Oxygen exchange in uranyl hydroxide via two "nonclassical" ions

Abstract

A recently proposed pathway for the scrambling of axial (uranyl) and equatorial 0 atoms in [UO2(OH)4]2- (1) is refined using Car-Parrinello molecular dynamics (CPMD) simulations in an explicit solvent (water) and with model counterions (NH4+). According to constrained CPMD/BLYP simulations and thermodynamic integration, 1 can be deprotonated to [UO3(OH)3]3- with a T-shaped UO3 group (Delta A = 7.1 kcal/mol), which in turn can undergo a solvent-assisted proton transfer via a cis-[UO2(OH)4]2-center dot OH- complex and a total overall barrier of Delta A(double dagger) = 12.5 kcal/mol. According to computed relative energies of trans- and cis-[UO2(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 Delta A(double dagger) approximate to 16 and 17 kcal/mol are obtained at the B3LYP and CCSD(T) levels, respectively, in excellent agreement with the experiment.