Nanocrystal [Ti14C13] to Metallocarbohedrene [Ti8C13]: Structural Principles and Mechanism

Gradient-corrected density functional calculations have characterized the geometric and electronic structures of the metal-carbon clusters [Ti 14 C 13 ] 0,+ and [Ti 8 C 13 ] 0,+ and the intermediates Ti x C 13 in the sequence of photoextrusions of Ti atoms from [Ti 14 C 13 ] + to [Ti 8 C 13 ] + . Ti 14 C 13 is a vertex-contracted “nanocrystal” fragment of the face-centered cubic TiC lattice, with low-lying spin states, while Ti 8 C 13 is a C c -centered tetra-Ti o -capped Ti i 4 tetrahedron with C 2 groups cradled in six Ti i 2 Ti o 2 butterflies and has a large HOMO-LUMO gap. This sequence of six photodissociations of Ti atoms is significant because it involves the transformation of a fragment of a non-molecular lattice structure to a molecular cluster structure and the formation of six C−C bonds on the surface. The transformation is most endergonic in the first stage to [Ti 13 C 13 ] +, and decreasingly endergonic to [Ti 12 C 13 ] + and [Ti 11 C 13 ] + . The geometry changes are concerted, with each intermediate partly prepared for the next extrusion of Ti, and the central C c atom plays a significant role in the substantial structural rearrangements. The high symmetry ( O h ) of [Ti 14 C 13 ] + is completely lost at [Ti 12 C 13 ] +, but there is recovery of C 3 v symmetry at the intermediate [Ti 11 C 13 ] + which already has in place the four C c −Ti i bonds and the Ti i (C 2 ) 3 (Ti o ) 3 face characteristic of [Ti 8 C 13 ] + . Energy changes follow the geometry changes.