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 [Ti14Cr13](0,+) and [Ti8C13](0,+) and the intermediates TixC13 in the sequence of photoextrusions of Ti atoms from [Ti14C13](+) to [Ti8C13](+). Ti14C13 is a vertex-contracted ''nanocrystal'' fragment of the face-centered cubic TiC lattice, with low-lying spin states, while Ti8C13 is a C-c-centered tetra-Ti-o-capped Ti-4(i) tetrahedron with C-2 groups cradled in six (Ti2Ti2o)-Ti-i 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 [Ti13C13](+), and decreasingly endergonic to [Ti12C13](+) and [Ti11C13](+). 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 [Ti14C13](+) is completely lost at [Ti12C13](+), but there is recovery of C-3v symmetry at the intermediate[Ti11C13](+) 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 [Ti8C13](+). Energy changes follow the geometry changes.