Feature

Buckyballs Bouncing On

The soccer-ball-shaped C60 molecule continues to fascinate chemists and physicists.

Robin Ferrier

None of us a handful of years ago had heard of the attention-grabbing name "buckyballs", a material which has taken the physical sciences world by storm. No other new material in history, it can be suggested, has attracted so much immediate attention, its distinctive features being the relative suddenness with which it burst on the scene, the enigmatic simplicity yet complexity of its structure, the familiarity yet bizarre novelty of that structure and its uniqueness as a form of matter. Claims to its discovery could be made only after a major piece of chemical detective work but, it turns out, it has probably been all around for all of human history not just on Earth but also in outer space.

Physicists and chemists have clamoured to investigate this new material and the key British member of the discovery team, Professor Harry Kroto, of University of Sussex, has already become a Knight of the Realm. Victoria University researchers are just one group keen to look at the possibilities provided by buckyballs.

There is a wonderful feature of the story that must be stressed, particularly these days when scientific research is seen by too many solely as a route to instant commercial gain. Thankfully it is still possible to make discoveries in very fundamental science that set people thinking a thousand new thoughts, some of which one day can lead to new practical advances, solutions to real problems and profit. The finding of buckyballs is such a discovery; the new thoughts have emerged, although the profits seemingly await the future -- notwithstanding immediate efforts to identify practical uses.

The name of the new material was coined after Buckminster Fuller, the American architect/philosopher who designed the geodesic dome that housed the US exhibit at Expo 67 held in Montreal. Once the molecular structure of the substance had been identified, the name appeared obvious, as it is a chemical analogue of the geodesic domes beloved by the 70s.

It is the third allotrope of carbon, and it must be a long time since a new regular form of such an important element was discovered. The others are diamond and graphite, which are made up, in turn, of a 3-dimensional network of "saturated" carbon (as in methane), and parallel flat sheets of six-membered rings of "unsaturated" carbon, each ring being akin to the ring of benzene. While in these cases there are effectively infinite arrays of carbon atoms and no discrete units that can be considered to be specific molecules, buckyballs each consist of 60 identical carbon atoms formed into the 20 hexagons and 12 pentagons seen most commonly in the traditional soccer ball. In science, "C₆₀" is now much more commonly used than "buckyballs", "buckminsterfullerene" or "footballane", and is known to all chemists and physicists as a new age material. Thanks to widespread media coverage it is also familiar to a much wider public.

From Space to the Laboratory

The discovery of C₆₀ begins in space. Following an interest in the ultra-esoteric study of the infrared spectroscopic and physical properties of such exotic species as polyynes (compounds containing several acetylenic triple bonds), Kroto's attention spread into the study of organic molecules that can be detected in the space between the stars, particularly in the black clouds of the Milky Way.

In the last 30 years, microwave spectroscopy has allowed the identification of such compounds and many have now been recognised -- initially simple ones, such as methanol (CH₃OH), but then others of increasing complexity. Quite recently glycine (H₂NCH₂COOH) has been detected, and this is of relevance to the questions of extraterrestrial life and the origin of life on Earth since it is the simplest of the a-amino acids which are the building blocks from which proteins are derived. Also, and of great importance to the C₆₀ story, polyyne-like compounds hae been detected in space.

In 1980, Kroto went to Rice University in Texas to work in Professor Richard Smalley's laboratory, where special equipment was available for laser vaporising materials and studying the products of this process. It wasn't until a further visit in 1985 that Kroto could get use of the equipment to study the making of complex compounds of carbon by this method, in the hope of finding molecules of the kind in which he was interested and which he knew to be present in interstellar space. Perhaps the work would lead to an appreciation of some of the chemistry occurring in red giant stars, which release carbon products in massive amounts.

He chose to laser energise graphite and to examine the products formed using the technique of mass spectrometry (now in general use for analytical work in organic chemistry, in particular for determining the molecular weights of compounds). A vital feature is its applicability to ultra-minute samples.

From graphite, a whole set of fragments was produced within the approximate molecular weight range 250-1,000 (relative to the 12 of the carbon atom). This indicated a set of clusters of carbon atoms containing between 20 and 80 atoms. A notable feature of these mixtures was the prominence of the member with molecular weight 720 -- C₆₀. Often it was only a little more abundant than other members, such as the more common C₇₀ form, but conditions for optimising its formation were found, and intense speculation on the structure of the main product ensued.

Within days of starting these experiments, a consensus had been reached within the Smalley-Kroto group at Rice on the basis only of the molecular weight evidence and conjecture, and they illustrated their November 1985 paper in Nature with the photograph of a soccer ball.

The Hunt Begins

Science does not accept this level of evidence as proof of structure, and the race was on to isolate a sample of C₆₀ and determine its structure by unambiguous means. New methods for preparation were required, such as arcing graphite in an inert atmosphere and extracting the soot formed with a solvent. The goal was reported in 1990, by a combined group from Tucson and Heidelberg, who were mainly interested in the problem from the astronomical viewpoint. They had been able to obtain a crystalline product which gave X-ray and electron diffraction data consistent with its constituent molecules having spherical structure. This was tantalisingly close to proof of structure, but one simple but definitive experiment remained to be done.

