Observation and Photoelectron Spectroscopic Study of Novel Mono ...

Mono- and Diiron Oxide Molecules J. Am. Chem. Soc., Vol. 118, No. 22, 1996 5301complex. The Fe 2 O 3 isomer may consist of a Fe 2 O-O 2complex or Fe 2 -O 3 complex. Similarly, the Fe 2 O 4 and Fe 2 O 5isomers may also consist of O 2 or O 3 units. All these isomersseem to yield rather broad features in the photoelectron spectra,consistent with their weakly bound nature. In some sense, theseO 2 complexes may be more interesting because they may mimicthe properties of those O 2 complexes formed in oxygen transportsystems or organometallic-O 2 complexes, that are precursorsfor the cleaving of the O-O bond. 17SummaryWe study two series of novel iron oxide molecules, FeO y (y) 1-4) and Fe 2 O y (y ) 1-5), using anion photoelectronspectroscopy. Important electronic and vibrational structuresare obtained, providing unique information about the structuresand bonding of these molecules. For both series, sequentialoxidation behaviors are observed, as inferred from the near linearincrease of the electron affinity versus the number of O atoms.An oxidation saturation is observed at FeO 3 , consistent withthe known maximum oxidation state of the Fe atom. In bothseries, the photoelectron spectra are observed to become simpleras the number of O atoms increases. This is consistent with acharge transfer picture where the electrons are sequentiallytransferred from the Fe atoms to the increasing O atoms, yieldinga simpler electronic structure for the higher oxides because theO-derived orbitals all lie deeper in energy and cannot beaccessed by the photon energies used. The structures of the(38) Goldberg, N.; Iraqi, M.; Koch, W.; Schwarz, H. Chem. Phys. Lett.1994, 225, 404-409. Bencivenni, L.; Pelino, M.; Ramondo, F. J. Mol.Struct. (Theochem) 1992, 253, 109-120. Snyder, L. C.; Raghavachari, K.J. Chem. Phys. 1984, 80, 5076-5079.(39) Nicholas, J. B.; Fan, J.; Wu, H.; Colson, S. D.; Wang, L. S. J. Chem.Phys. 1995, 102, 8277-8280.monoiron series are found to change from a C 2V FeO 2 to a D 3hFeO 3 , and finally to a C 2V FeO 4 that contains an O-O bondsince the oxidation state of Fe is already saturated at FeO 3 . ThatFeO 4 must contain an O-O unit is also evidenced by observationof photodissociation of the parent FeO - 4 anion, which canlose an O 2 under high laser fluences to give rise to a broadFeO - 2 feature. For the diiron series, all the spectra are observedto be quite similar, each containing a rather prominent groundstate peak, followed by one or two excited state features.Isomers are also observed for the Fe 2 O - y series when y > 1.These isomers are particularly abundant for Fe 2 O - 2 and Fe 2 O- 3and complicate the observed spectra. These isomers areproposed to be complexes involving O 2 or O 3 units. For themain oxide sequence, the electron affinity is observed to increasewith the number of O atoms. The oxidation is observed not tobe saturated at Fe 2 O 5 , agreeing with the maximum knownoxidation state of the Fe atom. Possible structures are proposedfor these oxide molecules, consistent with the experimentalobservations, which provide stringent tests for any theoreticalcalculations that can more completely elucidate the structureand bonding of these novel oxide molecules.Acknowledgment. We thank Dr. L. Lou for valuablediscussions and Pacific Northwest National Laboratory/U.S.Department of Energy, Basic Energy Sciences for the developmentof the cluster-PES apparatus. Support for this study fromthe National Science Foundation (Grant No. CHE-9404428) isgratefully acknowledged. This work was performed at PacificNorthwest National Laboratory, operated for the U.S. Departmentof Energy by Battelle under contract DE-AC06-76RLO1830.JA954153B