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 be en studied in detail using density functional theory (DFT) in the firs t systematic theoretical study of the compound. Scalar relativistic ef fects are included approximately by replacing the uranium core with a relativistic effective core potential. A total of nine stable structur es have been characterized. Four of them (I-IV) possess the usual line ar uranyl bond, and rapid exchange between these conformations is expe cted at finite temperatures. The uranyl and U-OH bond lengths of the m inimum energy structure, I, are calculated as 1.842 and 2.334 Angstrom , respectively. This compares well with the experimental crystal struc ture values of 1.824(3) Angstrom and 2.258(3) Angstrom, respectively. The existence of stable structures with a bent uranyl bond (''cis-uran yl'') is predicted for the first time (structures V-IX). These conform ers are only 18-19 kcal/mol higher in energy than the global energy mi nimum, and their uranyl bond angles cover a range of 113-132 degrees. Harmonic vibrational frequencies for all stable conformers, I-IX, were calculated. They are compared to experiment where possible. a mechani sm is suggested for the nonaqueous intramolecular oxygen ligand exchan ge 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.