Low-lying electronic states and molecular structure of FeO2 and FeO2−

An ab initio theoretical study on the structures, molecular bonding and harmonic vibrational frequencies of the low-lying electronic states of FeO2 and FeO2− has been carried out at the HF, MP2 and CCSD(T) levels of theory with the MIDI∗ and TZV∗ basis sets. For FeO2, the properties of the ground state strongly depend on the level of the calculation. At the UHF level, the inserted linear dioxide 3B1 state (C2v subgroup notation) in D∞h symmetry is found to be the most stable state. The inclusion of correlation energy by means of the MP2 and CCSD(T) methods results in an inserted dioxide 1A1 state in C2v symmetry as the lowest energy state. For FeO2−, the inserted linear dioxide 6A2g state (in D4h subgroup notation) is the ground state at the HF, MP2 and CCSD(T) levels of theory with the TZV∗ basis set. Other stable and first order saddle points with possible side-on, end-on and inserted structures have been located on the potential energy hypersurfaces of FeO2 and FeO2−.