A theoretical study of the qualitative reaction mechanism for the homogeneous disproportionation of pentavalent uranyl ions

There is significant interest in understanding the oxidation and reduction of aqueous uranium ions. In the literature, there is considerable variation in the results of attempts to reduce uranium depending on very small differences in reducing species, pH, spectator ions, mineral substrates, and other factors. Of particular curiosity is the fact that U(V) can be stabilized in the lab but is almost never found in the environment. In order to better understand these two phenomena, a direct computational study of homogenous, aqueous U(V) disproportionation was undertaken. Using a combination of Marcus Theory with Hartree–Fock and MP2 calculations, the rate constant of the electron-transfer reaction was calculated. Under the conditions studied, the electron-transfer reaction is slow (kobserved ∼ 10−1 M−1 s−1). This reduced rate is a result of a variety of factors: the +1 charge of the reactants, the large encounter complex distance, and the electronic reorganization energies associated with the proton transfers. Excluding the energy from the coupled proton transfers, all of the remaining factors could be eliminated by small alterations of the uranium’s environment.