Diffusion processes of intermediate radicals created by the photoinduced hydrogen abstraction reactions of benzoquinone (BQ) and acetophenone (AP) in ethanol-water mixed solvents are studied by using the transient grating (TG) method. The electrically neutral radicals and the anion radicals are created selectively by adding sodium hydroxide (NaOH) or sulfuric acid (H2SO4) to the solutions. The intermediate species and the chemical stability of the radicals were monitored by the transient absorption and time-resolved EPR methods. The diffusion constants (D) of the radicals and the parent molecules are determined simultaneously. D of the anion radicals are similar to those of the neutral radicals in any mixtures of the solvents. D of the neutral and anion radicals (DR) are much smaller than those of the parent molecule (DP) in ethanol (DP/DR ∼ 2.8 for BQ and DP/DR ∼ 2.1 for AP in ethanol), while DP become closer to DR with increasing water content in the solvent (DP/DR ∼ 1.3 for BQ and QP in 90% water solution). The temperature dependence of D was also investigated. A possible origin of this water effect is discussed from the point of view of hydrophobic hydration.
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