Substituent effects on benzyl radical hydrogen hyperfine coupling constants (hfc's).: Part 5. The comparison of electron spin resonance hfc's and the stabilization energy of π-radicals due to spin delocalization

Electron spin resonance (esr) hfc values of π-radicals are found to be linearly related to the stabilization energy due to spin delocalization. Application of this relationship to benzyl radicals permits the representation of σ•α values on an energy scale. Mulliken charges in the α- and β-spin systems are found to provide valuable information about factors governing the effects of substituents on spin delocalization in benzyl radicals. These effects can be rationalized by considering four types of spin–spin interactions involving σ- and π-electrons in the α- and β-spin systems. The Mulliken analysis shows that all substituents are stronger π α-acceptors (or weaker π α-donors) at the para position relative to the meta position. Similarly, in the 3 system the meta derivatives are the better acceptors. This may be understood in terms of the excess of π α-spin at the para-carbon and the excess of π β-spin at the meta-carbon. Similar effects are observed in the a framework.