THEORETICAL-STUDIES ON THE STABILITY OF M8C12 CLUSTERS
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Ab initio calculations with effective core potentials were completed on M8C12 (M = Y, Zr, Nb, and Mo) clusters. Three different isomers (Th, D3d, and Td) have been investigated. The Th and D3d isomers are derived from a dodecahedral geometry. In the former, the faces of a cube of the metal atoms are capped by ethylene-like C2 units, while in the latter the 12 carbons form a cylindrical, closed chain capped by two M4 units. In the Td isomer, six acetylene-like C2 units link the vertices of a tetracapped tetrahedral metal framework in a butterfly fashion. Our calculations show that the observed M8C12 (M = Ti, Zr, Hf, V, Mo or W) clusters most likely adopt a structure similar to the Td structure rather than one of the dodecahedral structures. The Td structure is more stable than the Th or D3d structure by more than 200 kcal/mol. Molecular orbital analyses of the bonding in the Td structure allow us to derive magic numbers of metal electrons, i.e., 18 and 36 when we formulate the cluster as (M8)12+(C22)6. The unique geometry and electronic structure are responsible for the observation of high stability of these metal-carbon clusters. In the group 4 clusters, a dicationic cluster, e.g., [Ti8C12]2, may be the best target to be isolated in the solid state as an ion. 1993, American Chemical Society. All rights reserved.