Unpaired Electrons and Radical Stability

In the majority of heteroaromatic radicals the unpaired electron is in a fully delocalized molecular 7t-orbital. The magnetic moment of the unpaired electron is quantized in an applied magnetic field, which fact gives rise to 

the hyperfine splitting conferred on the ESR spectrum of a 7t-radical by interaction of the electron spin with a proton is proportional to the 7t-spin population p, at the atom bearing the proton (the McConnell relationship).17 Thus, for a proton attached to carbon atom i the hyperfine splitting a(H),-is given by Eq. (1), where QCH is the spin polarization parameter for the C—H bond. 

In many examples it is found that the contribution to a(N) from adjacent carbon atoms is small values of Q vary somewhat with the molecular orbital 

Carrington and A. D. McLachlan, Introduction to Magnetic Resonance. Harper, New York, 1967. 

Ayscough, Electron Spin Resonance in Chemistry. Methuen, London, 1967. 

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