Showing a new, naturally occurring compound to a research chemist is, in a way, like throwing down a gauntlet. The unspoken challenge being issued — create this in the lab.
For Peter Rabideau, that gauntlet has been the buckyball — a curious hollow sphere formed by 60 atoms of carbon. Rabideau, an Ames Laboratory senior chemist, has moved a step closer to meeting that challenge by developing a practical means of producing bowl-shaped segments — buckybowls — that could eventually be pieced together to form the complete ball.
"Chemists are always interested in being able to synthesize things whether you need them or not," Rabideau says, pointing out the synthesis of cholesterol as just one example. "It's a big thing just to show you could do it, so there's an element of that to it.
"But there's also the practical side of being able to put things inside the buckyball, such as precursors for supertough coatings or drug-delivery systems, that may have some real novel properties if one could figure out how to do that," he says.
Buckyballs have intrigued chemists since the uniquely structured molecules were first discovered in 1985. The carbon atoms align to form a hollow structure similar to the pattern of panels found on a soccer ball. They get their name from architect/engineer/philosopher Buckminster "Bucky" Fuller, who pioneered the concept of the geodesic dome.
"It turns out that geodesic stability on the molecular level is also rather remarkable," says Rabideau, who also serves as dean of Iowa State University's College of Liberal Arts and Sciences. "Carbon atoms like to be arranged in this particular formation and are extremely stable."
Since the structure is very stable and hollow, chemists have envisioned a whole new array of applications if they could find a way to put other atoms or compounds inside the buckyball.
Unfortunately, the only way to produce C60 is to replicate the environment of interstellar space with a process that basically involves arcing carbon rods, but this reaction takes place only at high temperatures. The high temperatures make the reaction hard to control, so it's extremely difficult to try to produce buckyballs with something inside them. So the search turned to finding a way to "build" a buckyball from scratch.
"If you took a buckyball apart, it wouldn't be a stable entity because it would have dangling bonds," Rabideau explains. "But if you took it apart and put hydrogen atoms on the dangling bonds to stabilize the structure, you'd have a chemical compound that we call a buckybowl."
It's the curved shape of these compounds that Rabideau finds intriguing. Most polynuclear aromatic hydrocarbons, such as graphite, are flat. And it was these "flat" com-pounds that he spent the early part of his career studying at the University of Chicago, under the guidance of the man who literally wrote the book on the subject, Ron Harvey.
As Rabideau's research progressed, so did his advancement through the academic ranks. By 1990, he was a dean at Louisiana State University and had come to an impasse in his research.
"We had answered a lot of the questions that we originally asked, so it was a matter of where the research was going," Rabideau says. "I had also become a dean, so I had to do a reality check on whether or not I wanted to continue to do the active research.
"My feeling was that if I was going to remain active in research, we would have to revitalize our program and find some new things to do," he says. "About that time I saw a publication on corannulene (C20H10) and some of these curved-surface compounds, and decided that's the direction we would take."
Corannulene was first synthesized in 1966 at the University of Michigan by a long and difficult 17-step process that produced quantities weighing just a few milligrams. Then in the early 1990s, another group of researchers synthesized corannulene using pyrolysis — heating a material until it decomposes. It was this breakthrough that grabbed Rabideau's attention.
"The problem with this (gas-phase) technique is that you must do it in vacuum, so by definition you're still working with very small amounts of material," Rabideau says.
What Rabideau and fellow researcher Andrzej Sygula developed was a solution-phase synthesis using dilute sodium hydroxide in water and acetone that produces tetrabromocorranulene — a molecule of 20 carbon atoms with four bromine atoms attached. This process is detailed in the following article: Sygula, Andrzej and Rabideau, Peter W. 2000. A Practical Large Scale Synthesis of the Corannulene System. Journal of the American Chemical Society 122 (26): 6323-6324.
The process allows production of 25-gram samples, a thousandfold increase over the pyrolysis method.
"The process is really pretty simple," Rabideau says. "In fact, the final step could almost be done in your garage using household chemicals." Though it hasn't been a particular goal of his research, Rabideau adds that a feasible commercial synthesis could easily be developed.
The synthesis of tetrabromocorranulene has also had a silver lining. While the bromine atoms can easily be removed to form corannulene, it's advantageous to leave them on and add to them.
"We now have a way of elaborating that particular molecule in a lot of different ways," Radideau says, pointing to diagrams of just eight of the many variations. "It's better than if we'd discovered a way to go directly to corannulene because we can use the bromines to do other things, though our focus is on the fundamental chemistry involved."
Though building a buckyball is still in the distance, researchers now have an unlimited supply of bowl material to study. By unlocking the properties of these bowl-shaped compounds, Rabideau hopes to eventually discover a way to combine the bowls into a sphere.
"If we could figure out a few critical reactions, we might ultimately be able to synthesize C60," Rabideau says. "That would, in principle, allow us to build C60 with a hole in it so we could trap something inside, such as an atom or metal, and then close up the ball.
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