Ionic intermediates produced by gamma irradiation at 4 K: Halogenated naphthalene and benzene derivatives in ethanol

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Abstract
Optical absorption and ESR measurements have been performed for the irradiated halogenated benzene and naphthalene derivatives in ethanol at 4 K. The anions of both bromo− and chloronaphthalene derivatives were observed under steady state at 4 K, while the former anion at 77 K results in dissociation and the latter anion results in protonation. Both anions of benzene and fluorobenzene were stable at 4 K, whereas not at 77 K, indicating that protonation is inhibited at 4 K. Irradiated iodobenzene and bromobenzene solutions at 4 K showed a doublet band whose energy separation was 0.8−0.9 and 0.41−0.48 eV, which are close to the energy separation between 2P3/2 and 2P1/2 of I and Br, respectively. The transition energies of the long wavelength bands of these compounds and of chlorobenzene is linear with the ionization potentials of respective halide ions with a slope of close to unity. Therefore these bands may be assigned to an electron transfer band from halide ion to its counterpart benzene ring. As the electron acceptor, two models were discussed, one of which is the orbital of neutral benzene, and the other is the orbital of the phenyl radical. In solid phase, dissociative electron attachment to halogenated benzene goes through a state of metastable CT complex of a halide ion and its counterpart molecule. The irradiated p−C6H4BrI showed also a well resolved doublet band, suggesting that C−I bond rupture is more dominant that C−Br bond rupture. The absorption spectrum of irradiated p−C6H4BrCl in ethanol did not resemble either the irradiated chlorobenzene or bromobenzene derivatives, suggesting that both C−Br and C−Cl are ruptured at the same time.

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