Bucky Balls

Introduction
The Basics
Synthesis
Geometry
Natural Fullerenes
Uses of Fullerenes
Nanotubes
Conclusion

1. Introduction

Universally it is generally thought that carbon has two isotopes, graphite and diamond, however, this is wrong. Carbon actually has seven different crystaline forms:

two kinds of graphite
two kinds of diamond
chaoit (found in 1968)
carbon(VI) (found in 1972)
and FULLERENES.

The three naturally occuring forms of carbon can be seen in the table below:-

Diamond	Fullerene	Graphite

For a long time chemists had been unable to explain the persistent peaks occuring in the spectrum of soot around 720 amu (atomic mass units). That was until 1985 when a significant breakthrough was made by three scientists Prof. Harold Kroto (Univ. of Sussex) Prof. Richard Smalley and Prof. Robert Curl (Univ. Rice, Houston, Texas).

It was found that by shining a lazer onto a graphite block, some of the weakly bonded, hexagonal carbon layers would detach and react. On closer examination it was found that these products contained predominantly 60 carbon atoms (although 70 atom molecules were present). These 60 atom molecules consisted of a pure carbon sphere approximately 10Å in size.

To their makers -the term is used cautiously as it is debateable whether the molecules were discovered or invented- the spheres were reminscent of the geodesic domes designed by the late engineer R. Buckminster Fuller. Thus C₆₀ became known as buckminsterfullerene, or bucky balls for short. While the new family of molecules became known as fullerenes.

In recoginition of this discovery the three received the 1996 Nobel Prize for chemistry.

A useful insight is also given on Paul May's molecule of the month page.

Did You Know?

You could line up 25,000,000 C₆₀ molecules on a ruler before passing the inch mark.

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2. The Basics

Since bucky balls numerous other fullerenes have been developed. It is now estimated that they can contain between 30 and 980 carbon atoms in their structure, although theoretically they could contain an infinite number of atoms.
Fullerenes follow the net closing formula postulated by the mathmatician Euler. His proposition states that,

"for any polygon with n edges, (where n is an even number greater than 22), at least one polyhedron can be constructed with 12 pentagons and (n-20)/2 hexagons".

This may also be represented by the equation:-
12=3n3+2n4+1n5+0n6-1n7-2n8...-(6-m)nm
(where n=number of m-gons you want to form, for example if want a closed net consisting of x pentagons you need 12 of them: 12=1*x). All this in simple terms means is, that it is possible with one or more than one hexagon to form a polyhedron. Which of course bucky balls do using 20 hexagonal and 12 pentagonal rings.

You can now even build your own bucky balls.

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3. Synthesis

Fullerenes are formed when vapourised carbon condenses in an atmosphere of inert gas. The gaseous carbon is obtained e.g. by directing an intense pulse of lazer at a graphite surface. The released carbon atoms are mixed with a stream of heium gas and combine to form clusters of some few up to hundreds of atoms. The gas is then led into a vacum chamber where it expands and is cooled to a few degrees above absolute zero. The clusters can then be extracted and analysed.

Buckminsterfullerene can be made in the laboratory. Two students from Evergreen College in the U.S.A demostrate step by step instructions for synthesis of fullerenes.

However both these processess only yield small amounts of fullerenes(ca. 6%). A process to obtain higher yields(ca. 15%) was designed by W. Kratschmer and D.R. Huffman. They made C₆₀ by causing an arc between two graphite rods to burn in a helium atmosphere. The fullerenes are then extracted using an organic solvent such as toluene, the pure C₆₀ is extracted by pumping the solution through a column of activated charcoal and silica gel, the magenta C₆₀ comes off first followed by the red C₇₀. There are also numerous other methods of synthesis.

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4. Geometry

Technically the geometry of a bucky ball is a truncated icosahedron, with one carbon atom at each vertex. In english buky balls may be considered footballs. This makes it the most symetrical and most stable of all the fullerenes.

As the molecule orginates from graphite it would be correct to assume the molecule contains hexagons, but in order for the molecule to form a closed cage it is necessary to incorporate pentagons into the structure. Indeed this network of fused pentagons and hexagons is characteristic of all the fullerenes.

Are buckyballs aromatic?
The answer is no. At a first glance the molecule looks as if it should be "superaromatic" because of the benzene like rings incorporated in the structure. however they are not aromatic for two reasons:-

On closer examination it is possible to see the double bonds only occur at the fusion between hexagons. Pentagons play no part in any double bonding.
Secondly the curvature of the balls leads to less effective overlap of the sp² hybridised pi bonding orbitals, in turn leading to leading to a decrease in the stabalization that results from a delocalised molecular orbital.

