Theoretical Study of Stable Trans and Cis Isomers in [UO₂(OH)₄]^2- Using Relativistic Density Functional Theory

The title compound, uranyl(VI) tetrahydroxide [UO₂(OH)₄]^2-, has been studied in detail using density functional theory (DFT) in the first systematic theoretical study of the compound. Scalar relativistic effects are included approximately by replacing the uranium core with a relativistic effective core potential. A total of nine stable structures have been characterized. Four of them (I−IV) possess the usual linear uranyl bond, and rapid exchange between these conformations is expected at finite temperatures. The uranyl and U−OH bond lengths of the minimum energy structure, I, are calculated as 1.842 and 2.334 Å, respectively. This compares well with the experimental crystal structure values of 1.824(3) Å and 2.258(3) Å, respectively. The existence of stable structures with a bent uranyl bond (“cis-uranyl”) is predicted for the first time (structures V−IX). These conformers are only 18−19 kcal/mol higher in energy than the global energy minimum, and their uranyl bond angles cover a range of 113−132°. Harmonic vibrational frequencies for all stable conformers, I−IX, were calculated. They are compared to experiment where possible. A mechanism is suggested for the nonaqueous intramolecular oxygen ligand exchange in [UO₂(OH)₄]^2- between uranyl and hydroxide involving a “cis-uranyl” structure as a stable intermediate in a two-step process with a calculated activation energy of 38 kcal/mol.