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 a nd configuration interaction (CI) levels on Ti8C12, and at the SCF lev el on Ti14C13. For Ti8C12, the cage structure of tetracapped tetrahedr on 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 th e d metal electrons rather than the aufbau principle is defined as a d ecisive criterion for selecting the ground-state electronic configurat ion. A one-configuration SCF calculation accounting for this localizat ion criterion leads to a quintet ground state and to an optimal geomet ry characterized by d(c-c) = 1.343 Angstrom, d(Ti-C(end-on)) = 1.964 A ngstrom, 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 unp aired electrons lead to a totally symmetric singlet ground state, sepa rated from the quintet state by 10.5 kcal mol(-1). The electronic stru cture of the Ti-8 framework is discussed in terms of mixed valency, th e 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 pe ntagonal dedocahedron with T-h symmetry is subject to a considerable e nergy barrier. The energy minimum corresponding to the T-d symmetry is not unique, however. Six other minima, corresponding all to cage stru ctures with 36 Ti-C bonds have been characterized on the potential ene rgy hypersurface. CI calculations show that those secondary minima are less stable than the T-d form by 153 to 179 kcal mol(-1). The calcula tions carried out on the Ti14C13 cluster indicate that a proper locali zation of the d metal electrons requires the O-h symmetry of the fee c rystallite to be broken into T-d. This leads to metal-carbon bond leng ths comprised between 1.99 Angstrom and 2.12 Angstrom in the optimized structure. Breaking the O-h symmetry leads to a potential energy curv e with two equivalent minima separated by an energy barrier of 13 kcal mol(-1) associated with the totally symmetric O-h structure.