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

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journal contribution
posted on 20.03.1996, 00:00 by Ian Dance
Gradient-corrected density functional calculations have characterized the geometric and electronic structures of the metal-carbon clusters [Ti14C13]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 Cc-centered tetra-Tio-capped Tii4 tetrahedron with C2 groups cradled in six Tii2Tio2 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 Cc atom plays a significant role in the substantial structural rearrangements. The high symmetry (Oh) of [Ti14C13]+ is completely lost at [Ti12C13]+, but there is recovery of C3v symmetry at the intermediate [Ti11C13]+ which already has in place the four Cc−Tii bonds and the Tii(C2)3(Tio)3 face characteristic of [Ti8C13]+. Energy changes follow the geometry changes.