Pure Carbon-60

A C₆₀ molecule consists of 60 carbon atoms bonded in the nearly spherical configuration shown above (the red bonds are double bonds, the white ones single bonds). If you look carefully, you will see that each carbon atom is equivalent to all the others (ie, they all have the same number of neighbors, bonded at the same relative angles). This shape is called a truncated icosahedron, with 20 hexagons and 12 pentagons. (It's also the shape of a soccer ball). A C₆₀ molecule or "buckyball" is about 7 angstroms in diameter (4.5 times as large as a hydrogen atom, or a bit less than a billionth of a meter)and is the most common fullerene, and thus the most-researched.

(For a nifty Java buckyball that you can move around with your mouse, check out http://java.sun.com/applets/jdk/1.0/demo/MoleculeViewer/example2.html.)

C₆₀ could not be studied scientifically until 1990 when Kratschmer and Fostiropoulos found a way to synthesize reasonable quantities of it (grams). It was learned that C₆₀ molecules will combine to form a crystalline solid with interesting properties. The solid is cubic, weakly bound, and electrically insulating (for those who know what this means, the gap is about 2.3 eV). Above approximately -13 degrees C (260 Kelvin), the balls spin freely in their crystal positions, so they look like smooth spheres. At lower temperatures, they begin to "stick" at certain orientations, and the motion more closely resembles ratcheting. Eventually, below -183 C or 90 K (although this temperature depends on how fast the system is cooled), the balls become completely stuck. We still do not understand some aspects of this cooling process.

Chemically, C₆₀ is highly electronegative, meaning that it readily forms compounds with atoms that will donate electrons to it. An obvious combination to try is C₆₀ plus an alkali metal, since the alkali metals are highly electropositive. Indeed, such studies yielded a new set of fascinating materials which you can read about on the alkali-C₆₀ page.

But even pure C₆₀ is interesting. Visually, it is quite different from both graphite and diamond-- it is a yellow powder which turns pink when dissolved in certain solvents such as toluene. When exposed to strong ultraviolet light, such as from a laser, the buckyballs polymerize, forming bonds between adjacent balls. In the polymerized state, the C₆₀ no longer dissolves in toluene. The fact that it changes properties upon exposure to light makes C₆₀ "photosensitive," and it has been used as a photoresist in certain photolithographic processes.

Clearly we do not yet know everything there is to know about buckyballs. If you have an interest in this field, I recommend taking a look at the references I have noted. (Go ahead and dig in. There are thousands of articles about this material!).

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Email: kimall (at symbol) mindspring.com