Proton Hyperfine Interactions in Semiquinone Ions

The proton hyperfine structure in a series of methylsubstituted semiquinone ions has been shown to arise in a general sense from configuration interaction. A crude calculation of the proton hfs to be expected in a hypothetical solution of C2H4+ ions indicates that the σ—π interaction is responsible for the hfs of protons attached directly to an aromatic ring. The molecular orbital theory of hyperconjugation accounts for the detailed hfs associated with the methyl protons.