H/D ISOTOPE EFFECTS ON FEMTOSECOND ELECTRON REACTIVITY IN AQUEOUS-MEDIA

The effects of isotope substitution on the primary steps of electron reactivity in aqueous media have been investigated by using femtosecond near-infrared and visible spectroscopy. In neat deuterated water, a precursor of the hydrated electron has been identified. For both H2O and D2O, this localized state (e(prehyd)-) which absorbs in the infrared is distinct from the fully hydrated state (e(hyd-)). In neat D2O the dynamics of electron localization is slightly slower (9%) than in H2O; however, the lifetime of this transient electronic state (250 fs) remains similar to the analogue in light water. The absence of an H/D isotope effect on the electron hydration dynamics is confirmed in ionic aqueous media and in organized assemblies by using an anionic species (chloride ion) and a chromophore (phenothiazine), respectively, as the electron donor. In pure aqueous solutions, the changes due to isotope substitution are mainly observed during the early electron-radical pair recombination (e(hyd)- + X3O+, e(hyd)- + OX with X = H or D). The percentage of hydrated electrons involved in the fast recombination is increased in D2O. This moderate H/D isotope effect can be linked to a change in the initial spatial distribution of the electron and prototropic radicals. The analysis of the femtosecond kinetics provides evidence that the primary electron-radical pairs (e(hyd)-...X3O+, e(hyd)-...OX) execute a one-dimensional (1D) walk before undergoing recombination. The jump rate of the neutralization processes is found to be significantly influenced by H/D isotope substitution (0.45 x 10(12) s-1 in D2O and 0.83 x 10(12) s-1 in H2O). Moreover, the recombination dynamics in H2O and D2O, which have a time scale comparable to the H/D bond lifetime, suggest the existence of a dynamical protic solvent reorganization in the vicinity of hydrated electron during the electron-radical pair's neutralization.