AB-INITIO SCF AND CI INVESTIGATIONS ON TITANIUM CARBON CLUSTERS - METALLOCARBOHEDRENES TI8C12 AND FCC CRYSTALLITES TI14C13

Ab initio Hartree-Fock calculations have been carried out at the SCF and configuration interaction (CI) levels on Ti8C12, and at the SCF level on Ti14C13. For Ti8C12, the cage structure of tetracapped tetrahedron of metal atoms with T-d symmetry and 36 Ti-C bonds was found to be most stable at both levels of calculation. A proper localization of the d metal electrons rather than the aufbau principle is defined as a decisive criterion for selecting the ground-state electronic configuration. A one-configuration SCF calculation accounting for this localization criterion leads to a quintet ground state and to an optimal geometry characterized by d(c-c) = 1.343 Angstrom, d(Ti-C(end-on)) = 1.964 Angstrom, d(Ti-C(side-on)) = 2.265 Angstrom, and d(Ti-Ti) = 2.943 and 3.114 Angstrom. CI calculations allowing for a coupling of the four unpaired electrons lead to a totally symmetric singlet ground state, separated from the quintet state by 10.5 kcal mol(-1). The electronic structure of the Ti-8 framework is discussed in terms of mixed valency, the metal atoms belonging to either tetrahedral sets being assigned the formal oxidation states III or 0, respectively. An interchange process between both tetrahedral sets through an intermediate structure of pentagonal dedocahedron with T-h symmetry is subject to a considerable energy barrier. The energy minimum corresponding to the T-d symmetry is not unique, however. Six other minima, corresponding all to cage structures with 36 Ti-C bonds have been characterized on the potential energy hypersurface. CI calculations show that those secondary minima are less stable than the T-d form by 153 to 179 kcal mol(-1). The calculations carried out on the Ti14C13 cluster indicate that a proper localization of the d metal electrons requires the O-h symmetry of the fee crystallite to be broken into T-d. This leads to metal-carbon bond lengths comprised between 1.99 Angstrom and 2.12 Angstrom in the optimized structure. Breaking the O-h symmetry leads to a potential energy curve with two equivalent minima separated by an energy barrier of 13 kcal mol(-1) associated with the totally symmetric O-h structure.