Translational diffusion of transient radicals created by the photoinduced hydrogen abstraction reaction in solution: Anomalous size dependence in the radical diffusion

Diffusion coefficients (D) of various radicals created by the photoinduced hydrogen abstraction reactions from alcohols (ethanol and 2-propanol) are investigated by using the transient grating (TG) method. In all the reaction systems, D’s of the transient radicals, as well as those of the parent molecules, can be measured simultaneously. The results clearly show slower diffusive motions of the radicals, at least of the π radicals in the hydrogen abstraction reaction systems, compared with those of the parent molecules. D’s of the parent molecules usually agree well with the calculated values based on the Spernol and Wirtz modification of the Stokes–Einstein (SE) relation. Although the measured D’s of the radicals are closer to the values predicted by the simple SE equation, the agreements and the ratio of D between the radicals and its parent molecules depend on the molecular size. The ratio becomes closer to unity as the molecular size becomes large. Possible origins of this dependence are discussed.