ESR investigation of some free-radical adducts

The effect of steric strain on the esr hyperfine coupling constants of several free-radical adducts of perfluoropropene and perfluoro-2-butene were investigated. Silyl, hydroxyalkyl, thiyl and alkoxy free radicals, which were generated by photo-irradiation, reacted with these olefins to form free-radical adducts. Trends in the esr hfs were then looked for in each class of adducts.

The thiyl and alkoxy radicals were found to be unsuitable for this type of study due to the low degree of steric strain produced by these species in the adducts. Trends toward higher a-Fβ/CF₃ hfs and lower a^Fα hfs were observed for the silyl and hydroxyalkyl radical adducts as the steric strain in the adduct was increased. The ratio of the ^Fα to a-Fβ/CF₃ hfs was shown to be a good measure of the degree of steric strain in these radical adducts.

The possibility that the (Me₃Si)₂CHC(SiMe₃)₂ free radical, which has been reported to contain an "esr invisible" β-H, might be misidentified was investigated. Our conclusion was that the β-H is present and has not been replaced by a t-butoxy group.