Buckminsterfullerene is a type of fullerene with the formula . It has a cage-like fused-ring structure (truncated icosahedron) made of twenty hexagons and twelve pentagons, and resembles an association football. Each of its 60 carbon atoms is bonded to its three neighbors.
Buckminsterfullerene is a black solid that dissolves in hydrocarbon solvents to produce a purple solution. The substance was discovered in 1985 and has received intense study, although few real world applications have been found.
Molecules of buckminsterfullerene (or of fullerenes in general) are commonly nicknamed buckyballs.
Occurrence Buckminsterfullerene is the most common naturally occurring fullerene. Small quantities of it can be found in soot.
It also exists in space. Neutral has been observed in planetary nebulae and several types of star. The ionised form, , has been identified in the interstellar medium, where it is the cause of several absorption features known as diffuse interstellar bands in the near-infrared.
History
Theoretical predictions of buckminsterfullerene molecules appeared in the late 1960s and early 1970s. It was first generated in 1984 by Eric Rohlfing, Donald Cox, and Andrew Kaldor, using a laser to vaporize carbon in a supersonic helium beam, although the group did not realize that buckminsterfullerene had been produced. In 1985 their work was repeated by Harold Kroto, James R. Heath, Sean C. O'Brien, Robert Curl, and Richard Smalley at Rice University, who recognized the structure of as buckminsterfullerene.
Concurrent but unconnected to the Kroto-Smalley work, astrophysicists were working with spectroscopists to study infrared emissions from giant red carbon stars. Smalley and team were able to use a laser vaporization technique to create carbon clusters which could potentially emit infrared at the same wavelength as had been emitted by the red carbon star. Hence, the inspiration came to Smalley and team to use the laser technique on graphite to generate fullerenes.
Using laser evaporation of graphite the Smalley team found Cn clusters (where and even) of which the most common were and . A solid rotating graphite disk was used as the surface from which carbon was vaporized using a laser beam creating hot plasma that was then passed through a stream of high-density helium gas. The carbon species were subsequently cooled and ionized resulting in the formation of clusters. Clusters ranged in molecular masses, but Kroto and Smalley found predominance in a cluster that could be enhanced further by allowing the plasma to react longer. They also discovered that is a cage-like molecule, a regular truncated icosahedron.
The experimental evidence, a strong peak at 720 daltons, indicated that a carbon molecule with 60 carbon atoms was forming, but provided no 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, 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.
In 1989 physicists Wolfgang Krätschmer, Konstantinos Fostiropoulos, and Donald R. Huffman observed unusual optical absorptions in thin films of carbon dust (soot). The soot had been generated by an arc-process between two graphite electrodes in a helium atmosphere where the electrode material evaporates and condenses forming soot in the quenching atmosphere. Among other features, the IR spectra of the soot showed four discrete bands in close agreement to those proposed for .
Another paper on the characterization and verification of the molecular structure followed on in the same year (1990) from their thin film experiments, and detailed also the extraction of an evaporable as well as benzene-soluble material from the arc-generated soot. This extract had TEM and X-ray crystal analysis consistent with arrays of spherical molecules, approximately 1.0 nm in van der Waals diameter as well as the expected molecular mass of 720 Da for (and 840 Da for ) in their mass spectra. The method was simple and efficient to prepare the material in gram amounts per day (1990) which has boosted the fullerene research and is even today applied for the commercial production of fullerenes.
The discovery of practical routes to led to the exploration of a new field of chemistry involving the study of fullerenes.
Etymology The discoverers of the allotrope named the newfound molecule after American architect R. Buckminster Fuller, who designed many geodesic dome structures that look similar to and who had died in 1983, the year before discovery. Another common name for buckminsterfullerene is "buckyballs".
Synthesis Soot is produced by laser ablation of graphite or pyrolysis of aromatic hydrocarbons. Fullerenes are extracted from the soot with organic solvents using a Soxhlet extractor. This step yields a solution containing up to 75% of , as well as other fullerenes. These fractions are separated using chromatography. Generally, the fullerenes are dissolved in a hydrocarbon or halogenated hydrocarbon and separated using alumina columns.
Synthesis using the techniques of "classical organic chemistry" is possible, but not economic.
