The geometric structures, vibrational frequencies and redox properties of theactinyl coordination complexes ([An0_2(L)_n]~m; An = U, Pu, Np; L = H_2O, CO_3~(2-), CH_3CO_2-,OH~-) in aqueous solution, studied by densityfunctional theory methods

摘要

The geometric and electronic structures of the aqua, chloro, acetato, hydroxo and carbonato complexesof U, Np and Pu in both their (VI) and (V) oxidation states, and in an aqueous environment, have beenstudied using density functional theory methods. We have obtained micro-solvated structures derivedfrom molecular dynamics simulations and included the bulk solvent using a continuum model. We findthat two different hydrogen bonding patterns involving the axial actinyl oxygen atoms are sometimespossible, and may give rise to different An-0 bond lengths and vibrational frequencies. Thesealternative structures are reflected in the experimental An-0 bond lengths of the aqua and carbonatocomplexes. The variation of the redox potential of the uranyl complexes with the different ligands hasbeen studied using both BP86 and B3LYP functionals. The relative values for the four uraniumcomplexes having anionic ligands are in surprisingly good agreement with experiment, although theabsolute values are in error by –1 eV. The absolute error for the aqua species is much less, leading to anincorrect order of the redox potentials of the aqua and chloro species.