Electron transfer in neptunyl(VI)-neptunyl(V) complexes in solution

Area of Science:

• Inorganic Chemistry
• Quantum Chemistry
• Solution Chemistry

Background:

• Electron self-exchange reactions are fundamental in redox processes.
• Understanding Np(V)-Np(VI) and U(V)-U(VI) redox behavior is crucial for nuclear fuel cycles and waste management.

Purpose of the Study:

• To investigate the rates and mechanisms of electron self-exchange between Np(V) and Np(VI) in aqueous solution.
• To compare these findings with the analogous Uranium(V)-Uranium(VI) system.
• To elucidate the roles of outer-sphere and inner-sphere mechanisms in these redox reactions.

Main Methods:

• Quantum chemical calculations were employed to study electron transfer.
• Outer-sphere mechanisms were analyzed for aqua ions.
• Inner-sphere mechanisms were investigated for binuclear complexes with various bridging ligands (hydroxide, fluoride, carbonate).
• Solvent effects were modeled using the Marcus equation and a nonequilibrium PCM method.

Main Results:

• Calculated rate constant for Np(V)-Np(VI) self-exchange (k = 67 M⁻¹ s⁻¹ at 25°C) shows fair agreement with experimental values (0.0063–15 M⁻¹ s⁻¹).
• The nonequilibrium PCM method appeared to overestimate solvent effects for outer-sphere reactions.
• Differences between Np(V)-Np(VI) and U(V)-U(VI) electron self-exchange pairs are minimal.

Conclusions:

• Quantum chemical methods provide valuable insights into Np(V)-Np(VI) electron transfer mechanisms.
• Both outer-sphere and inner-sphere pathways contribute to the self-exchange reaction.
• The study highlights the similarities in redox behavior between Neptunium and Uranium in these oxidation states.