Carbon precursors wrap into buckyballs.

Amid all the current hoopla surrounding the buckyball, the soccer-ball-shaped molecule from the fullerene family, chemists continue to wonder exactly how all of its 60 carbon atoms come together to make this elegant structure.

Why, for instance, do the carbon atoms form a ball and not some other shape?

Now, experimental results back up a proposed explanation for this feat. Joanna M. Hunter and her colleagues, all chemists at Northwestern University in Evanston, Ill., describe a mechanism by which carbon atoms form "fullerene precursors." These subsequently wind themselves into spherical carbon cages.

Studying carbon clusters made up of 50 to 70 atoms each, the chemists find that the atoms tend first to form paired hexagonal rings, around which a long chain of carbon atoms extends. Under the right conditions, this chain will wind itself up to form the rest of the ball-shaped fullerene. Their results appear in the Feb. 17 JOURNAL OF PHYSICAL CHEMISTRY

"We've been after a specific mechanism to explain how carbon, which naturally forms rings, will convert into a spheroidal cage. So we've been measuring the amount of energy needed to drive this structural transformation," says Martin E Jarrold, a coauthor of the report. "We find that this transformation occurs fairly easily, driven by a remarkably small amount of heat. This efficiency appears to hold not just for [C.sub.60] but for other carbon clusters as well."

The proposed fullerene-forming mechanism begins when a process called Bergman cyclization causes two hexagonal carbon rings to close up, creating a "fullerene fragment." That fragment then serves as a sort of seed structure, allowing a dangling chain of carbon atoms to kink itself into other hexagons and pentagons. Once configured this way, the chain then "spirals around the fullerene fragment and zips up to form a spheroidal fullerene" the chemists explain.

"By understanding the mechanism that has these carbon rings collapse into fullerene spheres, maybe we can think about making other fullerene derivatives," says Eric J. Roskamp, another coauthor. "So this model may have practical uses, especially if you want to make a particular type of fullerene." Grasping the details of this mechanism may help chemists streamline the process by which ordinary flat carbon rings pull themselves together to form carbon cages, he adds.

Jarrold has fleshed out this model of buckyball formation by making "endohedral metallofullerenes," which house lanthanum metal atoms within the carbon cages. By zapping graphite rods and lanthanum oxide with a laser, Jarrold and his colleagues created an abundance of metal atoms trapped inside fullerenes, converting them with "remarkably high efficiency."

Based on their best results, Jarrold and his colleagues believe that the lanthanum itself is acting as a "nucleation center," around which the carbon rings arrange themselves before becoming a complete cage. These intriguing results will appear in a forthcoming NATURE.