Electron Paramagnetic Resonance Spectra of Semiquinones. VI. Spin and Electron Density Distributions in Bicyclo[2.2.1]heptyl and Bicyclo[2.2.2]octyl Derivatives of Semiquinone

The epr spectra of the semiquinones prepared from 3′,6′-dihydroxybenzobicyclo[2.2.1]hepta-2,5-diene (1-S), 3′,6′-dihydroxybenzobicyclo[2.2.1]hept-2-ene (6-S), 3′,6′-dihydroxybenzobicyclo[2.2.2]octa-2,5-diene (10-S), 3′,6′-dihydroxybenzobicyclo[2.2.2]oct-2-ene (13-S), and selected derivatives have been recorded and analyzed. This research reveals that the vinyl, anti, and syn protons of 1-S and 10-S are coupled, but that the bridgehead protons of these substances are not coupled perceptibly. Only the anti protons of 6-S and 13-S are coupled importantly. Symmetry considerations and the consequences of methyl group substitution suggest that spin density is transferred to the bicyclic fragment of the molecule without the transfer of electron density. Spin polarization, rather than electron delocalization, is apparently the important coupling mechanism in these bicyclic semiquinones.