Density functional study of FeO2, FeO2+ and FeO2-

The lowest energy structures of FeO2 were determined by means of density functional theory techniques as implemented in the program DGauss 3.0.1. The calculations performed were of the ah-electron type using two levels of theory, namely the local spin density approximation with the use of the Vosko-Wilk-Nusair (VWN) functional and the generalized gradient approximation (GGA) in the form of the Becke (1988) exchange and Perdew (1986) correlation functionals. Results were Visualized by means of the program UniChem. Bond distances and angles as well as total energies were calculated for several states of the moieties: Fe(O)(2), C-2v; Fe(O)(2), D-infinity h; Fe(eta(2)-O-2), C-2v; Fe(eta(1)-O-2), C-s; and Fe(eta(1)-O-2), C-infinity v. Molecular orbital and harmonic vibrational analyses were carried out for these species, in addition to Mulliken population analyses. Singly positive and negative charged species were also considered and fully geometry optimized in a self-consistent field (SCF) gradient method. Accurate ionization potentials and electron affinities (both vertical, v, and adiabatic, a, determinations) were thus able to be computed. The results show the following for the ground state (GS) Fe(O)(2), C-2v, M = 3: angle OFeO = 138.1 degrees (133.6 degrees) [values in parentheses are for local spin density approximation (LSDA)-VWN, while the others are at the GGA-B88/P86 level]. Here, R-e Fe-O = 1.60 Angstrom (1.57 Angstrom), ET = -1414.2064 au (-1,410.5047 au), EA(a) = 2.47 (2.60) eV, IPa = 10.6 (10.5) eV, EA(v) = 2.41 (2.20) eV, IPv = 10.67 (10.63) eV, and EA(exp) = 2.349 eV (in agreement with related studies). In the GS the dioxygen molecule is found to be dissociated, compared to those states which have coordination modes where the O-2 molecule formally persists. A 3d4sp configuration for the iron atom is found to be especially relevant in Fe-O bond formation. The iron-oxygen and oxygen-oxygen bonds involved are characterized. A direct relationship is observed between these electronic and structural properties, influencing also the total energy for a given molecule. (C) 2000 John Wiley & Sons, Inc.