Density functional theory study on the geometric and electronic structures of Fe2O2 and the reaction of Fe2 + O2

Abstract

Density functional theory calculations are performed on all possible structures of Fe2O2 using the hybrid B3LYP functional and the B3LYP functional combined with the broken-symmetry (BS) approach to obtain the most stable isomers. Based on the obtained stable isomers, the reaction mechanism of Fe2 + O2 toward rhombic Fe2(μ-O)2 is considered. The BS-singlet state of the rhombic Fe2(μ-O)22.1 is found to be the ground state of all Fe2O2 isomers. The 9A″ state of the open-cycle (η1-O)Fe2(µ-O) 2.11 and 3A state of the near-linear OFeOFe 1.4 are found to have the second and third lowest energy states, which are higher than the ground state 2.1 by 109.7 and 120.0 kJ mol−1, respectively. The lowest-lying energy states of the bare Fe2O2 clusters do not favor three-dimensional structures, but favor the linear and planar structures. Numerous electronic states are found for the symmetric optimized rhombic 2.1. The energy ordering of the BS-singlet ground state and the four lowest-lying states (9Ag, BS-septet, BS-triplet, and BS-quintet) at the B3LYP and BS-B3LYP levels agrees with the energy ordering obtained at the CCSD(T) level. The characteristic frequencies calculated for the stable isomers are similar to the previous theoretical and experimental studies. The singlet and nonet energy surfaces calculated for the reaction of Fe2 + O2 toward rhombic Fe2(μ-O)2 suggest that the reaction adiabatically proceeds with spin inversion from the nonet to singlet state and is expected to be fast because the reaction takes place with a very low barrier relative to that of the reactants (or without any activation energy). The NBO analyses show that the FeO bonds in the intermediates and final product exhibit both ionic and covalent characteristics and have highly polarities because of large charge transfer from Fe to O.