A comparison of classical and quantum-mechanical ‘ring-current’ theories

The proton resonance spectra of a series of 2,7-disubstituted-4,5-bis(2-pyridyl)phenanthrene-3,6-diols are examined with emphasis on the shielding effects produced by aromatic ‘ring currents.’ In this series, two pyridine rings are rigidly held less than 3.4 Å apart and with considerable overlap. The ‘ringcurrent’ shieldings found for above-the-plane positions are in good agreement with the Johnson-Bovey theory, whereas a recently proposed quantum-mechanical theory underestimates the shieldings by a factor of nearly three.