Structure Buckminsterfullerene is a truncated icosahedron with 60 vertices, 32 faces (20 hexagons and 12 pentagons where no pentagons share a vertex), and 90 edges (60 edges between 5-membered & 6-membered rings and 30 edges are shared between 6-membered & 6-membered rings), with a carbon atom at the vertices of each polygon and a bond along each polygon edge. The van der Waals diameter of a molecule is about 1.01 nanometers (nm). The nucleus to nucleus diameter of a molecule is about 0.71 nm. The 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 three others. A carbon atom in the can be substituted by a nitrogen or boron atom yielding a or respectively.
Properties + Orthogonal projectionsCentered byVertexEdge5–6Edge6–6FaceHexagonFacePentagonImageProjectivesymmetry For a time buckminsterfullerene was the largest known molecule observed to exhibit wave–particle duality. In 2020 the dye molecule phthalocyanine exhibited the duality that is more famously attributed to light, electrons and other small particles and molecules.
Solution
+ Solubility of Solvent Solubility(g/L) 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 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 aromatic solvents and carbon disulfide, but insoluble in water. Solutions of pure have a deep purple color which leaves a brown residue upon evaporation. 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 molecules. Thus individual molecules transmit some blue and red 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.
crystallises with some solvents in the lattice ("solvates"). For example, crystallization of from benzene solution yields triclinic crystals with the formula . Like other solvates, this one readily releases benzene to give the usual face-centred cubic . Millimeter-sized crystals of and can be grown from solution both for solvates and for pure fullerenes.
Solid
In solid buckminsterfullerene, the molecules adopt the fcc (face-centered cubic) motif. They start rotating at about −20 °C. This change is associated with a first-order phase transition to an fcc structure and a small, yet abrupt increase in the lattice constant from 1.411 to 1.4154 nm.
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). films and solution have strong non-linear optical properties; in particular, their optical absorption increases with light intensity (saturable absorption).
forms a brownish solid with an optical absorption threshold at ≈1.6 eV. 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. Fcc contains voids at its octahedral and tetrahedral sites which are sufficiently large (0.6 and 0.2 nm respectively) to accommodate impurity atoms. When alkali metals are doped into these voids, converts from a semiconductor into a conductor or even superconductor.
Chemical reactions and properties undergoes six reversible, one-electron reductions, ultimately generating . Its oxidation is irreversible. The first reduction occurs at ≈−1.0 V (Fc/), showing that is a reluctant electron acceptor. tends to avoid having double bonds in the pentagonal rings, which makes electron delocalization poor, and results in not being "superaromatic". behaves like an electron deficient alkene. For example, it reacts with some nucleophiles.
Hydrogenation exhibits a small degree of aromatic character, but it still reflects localized double and single C–C bond characters. Therefore, can undergo addition with hydrogen to give polyhydrofullerenes. also undergoes Birch reduction. For example, reacts with lithium in liquid ammonia, followed by tert-butanol to give a mixture of polyhydrofullerenes such as , , , with being the dominating product. This mixture of polyhydrofullerenes can be re-oxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone to give again.
A selective hydrogenation method exists. Reaction of with 9,9′,10,10′-dihydroanthracene under the same conditions, depending on the time of reaction, gives and respectively and selectively.
Halogenation Addition of fluorine, chlorine, and bromine occurs for . Fluorine atoms are small enough for a 1,2-addition, while and add to remote C atoms due to steric factors. For example, in and , the Br atoms are in 1,3- or 1,4-positions with respect to each other. Under various conditions a vast number of halogenated derivatives of can be produced, some with an extraordinary selectivity on one or two isomers over the other possible ones. Addition of fluorine and chlorine usually results in a flattening of the framework into a drum-shaped molecule.
Addition of oxygen atoms Solutions of can be oxygenated to the epoxide . Ozonation of in 1,2-xylene at 257K gives an intermediate ozonide , which can be decomposed into 2 forms of . Decomposition of at 296 K gives the epoxide, but photolysis gives a product in which the O atom bridges a 5,6-edge.
Cycloadditions The Diels–Alder reaction is commonly employed to functionalize . Reaction of with appropriate substituted diene gives the corresponding adduct.
The Diels–Alder reaction between and 3,6-diaryl-1,2,4,5-tetrazines affords. The has the structure in which a four-membered ring is surrounded by four six-membered rings.