Increased yields of radical cations by arene addition to irradiated 1,2-dichloroethane

Pulse radiolysis in chlorinated hydrocarbon liquids such as 1,2-dichloroethane is a versatile and effective method for the generation of solute radical cations. The addition of a large concentration of toluene or benzene to solutions of 1,2-dichloroethane is found to increase the yield of solute radical cations (G = 0.68 molecules 100 eV(-1) in 1,2-dichloroethane (J. Phys. Chem. 83(15) (1979) 1944) by a factor of 2.5. The increased yield is found for solutes which have a potential of similar to 1.1 V (vs. SCE) or below for the S+./S couple and is due to reaction of the chlorine atom:toluene (pi-Cl-.) complex with the solute. A similar species forms with benzene. pi-Cl-. is formed with a yield of G = 3.0, and arises principally as a result of geminate recombination of ions. It has an absorption in the visible with lambda(max) 460 nm, epsilon(max) = 1800 M-1 cm(-1) and decays with an observed first-order rate constant k = 1.12 x 10(6) s(-1). The rate of reaction of the pi-Cl-. with added solutes ranges from 2.5 to 5 x 10(9) M-1 s(-1). The other oxidant present in the 1,2-dichloroethane/toluene solutions is identified as the toluene cation dimer. This is formed from the 1,2-dichloroethane radical cation with bimolecular rate constant k = 1.35 x 10(10) M-1 s(-1) with a radiation chemical yield G = 0.5. The rate of reaction of this species with the added solutes is diffusion controlled, k = 1-2 x 10(10) M-1 s(-1). (C) 2004 Elsevier Ltd. All rights reserved.