Mixed uranium chloride fluorides UF6-nCln and methoxyuranium fluorides UF6-n(OCH3)(n): A theoretical study of equilibrium geometries, vibrational frequencies, and the role of the f orbitals

The title compounds, the uranium (VI) fluoride chlorides (UF6-nCln, n = 0-6 ) and methoxyuranium (VI) fluorides [UF6-n(OCH3)(n), n = 0-5], have been st udied using relativistic density functional theory. Applying the B3LYP hybr id functional and an effective core potential on uranium, equilibrium geome tries have been calculated for these molecules. In addition, harmonic vibra tional frequencies have been computed for the chloride fluorides. Calculate d frequencies have been compared to experiment where possible. All experime ntally observed bands have been assigned, based on these calculations. The average deviation between theoretical and experimental frequencies is 15.6 cm(-1) for 23 experimental modes. Theory always underestimates the experime ntal frequencies. This can be explained by the calculated bond lengths that are somewhat too long. The electronic structure of the uranium (VI) chlori de fluorides has been investigated using scalar relativistic calculations a nd the PW91 functional. Periodic trends in the role and bonding contributio n of the uranium 5f orbitals are discussed.