A 'metcar' molecule of twenty-one atoms, Ti s C 13, is predicted to be a chemically stable, electronically closed-shell species in an endohedral form with one carbon atom inside a Ti& cage of tetrahedral (Td) symmetry.
A class of transition-metal carbon clusters, metallocarbohedrenes, or metcars for short, has been observed by Castleman and co-workers in laser cluster beams [ 1,2 1. The smallest member in this class is of 8/ 12 metal-carbon composition ratio. One example is Ti&. The original structure proposed for the 8/ 12 metcar clusters is a dodecahedron with distorted pentagonal faces [ 11. Most previous calculations on Ti&, and other 8/ 12 metcar clusters with different transition metal elements, have assumed this atomic arrangement [ 3-10 1. Recently, another structure with DU symmetry has been found and calculated, using density-functional methods, for the Ti& and ZrsClz clusters [ 111. The DZd structure is significantly more stable than the Th structure, due to the increased number of carbon atoms which have become involved in bonding with the metal atoms. In the original T,, structure, the carbon-carbon pairs are arranged to point to the middle of the metal-metal bonds. In the Dti structure, however, the carbon pairs are oriented directly to the metal atoms and the result of this m-arrangement is a significant increase in binding energy. For TisC,2, the increase of binding energy is 0.68 eV per atom [ 111. A slightly different * Abstract published in Bulletin of The American Physical Society 39( 1) (1994).