Actinide redox couples

The most obvious future data needs concern the missing, uncertain, and conflicting data identified above. Additional experimental investigations are needed in the case of Fe(III) and Zr(IV) carbonate complexation, and in the case of the Sn(IV)/Sn(II) and the Se(0)/Se(-II) redox couples. The molecular structure of metal silicate complexes needs clarification in order to remove ambiguities in the speciation scheme of these complexes. A rather challenging topic concerns the supposed transformation of crystalline tetra-valent actinide oxides, AnOz(cr), to solids with an amorphous surface layer as soon as the An4+ ion hydrolyses. The consequences of such... 

Oxidation state. Differences among the potentials of the redox couples of the actinides account for much of the differences in their speciation and environmental transport. Detailed information about the redox potentials for these couples can be found in numerous references (e.g., Hobart, 1990 Silva and Nitsche, 1995 Runde, 2002). This information is not repeated here, but a few general points should be made. Important oxidation states for the actinides under environmental conditions are described in Table 4. Depending on the actinide, the potentials of the III/IV, IV/V, V/VI, and/or IV/VI redox couples can be important under near-surface environmental conditions. When the redox potentials between oxidation states are sufficiently different, then one or two redox states will predominate this is the case for uranium, neptunium, and americium (Runde, 2002). The behavior of uranium is controlled by the predominance of U(VI) species under... 

Steele H, GuiUaumont D, Moisy P. Density Functional Theory Calculations of the Redox Potentials of Actinide(VI)/Actinide( V) Couple in Water. J Phys Chem A. 2013 117 4500-4505. 

The incorporation of radicals is a highly promising way to construct larger magnetically coupled clusters in 4f chemistry [44, 64], but this is yet to happen for the actinides [43]. A promising way forward seems to be the use of redox-active ligands [65]. 

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

As presently under discussion, the SISAK system coupled to a recoil separator [4] (see Sect 2.2.3. and Experimental Techniques ) may provide an alternative approach for continuously separating and detecting two oxidation states in Sg. The flow electrolytic column chromatography developed by Toyoshima et al. [117], which was successfully applied in on-line redox experiments of the heaviest actinides [118], may be adaptable to SISAK and may provide an interesting alternative approach for an electrochemical reduction of Sg. 

Reduction potentials for the actinide elements are shown in Fig. 14.6. These show formal potentials, defined as the measured potentials relative to the hydrogen ion/hydrogen couple taken as zero volts. The redox potentials are corrected to unit concentration of the reactants, but are not corrected for activity. The measured potentials were determined by electrochemical cells, equilibria, and enthalpy of reaction measurements. The potentials for add solution were... 

Uranium (III) redox reactivity with small molecules always consists in the preliminary double reduction of the latter by two U(III) complexes. Thus, the reactivity concerns only U(IV) bimetallic complexes. DFT calculations can be used for this problem thanks to the recent development of 5f-in-core ECPs by Moritz et al. and a methodology to compute the preliminary redox step via the combination of small core and 5f-in-core calculations. Recent theoretical studies have mainly concerned the reduction of CO2 and CO, but attention has also to be paid to the reduction of CS2, COS, PhNCO, and PhNs, the C-C coupling of terminal bis-alkynes to form U(IV) vinyl complexes and the reduction of arenes. However, in order to have more elements to discuss about actinide properties, one must perform calculations at a multireference post Hartree-Fock level to take into account the important effect of electronic correlation in these systems however, there is the obstacle of the computational power to perform calculations of real systems at this level that stands still. 

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