Substituent effects on benzyl radical hyperfine coupling (hfc) constants. Part 3. Comparison of the α-hfc for substituted benzyl radicals with the β-hfc for substituted cumyl radicals

The electron spin resonance (esr) spectra of eighteen para-substituted cumyl radicals were analyzed. The relationship between the β-hyperfine coupling (β-hfc) constants of these cumyl radicals and the corresponding α-hfc constants of benzyl radicals was studied. Although there is a general trend for the α- and β-hfc values to vary in a similar manner, specific deviations from a linear correlation between these parameters were observed. These deviations were rationalized by considering charge effects on spin delcoalization. The correlation coefficient for the linear regression analysis of these α- and β-hfc values was found to significantly improve when β arameters reflecting charge effects on spin delocalization were included in an extended Hammett treatment of the spectral data.