A CASPT2 Description of the Electronic Structures of FeO3(-/0) in Relevance to the Anion Photoelectron Spectrum.

DFT and multireference methods were used to investigate the electronic structure of FeO3 and FeO3(-) clusters. Geometries of different spin multiplicities and conformations were optimized without any symmetry restrictions at the BP/QZVP level and further refined with the CASPT2 method. Although the latter type of calculations were performed by using the C2v point group, all low-lying states relevant to the photoelectron spectrum were found to correspond to or to resemble closely a planar D3h iron trioxide with no bonds between the oxygen atoms. Depending on the computational method used, the ground state of the FeO3(-) anion can be either (2)E'' or (4)A1'. The two lowest binding energy bands of the photoelectron spectrum of FeO3(-) can only be ascribed to electron detachments from the (2)E'' state. The first band is the result of a transition to the (1)A1' ground state of FeO3, whereas the second band originates from the first excited (3)E'' state. A harmonic vibrational analysis of the symmetric stretch shows that the observed vibrational progressions of these two bands in the photoelectron spectrum of FeO3(-) are also in line with the assignment. A molecular orbital analysis led to the conclusion that the electronic structures of the anionic and neutral clusters can formally be described by an oxidation state of iron of +5 and +6, respectively. A population analysis, on the contrary, points to an ionization that takes place on the oxygen atoms.