Unstable intermediates. Part 137.—α-Bromoalkyl radicals in irradiated organic bromides: an electron spin resonance study

Abstract

Exposure of a range of organic bromides to ⁶⁰Co γ-rays at 77 K, either pure or in perdeuteromethanol gave, amongst others, radicals having e.s.r. features thought to be characteristic of α-bromo radicals, R₂ĊBr. Analyses of such spectra are given, including the use of selected single crystal spectra and measurements at different microwave frequencies. The effect of the large bromine quadrupole interaction is indicated. It is concluded that the unpaired electron is delocalised onto bromine to the extent of ca. 19 %, this being relatively insensitive to the nature of the other substituents. Exposure of bromomaleic acid to hydrogen atoms gave an α-bromo radical exhibiting a large hyperfine coupling (38 G) to as single hydrogen, which was shown to be the added atom by use of deuterium in place of hydrogen. For the radical H₂CCHĊHBr structure H₂Ċ—CHCHBr makes only a minor contribution. Possible radiation mechanisms are discussed.