Substituent Effects on Benzyl Radical ESR Hyperfine Coupling Constants. The ?α*Scale Based upon Spin Delocalization

Article abstract of DOI:10.1021/ja00343a024

The electron spin resonance spectra of 21 para- and meta-substituted benzyl radicals have been analyzed.A substituent constant, ?_α*, has been defined from the benzylic α-hydrogen hyperfine coupling constant.The ?_α* constant reflects the component of energy that may be attributed to spin delocalization in a substituted benzyl radical in comparison to the unsubstituted radical.For the derivatives studied, para substitution is stabilizing, except when fluorine is the substituent; meta substitution destabilizes the benzyl radical.A series of radical reactions is reexamined with an extended Hammett relation.The relative importance of spin delocalization vs. polar effects is assessed by the ratio of ρ* to ρ.The importance of choosing substituents for which ?_a* and ? differ widely for such an assessment is emphasized.