Iron-carbon clusters: Geometric structures and interconversions

We report density functional theoretical investigations of the experiments described by Pilgrim and Duncan [J.S. Pilgrim, M.A. Duncan, J. Am. Chem. Soc., 115 (1993) 6958] in which [FexCy](+) clusters are generated by laser vapourisation of Fe atoms in a plasma containing acetylene. Many possible structures for the clusters [Fe12C12], [Fe10C12], [Fe8C12], [Fe6C12], [Fe4C12] [Fe3C12] and [Fe2C12] are evaluated, with the following conclusions: [Fe12C12] is a slightly distorted 2 x 3 x 4 nanocrystallite fragment of a face-centered cubic FeC lattice; [Fe10C12] is distorted [Fe12C12] depleted by two three-coordinate Fe atoms; [Fe8C12], with the prototypical composition for metallocarbohedrenes, has a C-3 nu structure containing only C-2 groups and with a short axial Fe-Fe bond; [Fe6C12] has the [Fe8C12] structure but without the two axial Fe atoms, such that each C-2 group has two end-on Fe-sigma-C-2 bonds and one side-on Fe-pi-C-2 interaction; at [Fe4C12] there are two cumulenic C-6 chains separated by four Fe atoms; [Fe3C12] is a monocyclic alternation of cumulenic C-4 chains and Fe atoms, while [Fe2C12] is a monocyclic alternation of cumulenic C-6 chains and Fe atoms. Mechanisms are proposed for the experiments in which photodissociation of Fe atoms converts [Fe12C12] to [Fe8C12], and, in which [Fe8C12] photodissociates to [Fe6C12], [Fe4C12], and [Fe2C12]. The formation of C-C bonds and of C-n chains initiated by the withdrawal of Fe atoms in these processes is related to the Fe catalysed formation of carbon nanotubes. (c) 2006 Elsevier Ltd. All rights reserved.