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.