Electron Transfer in Uranyl(Ⅵ)-Uranyl(Ⅴ) Complexes in Solution

摘要

The rates and mechanisms of the electron self-exchange between U(Ⅴ) and U(Ⅵ) in solution have been studied with quantum chemical methods. Both outer-sphere and inner-sphere mechanisms have been investigated; the former for the aqua ions, the latter for binuclear complexes containing hydroxide, fluoride, and carbonate as bridging ligand. The calculated rate constant for the self-exchange reaction UO_2~+(aq) + UO_2~(2+)(aq) <=> UO_2~(2+)(aq) + UO_2~+(aq), at 25 ℃, is k = 26 M~(-1) s~(-1). The lower limit of the rate of electron transfer in the inner-sphere complexes is estimated to be in the range 2 x 10~4 to 4 x 10~6 M~(-1) s~(-1), indicating that the rate for the overall exchange reaction may be determined by the rate of formation and dissociation of the binuclear complex. The activation energy for the outer-sphere model calculated from the Marcus model is nearly the same as that obtained by a direct calculation of the precursor- and transition-state energy. A simple model with one water ligand is shown to recover 60% of the reorganization energy. This finding is important because it indicates the possibility to carry out theoretical studies of electron-transfer reactions involving M~(3+) and W~(4+) actinide species that have eight or nine water ligands in the first coordination sphere.