Oxygen Exchange in Uranyl Hydroxide via Two “Nonclassical” Ions

A recently proposed pathway for the scrambling of axial (uranyl) and equatorial O atoms in [UO(2)(OH)(4)](2-) (1) is refined using Car-Parrinello molecular dynamics (CPMD) simulations in an explicit solvent (water) and with model counterions (NH(4)(+)). According to constrained CPMD/BLYP simulations and thermodynamic integration, 1 can be deprotonated to [UO(3)(OH)(3)](3-) with a T-shaped UO(3) group (DeltaA = 7.1 kcal/mol), which in turn can undergo a solvent-assisted proton transfer via a cis-[UO(2)(OH)(4)](2-).OH(-) complex and a total overall barrier of DeltaA(double dagger) = 12.5 kcal/mol. According to computed relative energies of trans- and cis-[UO(2)(OH)(4)](2-) in the gas phase and in a polarizable continuum, "pure" functionals such as BLYP underestimate this overall barrier somewhat, and estimates of DeltaA(double dagger) approximately 16 and 17 kcal/mol are obtained at the B3LYP and CCSD(T) levels, respectively, in excellent agreement with the experiment.