Radical cations of halogenobenzenes. An electron spin resonance study

Abstract

Exposure of dilute solutions of a range of halogenobenzene derivatives in fluorotrichloromethane to ⁶⁰Co γ-rays at 77 K gave the corresponding radical cations, characterized by their e.s.r. spectra. The approximate π spin densities on halogen estimated therefrom increased from ca. 8% for PhF⁺ to 23% for PhCl⁺, 30% for PhBr⁺ and 46% for PhI⁺, in accord with the fall in ionization potential for this series of halogens.

For p-fluorotoluene well defined methyl- and ring-proton hyperfine coupling was obtained, the methyl-proton coupling (19 G) being slightly greater than that for the toluene cation (18.5 G). For PhBr⁺, replacement of para-hydrogen by Br, OH and SH gave a steady fall in spin density on bromine, reflecting increasing π-delocalization onto the para-substituent.

For concentrated solutions of the chloro-, bromo- and iodo-derivatives dimer cations were detected.