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 n v ) of the isotope fractionation coefficient ( ε ) between U 235 - U 238 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 n v = 0.001 90 at 308 K, while the experimentally estimated value of ln K n v is 0.00224. We also discuss how the ligand affects the value of ln K n v , especially for the various structures of different compounds, and different ligands within the halogen ion series (F, Cl, and Br).