Ab initio study of the mechanism for photoinduced Yl-oxygen exchange in uranyl(VI) in acidic aqueous solution.
The mechanism for the photochemically induced isotope-exchange reaction U(17/18)O2(2+)(aq) + H2(16)O <==> U(16)O2(2+)(aq) + H2(17/18)O 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 A; 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 (approximately 36,000-40,000 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 UO2(2+)(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 approximately 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 UO2(2+)(aq) is the formation of a uranyl(V) species; our results indicate that the structure in the luminescent state has some similarity to that of UO2(+) but that the reactive species in the "pi" states is a cation radical with a distinctly different structure.
Réal
Florent
F
Department of Physics, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden.
Vallet
Valérie
V
Wahlgren
Ulf
U
Grenthe
Ingmar
I