Theoretical Investigations of Geometry, Electronic Structure and Stability of UO6: Octahedral Uranium Hexoxide and Its Isomersâ€.

The existence of a novel octahedral UO6complex had been suggested by Pyykkö et al. [Pyykkö, P.; Runeberg, N.; Straka, M.; Dyall, K. G. Chem. Phys. Lett.2000, 328, 415]. We have now investigated the stability, the geometric and electronic structures, and the vibrations of various UO6molecules, using spin−orbit density functional and scalar-relativistic coupled-cluster approaches. We find four different (meta-)stable species, namely 3D2h-UO2(η2-O2•)2at lowest energy, 3C2v-UO4•(η2-O2•) and 1D3-U(η2-O2)3at medium energies, and 1Oh-UO6at highest energy. The decay of Oh-UO6occurs via an activated spin-flip mechanism. The UO6species correspond to local minima on singlet and triplet energy surfaces and might be trapped in noble gas matrices. Experimentally, the four species might be identified through their vibrational spectra. Uranium is best described as coordinated by oxygen atoms in various oxidation states as oxo O2−, oxido(1) O•−, peroxido O22−, and superoxido O2•−ligands. The occurrence of monovalent oxygen is remarkable. The resulting characterization of the central ion as UVIin all four cases does not fully reflect the electronic differences, nor the “valence-activity” of the U-6p6semicore shell. [ABSTRACT FROM AUTHOR]