Journal of Nuclear Science and Technology, Vol. 37, No. 4, 2000: Pp. 344–348

The structural and kinetic studies of U(VI) complex with benzamidoxime (Hba) as ligand in CD₃COCD₃ have been studied by means of ¹H and¹³ C NMR. The Hba molecule was found to coordinate to UO²⁺₂ in the form of anionic benzamidoximate (ba), and the number of ba coordinated to UO²⁺₂ was determined to be 3 by analyzing the chemical shift of ¹³C NMR signal for Hba in the presence of UO²⁺₂. The exchangerate constants (k_ex) of ba in [UO₂(ba)₃]⁻ were determinedby the NMR line-broadening method. The kinetic parameterswere obtained as follows: k_ex(25°C)=1.3×10³ s^-1, ΔH^≠ =35.8±3.5kJ·mol^-1, and ΔS^≠=-65±13.7 J·K^-1·mol^-1. The UV-visible absorption spectra of solutionscontaining UO²⁺₂ and Hba were also measured. The molar extinction coefficient of the complex wasfound to be extremely large compared with those of UO₂(L)²⁺₅ (L=unidentate oxygen donor ligands) complexes. This is due to the strong electron withdrawing of UO²⁺₂ from Hba and suggests that an interaction between UO²⁺₂ and Hba is very strong. Such a high affinity of monomericamidoxime to UO²⁺₂ reasonably explains the high adsorptivity of amidoxime resin to U(VI) species, and is considered to result in the high recovery of U(VI) species from sea water using amidoxime resin.