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Structure puts cubane in a slanted box.

Boxes don't get much smaller than this. The molecule cubane consists of just eight carbon atoms bonded together in--as its name indicates--the shape of a cube, with hydrogen atoms at the corners. In the 33 years since cubane was first synthesized, scientists have learned a lot about its chemistry but not as much about its crystal structure.

Now, researchers at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., and the University of Chicago have determined the structure of solid cubane at high temperatures. The basic unit of the solid at room temperature, determined years ago, is a rhombohedron--"a cube squashed along the diagonals," says NIST's Peter M. Gehring. His team had expected that close to the melting point, the cube-shaped molecules would link to form larger cubes, as they do in other solids of that type. X-ray crystallography showed, however, that the basic unit of solid cubane is again a rhombohedron. The findings appear in the June 30 PHYSICAL REVIEW LETTERS.

Cubane's strained 90 [degrees] bonds store a lot of energy, so military scientists have tapped it as a potential high-powered explosive. Substituting eight nitrogen groups for the hydrogen, for example, would create a compound twice as powerful as TNT, Gehring predicts. So far, researchers have gotten as far as four substitutions, which make the cubane explosive. Other variations have shown antiviral and antitumor activity.

Solid cubane's structure at high temperatures remained elusive for so long because the compound sublimes readily: Crystals evaporate in minutes if left out on a table. Although sublimation is a "nuisance property," it doesn't generally cause much difficulty, says Chicago's Philip E. Eaton, who first synthesized cubane in 1964. "It's just a matter of putting a cork on the bottle. I still have some of the original samples."

By mixing amorphous carbon with cubane, study coauthor Tarter Yildirim of NIST found that he could effectively suppress the sublimation long enough to collect the data needed to obtain the crystal structure. He also modified the crystallography technique to improve the reliability of the results.
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Author:Wu, C.
Publication:Science News
Article Type:Brief Article
Geographic Code:1USA
Date:Jul 12, 1997
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