Ground State Structures of Fe2O4-6+ Clusters Probed by Reactions with N2

Reactions of small cationic iron oxide clusters (Fe2O4-6+) with N-2 are investigated by experiments and first principle calculations. The cationic iron oxide clusters are generated by reaction of laser ablated iron plasma with O-2 in a supersonic expansion, and are reacted with N-2 in a fast flow reactor at near room temperature conditions. Cluster cations are detected by a time-of-flight mass spectrometer. The substitution reaction Fe2On+ + N-2 -> Fe2On-2N2+ + O-2 is observed for n = 5 but not for n = 4 and 6. Density functional theory calculations predict that the low-lying energy structures of Fe2O4-6+ are with side-on (eta(1)-O-2) or end-on (eta(2)-O-2) bonded molecular oxygen unit(s). The calculations further predict that the substitution of eta(1)-O-2 and eta(2)-O-2 in Fe2O4.6+ Clusters by N-2 is exothermic and subject to negative and positive overall reaction barriers, respectively, at room temperature. We thus propose that the ground state structures of Fe2O4+ and Fe2O6+ contain eta(2)-O-2. In contrast, both the experiment and theory favor a eta(1)-O-2 in the ground state structure of Fe2O5+.