ELECTRON-TRANSFER FROM HALIDE-IONS TO UO2-2+ EXCITED-STATE IONS IN AQUEOUS-SOLUTION - FORMATION AND DECAY OF DIHALIDE RADICAL-ANIONS

The decay of the excited uranyl ion in water in the presence of Cl-, Br-, I-, and SCN-is studied by laser flash kinetic spectrophotometry. The four anions all quench *U022+by 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•-, are observed. However, kinetic studies indicate that these are not formed directly from *UO22+but probably come by bimolecular reaction of an intermediate uranium(V)/radical pair with the corresponding halide ion. The extinction coefficient for the *UO2 2+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. © 1990, American Chemical Society. All rights reserved.