Hydrogen/deuterium isotope effects on femtosecond electron reactivity in aqueous media

Abstract

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 H{sub 2}O and D{sub 2}O, this localized state (e{sub prehyd}{sup {minus}}) which absorbs in the infrared is distinct from the fully hydrated state (e{sub hyd}{sup {minus}}). In neat D{sub 2}O the dynamics of electron localization is slightly slower (9%) than in H{sub 2}O; 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{sub hyd}{sup {minus}} + X{sub 3}O{sub +}, e{sub hyd}{sup {minus}} + OX with X = H or D). The percentage of hydrated electrons involved in the fast recombination is increased in D{sub 2}O. This moderate H/D isotope effect can be linked tomore » 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{sub hyd}{sup {minus}}{hor ellipsis}X{sub 3}O{sup +}, e{sub hyd}{sup {minus}}{hor ellipsis}OX) execute a one-dimensional (1D) walk before undergoing recombination.« less