The effects of near-degeneracy in the electron spin resonance spectra of aromatic negative ions

The e.s.r. spectra of the anions of mono- and para-di-alkyl benzenes are described. The molecular orbitals which are degenerate in the case of the benzene negative ion are split apart by the introduction of the alkyl groups but the splitting is relatively small. It is suggested that the deviation of the proton hyperfine splitting constants from the values predicted by simple Hückel theory is due either to vibration-electronic interaction or to a mixing of electronic states through thermal agitation.