Iron-carbon clusters: Geometric structures and interconversions

Abstract

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.