Diffusion process of the benzyl radical created by photodissociation probed by the transient grating method

Translational diffusion processes of the benzyl radical (BR) created by the photodissociation from dibenzyl ketone (DBK) were studied by using the transient grating (TG) method in organic solvents (hexane, cyclohexane, ethanol, and 2-propanol). The values of the diffusion constants (D) of BR, DBK, and carbon monoxide (CO) and the rates for the self-termination reaction of BR in these solvents are accurately determined from the TG signals. The ratios of D of BR to that of DBK are independent of the solvent, and they are close to 1.25, which is expected from the ratio of the molecular volume of BR to that of DBK. This result is contrary to those of radicals created by the photoinduced hydrogen abstraction reaction, which show anomalously slow diffusions. A possible origin of this difference is discussed by comparison of the spin densities, dipole moments, and polarizabilities of BR and the radicals produced by hydrogen abstraction.