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E. Uzunova, H. Mikosch, G. Nikolov:
"Electronic Structure of Oxide, Peroxide and Superoxide Clusters (MOn: n=1, 2) of the 3d elements: a Comparative DFT Study";
Journal of Chemical Physics, 128 (2008), 9; S. 094307.

The 3d-element transition metal dioxide, peroxide and superoxide clusters with general composition MO2, M(O2) and MOO (M=Sc - Zn) are studied by DFT with the B1LYP functional. The reliability of the methods and basis sets employed was tested by a reinvestigation of the monoxides, for which a database of experimental data is available. The global minima on the M + O2 potential energy surfaces correspond to dioxide structure, the only exception being CuOO, with a superoxide structure. All Zn dioxygen clusters are thermodynamically unstable and even the Zn dioxide ground state is of higher energy than the dissociation limit to Zn + O2. Our B1LYP calculations are in favor of the high-spin states for the ground states for FeO2, CoO2 and NiO2, as well as CrO- clusters, which are still a subject of extensive theoretical and experimental studies. These assignments are confirmed by CCSD(T) calculations, except for NiO2. Based on the existence of a stable NiO2 monoanion in a 4B1 state however, it can be concluded that NiO2 in its 5A1 state should also be stable.
The vibrational frequencies are calculated for clusters entrapped in the cubic cell of solid Ar matrix and compared with those obtained for gas-phase clusters. The matrix has no influence on the vibrations of the monoxides and most of the dioxides; however Co and Ni - dioxo- clusters interact strongly with the atoms from the noble gas matrix. The most intense frequencies in the IR spectra are shifted to lower energies and the ordering of the low-lying electronic states by stability is also reversed.
According to the electrostatic potential maps, the oxygen atoms in the peroxides are more nucleophillic than those in the dioxides and superoxides. The terminal oxygen atom in superoxides is more nucleophilic than its M-bonded oxygen atom, though charge distribution analysis predicts a smaller negative charge on the terminal oxygen. TiO2 is the only dioxide in which nucleophilic character in the vicinity of the metal cation is induced.

http://dx.doi.org/10.1063/1.2831583

http://publik.tuwien.ac.at/files/pub-tch_8428.pdf