 4 mP2 1.141 1.1355 1.8355108.072 24 D2*, D2d Cubic Fmm 1.5817 1.5817 1.581790 19 35 46
In the table, "Num.Isom." is the number of possible isomers within the "isolated pentagon rule", which states that two pentagons in a fullerene should not share edges. "Mol.Symm." is the symmetry of the molecule, whereas "Cryst.Symm." is that of the crystalline framework in the solid state. Both are specified for the most experimentally abundant form(s). The asterisk * marks symmetries with more than one chiral form.
The smallest possible fullerene is the dodecahedral . There are no fullerenes with 22 vertices. The number of different fullerenes C2n grows with increasing n = 12, 13, 14, ..., roughly in proportion to n9 . For instance, there are 1812 non-isomorphic fullerenes . Note that only one form of , buckminsterfullerene, has no pair of adjacent pentagons (the smallest such fullerene). To further illustrate the growth, there are 214,127,713 non-isomorphic fullerenes , 15,655,672 of which have no adjacent pentagons. Optimized structures of many fullerene isomers are published and listed on the web.Fowler, P. W. and Manolopoulos, D. E. Fullerenes. nanotube.msu.edu
When or crystals are grown from toluene solution they have a monoclinic symmetry. The crystal structure contains toluene molecules packed between the spheres of the fullerene. However, evaporation of the solvent from transforms it into a face-centered cubic form. Both monoclinic and face-centered cubic (fcc) phases are known for better-characterized and fullerenes.
Buckminsterfullerene
Buckminsterfullerene is the smallest fullerene molecule containing pentagonal and hexagonal rings in which no two pentagons share an edge (which can be destabilizing, as in pentalene). It is also most common in terms of natural occurrence, as it can often be found in soot.
The empirical formula of buckminsterfullerene is and its structure is a truncated icosahedron, which resembles an association football 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 buckminsterfullerene molecule is about 1.1 nanometers (nm). The nucleus to nucleus diameter of a buckminsterfullerene molecule is about 0.71 nm.
The buckminsterfullerene molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered "double bonds" and are shorter (1.401 Å) than the 6:5 bonds (1.458 Å, between a hexagon and a pentagon). The weighted average bond length is 1.44 Å.
Another fairly common fullerene has empirical formula , but fullerenes with 72, 76, 84 and even up to 100 carbon atoms are commonly obtained. , , , and are chiral because they are D2-symmetric. Their enantiomers have been resolved.
Heterofullerenes and non-carbon fullerenes Many heterofullerenes have been synthesized (or studied theoretically) in which some or all the carbon atoms are replaced by other elements, such as nitrogen (azafullerenes). Buckyballs containing boron atoms (borafullerenes) have been observed, such as C40 and C80.
Inorganic (carbon-free) fullerene-type structures have been built with the molybdenum(IV) sulfide (MoS2), long used as a graphite-like lubricant, tungsten (WS2), titanium (TiS2) and niobium (NbS2). These materials were found to be stable up to at least 350 tons/cm2 (34.3 GPa).
Icosahedral or distorted-icosahedral fullerene-like complexes have also been prepared for germanium, tin, and lead; some of these complexes are spacious enough to hold most transition metal atoms.
Related polymeric materials Carbon nanotubes are described as open-ended cylinders are a major variant of fullerenes and have received much attention. 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. There are also cases in which the tube reduces in diameter before closing off. Their molecular structure results in extraordinary macroscopic properties, including high tensile strength, high electrical conductivity, high ductility, high heat conductivity, and relative chemical inactivity (as it is cylindrical and "planar" — that is, it has no "exposed" atoms that can be easily displaced).
Nested closed fullerenes have been named bucky onions, which have been proposed for use in lubricants. Nested carbon nanotubes, dubbed "carbon megatubes", have also been synthesized.
The bulk solid form of pure or mixed fullerenes is called fullerite.
Buckyballs can be connected, such as in linked "ball-and-chain" dimers (two buckyballs linked by a carbon chain) and rings of buckyballs linked together.
Non-carbon nanotubes have attracted attention.
Properties
Topology Schlegel diagrams are often used to clarify the 3D structure of closed-shell fullerenes, as 2D projections are often not ideal in this sense.
In mathematical terms, the combinatorial topology (that is, the carbon atoms and the bonds between them, ignoring their positions and distances) of a closed-shell fullerene with a simple sphere-like mean surface (orientable, genus zero) can be represented as a convex polyhedron; more precisely, its one-dimensional skeleton, consisting of its vertices and edges. The Schlegel diagram is a projection of that skeleton onto one of the faces of the polyhedron, through a point just outside that face; so that all other vertices project inside that face.
The Schlegel diagram of a closed fullerene is a graph that is planar and 3-regular (or "cubic"; meaning that all vertices have degree 3).