Evidence of Oxidation of Aromatic Hydrocarbons by Chloromethyl Radicals: Reinvestigation of Intersolute Hole Transfer Using Pulse Radiolysis

Abstract

Intersolute hole transfer (A•+ + B → A + B•+) was reinvestigated using nanosecond pulse radiolysis in fluid solutions of dichloromethane (A, biphenyl; B, naphthacene, rubrene). Under our new experimental conditions where the concentration of the second solute [B] was kept higher than 0.5 mM, the kinetics of the hole transfer was observed within 500 ns after irradiation, at which time the geminate recombination process with counterpart anionic species was also observed. On the basis of the analysis with the Smoluchowski equation, the second-order rate constants of hole transfers were successfully obtained. Moreover, we confirmed the existence of another slow oxidation of the second solute B by the solvent neutral radical CH2Cl• formed from the electrons ejected during the initial ionizing process. This reaction is a kind of electron transfer with a charge separation that has never been confirmed, but should generally exist in an irradiated solution of halogenated compounds. In our present case, more than 30% of B•+'s were formed through this side reaction. We clarify the formation of two ion pairs on one ionizing event, which will have a great impact on the understanding of the results of product analyses in radiation chemistry. Examination using ab initio molecular orbital calculation of two dissociative electron transfers is also presented.