Electron transfer from halide ios to uranyl(2+) excited-state ions in aqueous solution: formation and decay of dihalide radical anions

Abstract

The decay of the excited uranyl ion in water in the presence of CI-, Br-, I-, and SCN- is studied by laser flash kinetic spectrophotometry. The four anions all quench *U022b+y a bimolecular process suggested to involve electron or charge transfer. Steady-state luminescence studies suggest that there is also some static contribution to the quenching. For Br-, I-, and SCN-, radical anions, X2*-a,r e observed. However, kinetic studies indicate that these are not formed directly from *UO:+ but probably come by bimolecular reaction of an intermediate uranium(V)/radical pair with the corresponding halide ion. The extinction coefficient for the *UO?+ absorption is reported, and by use of this and transient absorbance data radical-anion yields are obtained and found to depend linearly upon the halide anion concentration. The variation of the yield in the series I-, SCN-, and Br- is interpreted in terms of the effect of overall free energy change on back-electron-transfer in the uranium(V)/radical pair. With the chloride system no significant yield of radical anion was observed. The decay of X2’- was found to be unaffected by the presence of uranyl ion, supporting the previously suggested mechanism for radical-anion decay in these systems.