title: The vibrational spectrum of FeO2(+) isomers--theoretical benchmark and experiment.
abstract: Infrared photodissociation is used to record the vibrational spectrum of FeO2 (+)(He)2-4 which shows three bands at 1035, 980, and 506 cm(-1). Quantum chemical multi-reference configuration interaction calculations (MRCISD) of structures and harmonic frequencies show that these bands are due to two different isomers, an inserted dioxo complex with Fe in the +V oxidation state and a side-on superoxo complex with Fe in the +II oxidation state. These two are separated by a substantial barrier, 53 kJ/mol, whereas the third isomer, an end-on complex between Fe(+) and an O2 molecule, is easily converted into the side-on complex. For all three isomers, states of different spin multiplicity have been considered. Our best energies are computed at the MRCISD+Q level, including corrections for complete active space and basis set extension, core-valence correlation, relativistic effects, and zero-point vibrational energy. The average coupled pair functional (ACPF) yields very similar energies. Density functional theory (DFT) differs significantly from our best estimates for this system, with the TPSS functional yielding the best results. The other functionals tested are BP86, PBE, B3LYP, TPSSh, and B2PLYP. Complete active space second order perturbation theory (CASPT2) performs better than DFT, but less good than ACPF.
authors.affiliation: Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, D 10099 Berlin, Germany.
authors.forename: Toni M
authors.initials: TM
authors.lastname: Maier
authors.affiliation: Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, D 10099 Berlin, Germany.
authors.forename: A Daniel
authors.initials: AD
authors.lastname: Boese
authors.affiliation: Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, D 10099 Berlin, Germany.
authors.forename: Joachim
authors.initials: J
authors.lastname: Sauer
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D 14195 Berlin, Germany.
authors.forename: Torsten
authors.initials: T
authors.lastname: Wende
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D 14195 Berlin, Germany.
authors.forename: Matias
authors.initials: M
authors.lastname: Fagiani
authors.affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D 14195 Berlin, Germany.
authors.forename: Knut R
authors.initials: KR
authors.lastname: Asmis
mesh_terms: D005290:Ferric Compounds / Q000737:chemistry*; D007536:Isomerism; D008956:Models, Chemical; D011789:Quantum Theory; D013816:Thermodynamics; D014732:Vibration
chemical_list: D005290:Ferric Compounds; C000499:ferric oxide