Explaining Carbon-Cluster Magic Numbers

By Weisburd, Stefi | Science News, August 15, 1987 | Go to article overview

Explaining Carbon-Cluster Magic Numbers


Weisburd, Stefi, Science News


Explaining carbon-cluster magic numbers

When graphite is vaporized by a laser, the liberated carbon atoms are found to be clumped together in remarkably specific numbers: If more than 40 atoms make up a cluster, then an even number will be in the clump, while in smaller clusters, certain "magic numbers'--11, 15, 19 and 23--are most common. For years chemists have debated the origin and structure of these clusters, especially those in the 40-plus range. To explain the high prevalence of C60 clusters, for example, some researchers have proposed a soccer-ball-like structure called buckminsterfullerene (SN: 11/23/85, p.325).

Now two scientists at Oregon State University in Corvallis have come up with a carbon structure that they believe may explain the magic numbers of the smaller clusters. Materials scientist James A. Van Vechten and chemist Douglas A. Keszler also suggest that their new structure could form the basis of a novel and industrially important material that would have all the strength of graphite, but none of its brittleness. Their work will appear in the Sept. 15 PHYSICAL REVIEW B.

Van Vechten and Keszler happened upon the structure when they were analyzing fine "whiskers' of carbon that they had made by "sputtering' or bombarding a graphite target with ions. Transmission electron microscopy revealed that the whisker material is crystalline, but that the carbon atoms in it are arranged in neither a diamond's nor graphite's pattern. Van Vechten says that after spending months thinking up atomic arrangements that could be reconciled with the experimental data, he found only one model that would fit: an 11-atom, paddle-wheel-like structure consisting of two "hub' atoms along an axle surrounded by three paddles, each containing an additional three atoms.

In the midst of the whisker work, Van Vechten realized that this structure could also be "a natural explanation for the strong prominence of 11-atom clusters in laser [vaporization],' since that process is as violent as sputtering. Support for the stability of the structure comes from the recent synthesis of similarly shaped molecules such as propellahexaene (SN: 6/6/87, p.357).

To explain the other carbon-cluster magic numbers, Van Vechten says each addition of four carbon atoms to the 11-atom molecule would allow a stable graphite six-member ring to form along the side of one paddle. …

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