THEORETICAL-STUDIES OF M8C12 SPECIES

The Ti8C12 species recently observed in mass spectra is investigated using ab initio electronic structure techniques along with the as yet unobserved Sc8C12 species. Assuming the highly symmetric structure recently proposed for M8C12, geometries are determined in small basis sets for both species. Sc8C12 adopts a closed-shell structure. At the SCF level, the lowest energy configuration for Ti8C12 corresponds to a high-spin ground state with eight unpaired electrons; a dense manifold of low-lying states arises from other spin couplings. Electronic states corresponding to 20-pi-electron structures are calculated to lie at much higher energies at the SCF level. The apparent stability of these species appears to arise from the topology of the metal-carbon cage rather than from any special feature regarding electron count.