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 × 3 × 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 C3v structure containing only C2 groups and with a short axial FeFe bond; [Fe6C12] has the [Fe8C12] structure but without the two axial Fe atoms, such that each C2 group has two end-on FeσC2 bonds and one side-on FeπC2 interaction; at [Fe4C12] there are two cumulenic C6 chains separated by four Fe atoms; [Fe3C12] is a monocyclic alternation of cumulenic C4 chains and Fe atoms, while [Fe2C12] is a monocyclic alternation of cumulenic C6 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 CC bonds and of Cn chains initiated by the withdrawal of Fe atoms in these processes is related to the Fe catalysed formation of carbon nanotubes.

Possible and probable structures are reported for the experimentally observed ions [Fe12C12]+, [Fe10C12]+, [Fe8C12]+, [Fe6C12]+, [Fe4C12]+, [Fe3C12]+ and [Fe2C12]+, together with mechanisms for the observed photodissociation of [Fe12C12]+ to [Fe8C12]+, and sequential photodissociations of [Fe8C12]+ to [Fe2C12]+.Figure optionsDownload as PowerPoint slide