Small iron doped carbon clusters: a comparison with early and late first-row transition metal doped clusters

A systematic study of the three lowest-lying structures, namely, linear, fan, and cyclic, of neutral FeC(n) (n = 1-8) clusters on the singlet, triplet, quintet, and septet potential energy surfaces has been carried out. Predictions for their electronic energies, rotational constants, dipole moments, and vibrational frequencies have been made using the B3LYP method in conjunction with the 6-311+G(d) basis set. Triplet or quintet states are found as the lowest-lying ones for FeC(n) clusters, and the septet states are found more stable than the singlet ones. The incremental binding energies show an even-odd parity effect, with n-even clusters being more stable than the n-odd ones in the linear and fan clusters, whereas a decrease with n is found for cyclic ones. The most stable isomers for FeC(n) clusters correspond to a fan-type structure for n < or = 4, whereas for n > or = 6 cyclic structures are the most stable. Only in the case of n = 5 the most stable isomer is the linear structure. Comparing the results of FeC(n) clusters with the previously studied TMC(n) (TM = Sc, Ti, V, Co, and Zn) systems, we can see that, as it should be expected, iron doped carbon clusters present an intermediate behavior between early and late first-row transition metal doped clusters.