Interaction of carbon atoms with Fen, Fen-, and Fen+ clusters (n=1-6)

The electronic and geometrical structures of the ground and exited states of FenC, FenC-, and FenC+ (n = 1-6) are computed using density functional theory (DFT). Several exchange-correlation functionals are tested on FeC, for which a number of experimental results are available for comparison. Fe2C and Fe2C- have triangular high-spin ground states, while the ground state of Fe2C+ is a linear asymmetric Fe-C-Fe doublet whose spins at the iron sites are antiferromagnetically coupled. The natural bond analysis (NBO) allows us to qualitatively describe the nature of super exchange coupling in Fe,C.. There is a competition between twofold and threefold sites in the Fe3C species. Carbon is threefold coordinated in the Fe4C and Fe6C species, while it is fourfold coordinated in the Fe5C clusters. Generally, attachment of a carbon atom reduces the number of unpaired electrons of an iron cluster. The maximum reduction of four is found for the largest cluster, Fe6C. The C-Fe-n bond energies in the neutral clusters are rather high, ranging from 5.5 (Fe2C) to 6.5 eV (Fe6C). These energies are not significantly affected by attachment or detachment of an electron to/ from a Fe,C cluster. Surprisingly, the Fe-Fen-1, Fe-Fen-1C, and Fe-FenC bond energies are found to be nearly the same for given it. The results of our computations are in reasonable agreement with experimental data. (C) 2003 Elsevier Science B.V. All rights reserved.