Unimolecular dissociation of benzyl halide radical anions studied by low- temperature pulse radiolysis

The one-electron reduction of benzyl halides (BzX; X=F, Cl, Br) by solvated electrons in 2-methyltetrahydrofuran has been investigated by using a pulse radiolysis technique at different temperatures above 77 K. The radical anions of BzX (λ(max)=300 nm) formed by the capture of an electron in the π*-orbital of the benzene ring dissociate into Bz· (λ(max)=320 nm) and X^- via Bz·/X^- complexes (λ(max)=340 nm for BzCl and 370 nm for BzBr). The corresponding species were observed with 4-phenylbenzyl halide at 85 K and below. A spectral change observed at 95 K has been attributed to a structural change of the biphenylyl group. The dissociation rate of the radical anions increased in the order BzF<BzCl<BzBr. On the basis of these results, the reaction mechanism is discussed.