In reality the balls behave like electron deficient alkenes e.g. they undergo addition reactions across the double bond. Indeed the X-ray structure of the product of one such reaction was used to prove the football shape of C₆₀.

For a more visual look at buckyballs link to J. Lauhers structure library or to Boris Pevzners image gallery.

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5. Uses of Fullerenes

Since their discovery in 1985 there has been much excitement about the possible uses of buckyballs. A range of ideas has been put forward varying from molecular ball bearings, to footballs in a molecular match.

Believe it or not bucky balls could be of great significance to the military. The Times (30 ^th October 1994) states that buckminsterfullerene is impervious to lazer beams. So in futuristic warfare these minute little balls could save your life!

The Times (21^st November 1985) reported on what may be the most important use for fullerenes, fighting the virus HIV which can lead to AIDS.
It was found by Kenyan and Wudl that water soluable C₆₀ derivatives inhibit the HIV-1 protease. The function of this enzyme is critical to the development of the virus. A very in depth account of this and other biological uses of C₆₀ is given by Prof. Rubin in his current research interests page.

There are three available sites for bonding on a bucky ball, namely one octahedral site and two tetrahedral sites, so called because of the shape the nearest neighbour atoms take.
Alkali metals can bond into these octahedral sites, leading to A₁C₆₀ molecules. Where the term A refers to the number of atoms of Alkali metal. Cooling these A₁ molecules can lead to polymer chains of buckyballs.
Here is an image of Rb₁C₆₀

When the tetrahedral sites are also filled A₃C₆₀ molecules are obtained. This molecule is of industrial significance since it is a superconductor. Conversly A₆C₆₀ is an insulator since the six electron band in C₆₀ is full.

Finally, fact or merely science fiction. Arthur C. Clark a science fiction writer with a large grasp of the science world, in his futuristic best selling book Galaxy 2000 can envisage a space elevator but in his opinion the only substance strong enough, cheap enough and available in megaton quantities is buckminsterfullerene.

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6. Natural Fullerenes

Because laboratory synthesis gives such low yields people began hunting for naturally occuring reserves of the chemical, to their economic dismay a massive reservoir of fullerenes has not yet been found.

First evidence of naturally occuring fullerenes was found by Arizona State University researchers Semeon Tsipursky and Peter Buseck when they examined a sample of shiny black rock known as shungite, from the Karelian Republic in Northern Russia.
Shungite is a rare, carbon rich variety of rock believed to have been formed between 600 million and 4 billion years ago, although how it was formed is debateable.
Electron microscopy of shungite revealed an almost identical pattern as Tspursky had seen with fullerenes. To confirm their suspicions the two sent a sample of rock for mass spectroscopy analysis, where it was found the sample did indeed contain traces of fullerenes, inparticular C₆₀ and C₇₀.
In order to anlyse the sample it was necessary to vapourize the rock using a lazer. Many people argued that this was responsible for forming the fullerenes. So a second test was carried out this time using a 400^oC stainless steel probe to vapourize the sample, identical results were seen. So fullerenes did exist in nature.

More recently fullerenes have been found in a sample of glassy rock known as fulgerite, in the mountains of Colarado. Fulgerite forms when lightening strikes the ground. In the laboratory fullerenes are grown in the gas phase, so this discovery provides evidence that fullerenes can be formed naturally in the solid phase.

Minor samples have also been found in carbonaceous Precambian rock formed at the boundary between the Cretaceous and Tertiary eras 65 million years ago, just as dinosaurs were becoming extinct.

Perhaps most interesting is the evidence of ALIEN FULLERENES put forward in the Times (12^th April 1996). It suggests that samples of meterorites, found in Sudbury Crater, which hit earth 2 billion years ago contained traces of fullerenes, stuffed with helium isotopes in a ratio not seen on earth but which is seen in other meterorites. So perhaps there exists a whole range of alien fullerenes.

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7. Nanotubes

Close relations of the fullerenes are carbon nanotubes(CNT's). CNT's consist of concentric shells of graphite, they can almost be thought of as rolled layers of graphite.
Their basic structure can be seen below:-

The number of shells in a CNT camn vary from one to as many as fifty, with the spacing between the layers closley matching the layer spacing in graphite(approx 0.34nm).
CNT's are usually about 1-50nm and typically a few microns long although they have been grown up to 300 microns.

There are alot of pages out there specializing in CNT's but perhaps some of the best are Milo Shaffer's pages. Where he discusses CNT's, their properties and production.

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8. Conclusion

Fullerenes are radical, new and exciting and it is clear to see from the information presented in this page, which is a mere fraction of the information that is currently out there, that research has flourished in only a twelve year period so far, the scope for the future is inconceivable.

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