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On the Electronic and Geometric Structures of FeO2(-/0) and the Assignment of the Anion Photoelectron Spectrum.

The photoelectron spectrum of FeO2(-) has been assigned by performing geometry optimizations at the CASPT2 and RCCSD(T) levels of computation. All relevant states are found to possess floppy C2v geometrical structures as the Renner-Teller splittings of the linear states are extremely small and the corresponding energy barriers for the OFeO bond angle inversions are calculated in the range of a few hundred wavenumbers. In this sense, the description of the electronic structure in terms of the D∞h point group is acceptable, and the experimentally proposed linear structure for FeO2(-) is theoretically confirmed. High accuracy single-point multireference RASPT2 and single-reference RCCSD(T) calculations support a (2)Δg as the ground state of the anion, even though the energy differences between the (4)Πg and (6)Σg(+) states are smaller than 0.2 eV. After this identification of the doublet ground state, the photoelectron spectra of FeO2(-) could be assigned in all aspects. The (2)Δg→(3)Δg ionization appears to be at the origin of the X band at 2.36 eV, while the A band at 3.31 eV should be ascribed to the (2)Δg→(3)Σg(+) ionization. This assignment is substantiated by Franck-Condon factors for which BP86 optimized geometries and harmonic vibrational frequencies were employed. Indeed, no pronounced vibrational progression should be observed since both bands involve electron detachments out of nonbonding mainly 3d iron molecular orbitals.