Diffusion Process of the Benzyl Radical Created by Photodissociation Probed by the Transient Grating Method

ion reactions of ketones, quinones, and N-hetero aromatic molecules from alcoholic solvents are 2-3 times smaller than those of the parent molecules, even though the radicals and parent molecules possess nearly the same sizes and the same shapes.17,18 Such an anomalously slow diffusion of the radicals suggests a strong intermolecular interaction between the radicals and the surrounding molecules. Extended researches have been reported, such as the solvent dependence,19 the solute size dependence,20 the temperature dependence,21 the effect of a charge in a radical,22 the effect of chemical stabilities of radicals,23 and transient Raman studies for the intermolecular interaction.24 The origin of such a strong molecular interaction of the radicals is still unclear. However, the difference between the radicals and the parent molecules is only the unpaired electron. That electron should lead to the anomalously slow diffusion process of the radicals. We must examine the role of the unpaired electron in affecting the diffusion process in solution. So far we have mostly studied the diffusion of the radicals created by photoinduced hydrogen abstraction reactions.17-21 In this study, we measure D of the benzyl radical (BR), which is created by the photodissociation reaction from dibenzyl ketone (DBK). We have two aims in this study. First, since BR frequently appears in chemical reactions as an intermediate radical, it would be interesting and important to know the diffusion constant of BR for the analysis of the chemical reaction. Second, D of BR is compared with those of other transient radicals created by the hydrogen abstraction reactions to see if there is a noticeable difference in D. If there is a X Abstract published in AdVance ACS Abstracts, July 1, 1997. DSE ) kBT/6πrAη (1) 5269 J. Phys. Chem. A 1997, 101, 5269-5277 S1089-5639(97)00911-0 CCC: $14.00 © 1997 American Chemical Society difference, a detailed comparison of the molecular character could provide an insight into the mechanism of the slow diffusion of many transient radicals. The photodissociation process of DBK has been studied extensively in various solvents.25-35 The reaction scheme is shown in Scheme 1. The lowest excited triplet (T1) state of DBK is created by the intersystem crossing from the lowest excited singlet (S1) state within a picosecond time scale after the UV irradiation (process a-c). The R cleavage of the C-CO bond (Norish type 1) occurs from the excited triplet state of DBK and brings BR and the phenylacetyl radical within a few nanoseconds (process d).25 Successively, carbon monoxide (CO) is separated from the phenylacetyl radical in a few hundred nanoseconds at room temperature in the solution phase and another BR is produced (process e).26 The quantum yield of the photodissociation of DBK has been reported to be ∼0.7.27 BR is known as a relatively stable radical because the unpaired electron of BR is delocalized into the phenyl ring.13 The recombination reaction of two BR to bibenzyl is a dominant subsequent reaction compared with a reaction between BR and the solvent molecules (process f). It has been reported that the reaction process of the self-termination of BR is a pseudodiffusion-controlled reaction and the steric factor of this reaction is 0.8.28 The rate constant (2k2) of such a second ordered reaction has been measured in various solvents and it is reported to be ∼109 M-1 s-1 as discussed in a later section.29-35