The Effects of α-Halogen Substitution on the Configuration of Alkyl Radicals in Solution by Electron Spin Resonance

The e.s.r. spectra of a variety of alkyl radicals with α-halogen substituents have been examined in solution. The configurations of α-halogen substituted ethyl and β,β,β-trifluoroethyl radicals are correlated with the hyperfine splittings of α-methyl protons and α-trifluoromethyl fluorines, respectively. For an α-fluorine substituent, electronegativity plays an important role in determining the pyramidal configuration of the radical center, whereas size appears to be a more important factor with chlorine and bromine in promoting a less pyramidal configuration at the radical center. Two α-halogen substituents have a disproportionately greater effect than one halogen in influencing the configuration at the radical site. The g-factors of fluoroalkyl radicals are qualitatively analyzed in order to discuss possible mechanisms of β-fluorine hyperfine interactions and the effect of trifluoromethyl substitution on the configuration of the radical site.