Chemistry

Unconventional Clusters

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Science  29 Jun 2001:
Vol. 292, Issue 5526, pp. 2401
DOI: 10.1126/science.292.5526.2401b

Atoms in small clusters often have different structures than they do in bulk, because their much greater surface energy must be minimized to reduce the overall energy. Different elements solve this problem in different ways, sometimes with spectacular results. For example, fullerenes such as C60 form the robust “football” structure of hexagons and pentagons. Troyanov et al. show, using x-ray crystallography, that C60 literally caves in when enough fluorine atoms (48 of them) are attached. The resulting molecule has very low reactivity because of shielding by the fluorine atoms.

For many other elements, especially metals, cluster sizes often follow “magic numbers”—certain cluster sizes are particularly stable. A well-known example is the Mackay icosahedra, which are complete at sizes of 13, 55, 147, … atoms. Recent studies indicate that cobalt may form polytetrahedral clusters, which can be assembled entirely from tetrahedra with atoms at their vertices. However, regular tetrahedra cannot fill the entire space, and the resulting strain must be reduced through local icosahedral coordination. Doye and Wales have discovered a series of magic numbers for polytetrahedral clusters. This series allows large polytetrahedral clusters to be built without serious strain. The structures are based on internal networks of linelike defects called disclinations. — JU

Angew. Chem. Int. Ed.40, 2285 (2001); Phys. Rev. Lett.86, 5719 (2001).

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