The 3d-element transition metal dioxide MO(2), peroxide M(O(2)), and superoxide MOO clusters (M=Sc-Zn), are studied by density functional theory 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+O(2) potential energy surfaces correspond to dioxide structure, the only exception being CuOO, with a superoxide structure. All Zn dioxygen clusters are thermodynamically unstable-their ground states lie higher than the dissociation limit to Zn+O(2). Our calculations are in favor of the high-spin configurations for the FeO(2), CoO(2), and NiO(2) ground states, which are still a subject of extensive theoretical and experimental studies. These assignments are confirmed by the coupled-cluster method, CCSD(T), except for NiO(2). Based on the existence of a stable NiO(2) monoanion in a (4)B(1) state, however, it can be concluded that NiO(2) in its (5)A(1) 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-dioxoclusters 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 nucleophilic 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. TiO(2) is the only dioxide in which nucleophilic character in the vicinity of the metal cation is induced.

The stationary points on the potential energy surfaces (PES) of cobalt oxide clusters were studied by the density functional theory, with the B1LYP exchange-correlation functional. A number of local minima were detected on the doublet, quartet, and sextet PES of CoO n (n = 1−4) and the singlet, triplet, and quintet PES of the corresponding anions. The normal vibrations of all optimized structures were calculated and interpreted in terms of group theory. The global minima and the low-lying local minima have been also examined by the coupled-cluster method with single and double substitutions and perturbational estimate of triple excitations, CCSD(T). The ground state of CoO is a quartet (4Δ), in agreement with previous assignments, while the ground state of CoO - is a quintet (5Δ). The oxides CoO2 are more stable than the peroxides Co(O2). The neutral oxide CoO2 in its ground state 6A1 is quasilinear; the monoanion in its 5Δg ground state is linear. Oxoperoxides OCo(O2) are more stable than oxides CoO3 and superoxides OCoOO. Diperoxides are the stable structures for the uncharged CoO4. In general, the monoanions are more stable than the neutral clusters; however, electron attachment to peroxide destabilizes the O−O bond. The ground states of CoO n (n = 2−4) and the ground states of their anions [CoO n ] - are high-spin statessextets for the neutral species and quintets for the anions. The thermodynamic stability of different structures was examined for possible fragmentation paths. The CoO n clusters and their monoanions dissociate preferably with the release of a dioxygen molecule.

The 3d-transition-metal dioxo-, peroxo-, and superoxoclusters with the general composition MO2, M(O2), and MOO (M = Mn, Fe, Co, and Ni) were studied by DFT by the B1LYP functional. The dioxides in their ground states represent the global minima for the M + O2 system. Both ground-state dioxides and the lowest-energy peroxides are in their (d-only) highest spin states. The 6A1 state of Co(O2) exceeds the d-only spin-multiplicity value (quartet), being nearly isoenergetic with the 4A1 state of Co(O2). The energy gain on transforming the peroxides to the corresponding dioxides decreases in the order Mn(O2) > Fe(O2) > Co(O2) > Ni(O2) and varies in the range 0.27-1.8 eV. The dissociation energy to M + O2 for all studied peroxides is less than 1 eV being the lowest (0.47 eV) for Mn(O2). The Mn and Fe peroxides need less than 0.3 eV to rupture one of the MO bonds to form the corresponding superoxide. Mn and Fe superoxides are less stable than the corresponding peroxides; the superoxide of Co is more stable than its peroxide, while Ni superoxide is unstable--its energy is above the limit of dissociation to Ni + O2. According to the electrostatic potential maps, the oxygen atoms in the peroxides are more nucleophilic than those in the dioxides and superoxides, in which the terminal oxygen atom is more nucleophilic than the M-bonded oxygen atom. This result differs from the expectations based on charge-distribution analysis.