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March 20, 2001 Press Contact: Steve Koppes
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Octanitrocubane: Easier said than done

American Chemical Society lauds difficult synthesis of new compound

Some chemists once thought that octanitrocubane, potentially the world’s most powerful non-nuclear explosive, would be impossible to create. Last year Philip Eaton, Professor in Chemistry at the University of Chicago, proved them wrong. Now the American Chemical Society’s Chemistry magazine is listing Eaton’s synthesis of octanitrocubane as one of the chemistry-related milestones of the year 2000.

“Octanitrocubane is not explosive under ordinary conditions,” Eaton said. “It’s a very stable compound kinetically. Such compounds need to be purposefully detonated and could even become large construction site regulars if they were cheap enough.”

Eaton and his research associate, Mao-Xi Zhang, described their successful synthesis of the compound in the Jan. 17, 2001, international edition of the journal Angewandte Chemie.

“I never had any doubt that octanitrocubane could exist, although many people did,” Eaton said. “But we did know from the beginning that we were going to have to learn a lot about fundamental chemistry before we could do this.”

Eaton and his colleagues describe the work as an advance in fundamental chemistry.

“Octanitrocubane is a beautiful molecule in the symmetry of its structure and the energy content of its bonds,” said David Oxtoby, the William Rainey Harper Professor in Chemistry and Dean of Physical Sciences at the University of Chicago. “It is also a triumph of synthetic chemistry. Phil Eaton and his research group have developed new methods of synthesis that will be useful for making other new molecules in the future.”

Zhang did all of the experimental work and contributed a large part of the intellectual input, Eaton said. “Without him, the project would never have come to fruition.”

Eaton credits scientists at the U.S. Army arsenal in Picatinny, N.J., for coming to him decades ago with the idea that led to the synthesis of octanitrocubane. The standard military explosive today, HMX, is less powerful than octanitrocubane and is difficult to safely manufacture because of its shock sensitivity. Part of the military’s interest in the research stemmed from the belief that octanitrocubane would be insensitive to shock.

“In fact you can hit octanitrocubane with a hammer and nothing happens,” Eaton said.

Nevertheless, the compound is probably too expensive to synthesize to make it a practical military explosive, Eaton said. And that is just fine with him.

“I’m not militaristic and have absolutely no desire to make bombs or explosives,” he said. “I resisted working on this project initially because of my own prejudices about explosives and more honestly because I hadn’t the slightest idea how to do it.”

Then Eaton realized that in order to make octanitrocubane, he would first have to understand the fundamental chemistry of its parent compound, cubane. That made it an important problem.

Chemists formerly regarded cubane as impossible to synthesize because of its unusual geometry.

Cubane is cube-shaped, with a carbon atom at each of its eight corners and a hydrogen atom attached to each carbon. But carbon atoms naturally bond at 109-degree angles. Skeptics assumed that the 90-degree bonding angles required cubane would strain the molecule beyond its breaking point. Nevertheless, in 1964, early in Eaton’s career at Chicago, he managed to synthesize cubane, which proved to be remarkably stable.

Eaton went on to other projects for a number of years, but the opportunity to further investigate cubane’s surprising chemistry lured him back. In the process of understanding cubane’s fundamental chemistry, Eaton’s group made a series of seminal discoveries that had nothing to do with explosives, but with the limits of bonding and structure in organic chemistry. This work was funded by the National Science Foundation, the Army and the Navy.

“It turned out that we could use cubane as a starting material for looking at really new and different structures that challenged the accepted limits of inorganic chemistry,” Eaton explained. “We found all kinds of things that nobody expected.”

In one of his experiments, Eaton came up with a result that differed from the theoretically predicted result by a factor of a quadrillion. “I think that that is the largest discrepancy between theory and experiment that’s ever been found,” Eaton said. “The theory was looked at and corrected appropriately, and now we have a better theory.”

Eaton’s latest achievement was to find ways to attach nitro groups, each consisting of a nitrogen atom connected to two oxygens, to each of cubane’s carbons. These nitro groups are a component of almost every explosive, including TNT and nitroglycerin. But attaching them to cubane required the development of entirely new techniques because of its special chemistry.

With octanitrocubane now “in the bottle,” the Army now is considering how to make the compound in its laboratories in sufficient quantity for real-world testing. “Whether the original theory adequately predicts octanitrocubane’s real explosive properties remains to be seen,” Eaton said.

Much of Eaton’s work now focuses on polycubyls, in which cubanes are strung together diagonally in corner-to-corner structures. These long, rigid rods are of interest for potential applications in technology at the nanoscale (devices built at the scale of atoms and molecules), and as models of cell bilayers (layers with the thickness of only two molecules ).

Some of the discoveries that flowed from cubane chemistry hold potential for making it biologically active materials. The cubane molecule, for example, is similar in size to some compounds derived from benzene, from which many pharmaceuticals and many practical compounds are derived.

The pharmaceutical implications of such research are uncertain at this stage, according to Eaton.

“Twenty years from now, that may be very different,” he said.

 

http://www-news.uchicago.edu/releases/01/010320.explosive.shtml
Last modified at 05:16 PM CST on Thursday, April 19, 2001.

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