Proton-Deuteron Hyperfine Structure in Paramagnetic Resonance: A π-δ Interaction

The mechanism of the hyperfine splitting observed in paramagnetic resonance spectra of certain aromatic free radicals containing protons adjacent to the aromatic ring is examined. It is shown that the magnitude of the interaction between an unpaired electron in a π orbital and a vibrating hydrogen atom is insufficient to account for the observed splitting. Such a vibrational mechanism is considered to be untenable: (i) on the basis of quantitative calculations, (ii) by the failure to observe lines corresponding to the first excited vibrational state, and (iii) by a comparison of the splitting caused by protons and deuterons. It is suggested that the unpaired electron is not in a purely π state and that the splitting may be accounted for by configuration interaction between π and δ states.