Ligand effect on uranium isotope fractionations caused by nuclear volume effects: An ab initio relativistic molecular orbital study.

We have calculated the nuclear volume term (ln K(nv)) of the isotope fractionation coefficient (epsilon) between (235)U-(238)U isotope pairs by considering the effect of ligand coordination in a U(IV)-U(VI) reaction system. The reactants were modeled as [UO(2)Cl(3)](-) and [UO(2)Cl(4)](2-) for U(VI), and UCl(4) for U(IV). We adopted the Dirac-Coulomb Hartree-Fock method with the Gaussian-type finite nucleus model. The result obtained was ln K(nv)=0.001 90 at 308 K, while the experimentally estimated value of ln K(nv) is 0.002 24. We also discuss how the ligand affects the value of ln K(nv), especially for the various structures of different compounds, and different ligands within the halogen ion series (F, Cl, and Br).