Polychlorinated biphenyl radical cations: a pulse radiolysis investigation

Abstract

Pulse irradiation of polychlorinated biphenyls (PCBs) in 1,2-dichloroethane leads to the formation of transient radical cations PCB^+˙. The latter exhibit optical absorption spectra with maxima in the visible range (370–450 nm) and in the i.r. (at 800 nm). Investigations have been carried out with 3-chloro-, 4-chloro-, 3,4-dichloro-, 3,5-dichloro-, 4,4′-dichloro-, 3,3′,4,4′-tetrachloro-, 2,2′,5,5′-tetrachloro-, 2,3,4,4′5-pentachloro-, and 2,2′,4,4′,5,5′-hexachloro-biphenyl. Two types of radical cations have been identified through their different optical and kinetic characteristics. Their structures are considered to be either planar or twisted with respect to the two aromatic rings. The relative yields are inversely related to the number of chlorine atoms ortho to the pivot bond, i.e. the 2-, 2′-, 6-, and 6′-positions. The PCB^+˙ radical cations have oxidizing properties and react readily with, for example, promethazine (PMZ) in the general reaction PCB^+˙+ PMZ → PCB + PMZ^+˙ with bimolecular rate constants of the order of 10⁹–10¹⁰ mol^–1 dm³s^–1. On the basis of these reactions it was possible to calculate the yield of PCB^+˙(which depends on the PCB concentration) and to estimate lower limits for the extinction coefficients of the PCB^+˙ radical cations.