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  • 1.
    Privalov, Timofei
    et al.
    KTH, Superseded Departments, Chemistry.
    Macak, Peter
    KTH, Superseded Departments, Physics.
    Schimmelpfennig, B.
    Fromager, E.
    Grenthe, I.
    Wahlgren, Ulf
    KTH, Superseded Departments, Physics.
    Electron transfer in uranyl(VI)-uranyl(V) complexes in solution2004In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 126, no 31, p. 9801-9808Article in journal (Refereed)
    Abstract [en]

    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 UO2+(aq) + UO22+(aq)UO22+(aq) + UO2+(aq), at 25 degreesC, 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 M3+ and M4+ actinide species that have eight or nine water ligands in the first coordination sphere.

  • 2. Real, Florent
    et al.
    Vallet, Valerie
    Wahlgren, Ulf
    Grenthe, Ingmar
    KTH, School of Chemical Science and Engineering (CHE), Chemistry, Inorganic Chemistry.
    Ab initio study of the mechanism for photoinduced yl-oxygen exchange in uranyl(VI) in acidic aqueous solution2008In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 130, no 35, p. 11742-11751Article in journal (Refereed)
    Abstract [en]

    The mechanism for the photochemically induced isotope-exchange reaction (UO22+)-O-17/18(aq) + (H2O)-O-16 reversible arrow (UO22+)-O-16 (aq) + (H2O)-O-17/18 has been studied using quantum-chemical methods. There is a dense manifold of states between 22 000 and 54 000 cm(-1) that results from excitations from the sigma(u) and pi(u) bonding orbitals in the (1)Sigma(+)(g) ground state to the nonbonding f(delta) and f(phi) orbitals localized on uranium. On the basis of investigations of the reaction profile in the (1)Sigma(+)(g) ground state and the excited states (3)Delta(g) (the lowest triplet state) and (3)Gamma(g) (one of the several higher triplet states), the latter two of which have the electron configurations sigma(u)f(delta) and pi(u)f(phi) respectively, we suggest that the isotope exchange takes place in one of the higher triplet states, of which the (3)Gamma(g) state was used as a representative. The geometries of the luminescent (3)Delta(g) state, the lowest in the sigma(u)f(delta,phi) manifold (the "sigma" states), and the (1)Sigma(+)(g) ground state are very similar, except that the bond distances are slightly longer in the former. This is presumably a result of transfer of a bonding electron to a nonbonding f orbital, which makes the excited state in some respects similar to uranyl(V). As is the case for all of the states of the pi(u)f(delta,phi) manifold (the "pi" states), the geometry of the (3)Gamma(g) state is very different from that of the (3)Delta(g) "sigma" state and has nonequivalent U-O-yl distances of 1.982 and 1.763 angstrom; in the (3)Gamma(g) state, the yl-exchange takes place by transfer of a proton or hydrogen from water to the more distant yl-oxygen. The activation barriers for proton/hydrogen transfer in the ground state and the (3)Delta(g) and (3)Gamma(g) states are 186, 219, and 84 kJ/mol, respectively. The relaxation energy for the (3)Gamma(g) state in the solvent after photoexcitation is -86 kJ/mol, indicating that the energy barrier can be overcome; the "pi" states are therefore the most probable route for proton/hydrogen transfer. They can be populated after UV irradiation but are too high in energy (similar to 36000-40000 cm(-1)) to be reached by a single-photon absorption at 436 nm (22 900 cm(-1)), where experimental data have demonstrated that exchange can take place. Okuyama et al. [Bull. Res. Lab. Nucl. React. (Tokyo Inst. Technol.) 1978, 3, 39-50] have demonstrated that an intermediate is formed when an acidic solution of UO22+ (aq) is flash-photolyzed in the UV range. The absorption spectrum of this short-lived intermediate (which has a maximum at 560 nm) indicates that this species arises from 436 nm excitation of the luminescent (3)Delta(g) state (which has a lifetime of similar to 2 x 10(-6) s); this is sufficient to reach the reactive "pi" states. It has been speculated that the primary reaction in acidic solutions of UO22+(aq) is the formation of a uranyl(V) species; our resuls indicate that the structure in the luminescent state has some similarity to that of UO2+ but that the reactive species in the states is a cation radical with a distinctly different structure.

  • 3.
    Real, Florent
    et al.
    Univ Sci & Techbol Lille, Lab Phys Lasers Atomes & Mol, CNRS, UMR 8523,UFR Phys, F-59655 Villeneuve Dascq, France..
    Vallet, Valerie
    Univ Sci & Technol Lille, Lab Phys Lasers Atomes & Mol, CNRS, UMR 8523, F-59655 Villeneuve Dascq, France..
    Wahlgren, Ulf
    KTH.
    Grenthe, Ingmar
    KTH.
    Ab initio study of the photoinduced proton transfert on the uranyl in aqueous solution2007In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 234Article in journal (Other academic)
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