Soccer Ball Molecule – Buckminsterfullerene
There is probably no molecule that looks better in this Organic Chemistry Museum than buckminsterfullerene, also known as C60. or fullerene. It is a miraculously beautiful molecule made up of sixty carbon atoms assembling to form the shape of a soccer ball (Figure 1). While the unique shape and properties of the molecule have been drawing the attention of scientists in many different fields, its original discovery was a pure serendipity.
Fig1 buckminsterfullerene (C60)
C60 was discovered in 1985 by the joint team of the UK/US scientists led by Professors Harold Kroto, Richard Smalley, and Robert Curl. Their original goal wasn’t finding fullerenes, but their initial research theme was to study “carbon clusters,” a special type of molecules known to exist only in the outer space. In an attempt to observe it on the earth, they were trying the experiments in which they vaporized graphite (carbon atoms arranged like a beehive, Figure 2) by hitting it with a laser beam under vacuum. The energy of a laser beam can shatter graphite into the fragments or the clusters of a few to dozens of carbon atoms, so this process was being examined.
Fig 2 graphite
One day, a graduate student working on an experiment noticed that the fragments
of sixty carbon atoms were formed in far greater amount than the others.
After the careful changes of experimental conditions the percentage of
C60 formation increased, and eventually even a condition that gave only C60 was found. But why was it sixty, not fifty or a hundred?
Assuming that C60 possesses a special structural stability, the research team went on to have more discussions and experiments in order to find out what it really was.
A hexagon is the most stable shape among carbon ring structures, and graphite is also made of a beehive-like network of carbon hexagons. So, might C60 also have a hexagonal structure? Based on this idea, Smalley made some model structures by joining hexagonally cut papers. However, he could not come up with a good shape by using only hexagons. Kroto then mentioned, “The stardome (cardboard starmap) in my home had pentagons too.” When a model was made again this time using pentagons as well as hexagons, an impressive polyhedral structure with sixty vertices and a perfect symmetry emerged. It had to be right. They knew it intuitively. The evidences soon followed, supporting the validity of the structure.
A fullerene contains sixty carbon atoms, sixty carbon-carbon single bonds, thirty double bonds, twenty hexagons, and twelve pentagons. Every carbon atom uses all four of its bonding arms to leave nothing unused, and bond strains are distributed evenly through the entire molecule, making it an extremely stable structure.
The soccer ball molecule ultimately received the name fullerene, although names like “soccerene” and “footballene” were also suggested. The name honors an architect Richard Buckminster Fuller, who invented a dome consisting of pentagons and hexagons. The report on the discovery of C60 was published on Nature, a scientific journal of the world’s highest authority. The beautiful structure of the molecule had an honor of decorating the title page of the journal.
The Dawn of Fullerene Fever
After making a spectacular debut, fullerene research did not see a huge progress for a while because only a tiny amount could be obtained by the original laser method. A big breakthrough arrived in 1990, when Professors Wolfgang Krätschmer and Donald Huffman succeeded in the large scale synthesis of fullerene by using arc-discharge instead of laser. The discovery had such an impact that there is even an episode that the research conference hall became nearly empty when it was first presented, because all the scholars left for their labs to try the same experiment as soon as they saw it. The large scale production of fullerene then started in all over the world, and that was the beginning of the fullerene fever which continues to this day (By the way, unnoticed in the arc-discharge experiment was the formation of carbon nanotube, a discovery worthy of another Nobel Prize).
As research progressed with now an abundant resource, it became more and more clear that C60 is not just good-looking, but also has fascinating properties. In 1991, a mixture of fullerene and a metal such as potassium (prepared by a process called doping) was found to show superconductivity at low temperatures. This was the result of the potassium ions taken inside the spaces between the regularly aligned fullerene molecules. Superconductivity is a phenomenon in which electrical resistance becomes zero, and there are few examples of organic compounds that display this kind of property.
Fullerenes are a hollow sphere as mentioned earlier, so putting an atom inside is also possible (Figure 3). In this case it’s hard to plunge a big atom into an already-made fullerene, so the atom has to be trapped inside during the ball is being made. Specifically, a metal is pre-mixed in the powder of graphite, which is an ingredient of fullerene, and the mixture is vaporized together by a laser beam.
The atoms enclosed within a fullerene have been mostly metals, but the fullerenes containing a nitrogen atom and the ones containing multiple atoms have been reported as well.
Fig 3 Metal atom within C60
Good for AIDS?
Astonishingly, there have been reports that fullerenes are potentially effective as AIDS treatment. The AIDS virus makes a protein called HIV protease to multiply, and the idea is that a fullerene can inhibit the function of the enzyme by fitting itself tightly in the round opening of the enzyme (Figure 4). A Canada-based company C Sixty Inc. is currently developing fullerene derivatives, some of which are already in the stages of clinical trial.
Besides as AIDS treatment, fullerenes are already found in cosmetic products because of their known antioxidant and radical-removing properties. The robust, uniform, and spherical fullerenes can act as “molecular ball bearings,” and are used in bowling balls, golf clubs, eyeglass frames, tennis rackets, and the oil for air conditioners.
Fig 4 C60 inhibits HIV protease
Moreover, fullerenes have been studied as the carriers of DNA into a cell, meaning that there are big potentials for them as pharmaceutical materials. It was therefore no surprise, that the three discoverers of these fullerenes, Kroto, Smalley, and Curl, were awarded the 1996 Nobel Prize in Chemistry.
After the Story
Just like other great discoveries, the tale of C60 discovery has a few stories behind the story. For example, the existence of C60 molecule had been predicted by a Japanese scientist fifteens years before the discovery by Kroto and others. The person was Professor Eiji Ohsawa of Toyohashi University of Technology. Back in 1970, Ohsawa was interested in the structure of corannulene (Figure 5), which had just been synthesized, and was working on theoretical calculations on the compound. He noticed that it was a part of the surface motif of a soccer ball, and thought that perhaps C60 could also exist as a stable molecule. The result of his calculation appeared only on Japanese journals and books, so unfortunately it remained unnoticed by the scientists overseas, never seeing the sunlight until its “re-discovery” fifteens years later.
Fig 5 corannulene
Professor Chapman in the United States was also aware of the structure independently around 1981, and was trying for total synthesis by the methods of organic chemistry. The episode tells that similar ideas are generated by the people around the world, making you realize how difficult it is to come up with a truly original idea.
The fullerene research continues, but a fundamental question remains as a mystery. Why, after being shattered into pieces by laser or arc discharge, do the carbon atoms re-assemble themselves into this impressive polyhedral shape? An analogy would be throwing all the parts of a plastic model into a wall together, then the parts clicking by chance in the air, and landing as a complete shape of car. The phenomenon is simply a miracle of nature.
The science of fullerenes has a lot of potentials, as well as many questions to be answered. It is amazing to think that carbon, the element which humans have companied with since the dawn of history, had the unexpected form and mystery. What kind of exciting world is hidden behind the beautiful structure with the world’s best symmetry? Fullerenes will keep attracting the interest of researchers, and seem unlikely to let it go.