Dec. 11, 2000 A computational chemist at the University of Georgia has found an entirely new bonding arrangement for carbon molecules, a discovery that could open new ideas about life’s most basic element.
The research by Dr. Paul von Ragué Schleyer of UGA’s Center for Computational Quantum Chemistry was published today in the journal Science.
Scientists have known for well over a century that carbon normally binds with four other atoms or groups tetrahedrally to produce three-dimensional structures that have low energy requirements and therefore are naturally stable.
But a few chemists, including Schleyer, have used computational methods to predict that four groups around carbon molecules sometimes can lie in a plane. These weird flat molecules were only a theoretical idea until several examples were created in the laboratory. Now, Schleyer has taken the idea a step further: He and post-doctoral associate Kai Exner have put forward evidence for the first time that hexacoordinate, or six-sided flat carbon molecules, are theoretically possible.
“We have shown that planar configurations with more groups than anyone imagined before can exist,” said Schleyer. “What makes this discovery remarkable is that it violates two basic tenets of carbon chemistry–that carbon should have four bonds or neighbors, not six, and that molecules should be in 3-D arrangements–simultaneously.”
Nearly 125 years ago, chemists discovered that carbon has four bonds arranged in a tetrahedral manner. The researchers took for granted that the structure not only was tetrahedral but was rigidly tetrahedral, since any other configuration would have considerably more energy. The vast majority of carbon compounds, which form the basis of life, follow these expectations. Exceptions, which would mean that designs different from what scientists firmly believed, were not known.
That foundation was shaken in the early 1970s, when chemists like Roald Hoffmann of Cornell and then Schleyer unleashed the idea that flat configurations of carbon molecules are possible. Substantiated subsequently using increasingly powerful computers and software, Schleyer was able to predict with certainty that a number of such flat molecules might actually be able to exist. Such computations have been verified in the laboratory, most recently in two papers published in the Journal of the American Chemical Society in 1999 and 2000.
In the first paper, chemists at Washington State University and the University of Utah synthesized a simple flat or planar molecule with one carbon atom at its center and four aluminum and silicon atoms bound to it. That group predicted that other five-atom molecules could exist in planar arrangements, and they subsequently proved it experimentally. The latter research was published earlier this year.
“Each new example . . . is a milestone, not just in validating theory, but in establishing the limits of such seemingly outlandish structures,” Schleyer told Chemical and Engineering News in August.
While experimental examples of flat tetracoordinate carbon molecules are now accumulating, the very idea that a hexacoordinate version might exist is, Schleyer admits, “incredible.” Nevertheless, he and Exner now predict the existence of such bizarre molecules, which could shed entirely new light on how carbon can bond with atoms to form molecules.
The team specifically used computational techniques to investigate the possibility of hexacoordinate molecules with a carbon in the center of six-atom rings. They designed the boron and carbon compounds by fitting the atoms together in optimal ways, making sure that all bond lengths were in just the right ranges. They then checked their predicted planar hexacoordinate structures to make sure they would be stable.
“The predicted existence of compounds with a planar hexacoordinate carbon, which has never been envisioned before, is exciting,” the authors wrote in their Science paper. While Schleyer calls the new discovery pure research and says that there can be no immediate use for truly new discoveries like these flat molecules. He points out that the laser was invented before any uses were known for it, but it’s now a ubiquitous part of the consumer world.
Maybe some day flat carbon molecules will find their way into the marketplace as well.
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