Since all of the carbon atoms of C60 in the proposed soccer ball orientation are identical, any method that clearly showed them to be so would clinch the point, and nuclear magnetic resonance spectrometry (NMR), the "seeing eye" of organic chemistry since the late 1950s, provided the means. The spectrum given by a sample of pure C₆₀ made in the University of Sussex showed just the one signal for carbon, establishing that all 60 atoms were identical. The Kroto group's final proof also appeared in 1990. The football really did exist!

Since the isolation and formal characterisation of C₆₀ six years ago, it has become commercially available with the current amazingly low price being about $1/g for material containing about 20% C₇₀. Pure C₆₀ may also be bought, but it is very much more expensive. With the material now available, all manner of solid state and molecular physicists, chemists, material scientists and even biologists in industrial and in academic science have striven to claim a piece of the action.

Chemists have found several ways of modifying C₆₀, for example by making derivatives with metal atoms trapped within the sphere or alkali metal-doped derivatives. One such substance is K₃C₆₀, which has the potassium atoms (K) occupying sites between the spheres in the crystal lattice. The surfaces of the spheres have revealed the presence of double bonds which are very reactive and deficient of electrons, so they can readily take up other species such as amines, halogens and conjugated dienes.

There is now an extensive range of modified C₆₀-based species which offer innumerable opportunities to further modify the material (by, for example, linking spheres together to give chains or snipping them open in a specific manner) and produce an ever increasing set of compounds for study by physicists and biologists. Metal-doped C₆₀ derivatives can be insulators, conductors or superconductors; for the latter they offer opportunities to access materials with ever-increasing transition temperatures, bringing superconductivity closer to practical usage.

For biological and medical studies, it is now possible to use water suspensions of C₆₀, and reports of the inhibition of the AIDS virus have appeared together with indications that C₆₀ itself has no acute toxicity. The inhibition is in keeping with the finding that the sphere of C60 fits snugly into the active site of a key enzyme of the virus. It is, of course, too early to claim any major beneficial property of an applicable sort in medicine, discoveries in this field taking particularly long to establish.

Six years ago when C₆₀ was identified, all manner of potential uses were proposed: new polymers, better batteries, catalysts, superconductors, drug-delivery vehicles, but six years on we can read articles entitled "Have buckyballs lost their bounce?" and "The commercial use of fullerenes slow to develop".

In 1996 it is safe to conclude that C₆₀ has not been the instant solution to any problem; no significant C₆₀ products appear to have been commercialised, but opportunities still exist. For example, under very high pressure, C₆₀ is converted to polycrystalline (industrial) diamond, so we should not write it off as a one-day wonder! Of course not, we are six years into the effective life of this material, and its derivatives are only just becoming available -- surely we are many years from being able to assess the practical value of its discovery.

Interest in C₆₀ at Victoria University centres on a very different aspect -- its formal synthesis from simple compounds. There have been several attempts to do this, but so far the objective has not been achieved. PhD student Steven Holden is following a new route which, on paper, appears relatively simple; completing it in practice is a different matter, however, but he is making good progress.

We believe it will be possible to link four molecules of a C₁₅ hydrocarbon which represents repeating "quarters" of the C₆₀ structure. To date, Steven has linked two such units and has a C₃₀H₃₀ "half' which has the appropriate framework, and he is working on a second "half' designed to bond to the first to give a product with approximate molecular formula C₆₀H₆₀ and with the 60 carbon atoms also appropriately linked. Removal of hydrogen to close the sphere and form 16 new benzene-like rings will then be required. It's tricky work, but interesting.

Space-Faring Substance

Although the subject has burst on the scene with amazing impact, science was slow to recognise that C₆₀ is all around us, and has been for all time. It is present in the smoke formed by the burning of benzene, and by the pyrolysis of other hydrocarbons, and has been detected in the products of laser destruction of such materials as coal and synthetic polymers.

If C₆₀ is a ubiquitous species, we can return to the topic from which its discovery emerged -- chemistry in outer space -- and ask the question "is it possible that C₆₀ exists extraterrestrially?" There is evidence that it does. Spectroscopic data has suggested such evidence, but most recent results provide direct information of a very different kind. C₆₀ (again with C₇₀) has been found in meteorites, and questions arise as to whether the carbon involved was extraterrestrial, and if so whether the C₆₀ was formed prior to the arrival of the meteorite or during its impact with Earth.

The Sudbury meteorite contains not just C₆₀ (molecular weight 720), but C₆₀ containing a helium atom within the sphere (molecular weight 724). Furthermore, this helium appears not to be of terrestrial origin, since it has an isotopic ratio ³He:⁴He of nearly 6 x 10^-4, while the value for solar wind helium is 4.4 x 10^-4 and the maximum value for terrestrial helium is 4.4 x 10^-5. The implications are that the gas was not trapped from Earth's atmosphere and that the C₆₀ was extraterrestrial.

A further, more general, conclusion follows from these observations: since the complex of C₆₀ containing helium breaks down at 1,000^oC, this temperature must not have been attained during the meteorite's approach to Earth or during impact, and therefore the delivery of organic material to Earth from space need not subject it to the extreme conditions previously assumed.

Dr Robin Ferrier is in the Department of Chemistry at Victoria University.