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13C NMR spectra of substituted pyridine N‐oxides. Substituent and hydrogen bonding effects

Aromatic 13C chemical shifts of 4‐ and 3‐substituted pyridine N‐oxides in aprotic and protic solvents are reported and discussed. Changes in chemical shifts caused by hydrogen bonding and protonation are much smaller than those induced by a substituent, and are not related to the pKa values of N‐oxides. Since the effect of hydrogen bonding is not in the same direction as that due to the substituent effect, the estimation of proton transfer from measured chemical shifts is not possible in complexes of pyridine N‐oxides with trifluoroacetic acid. A linear correlation is obtained between the ipso carbon substituent chemical shifts of 4‐ and 3‐substituted pyridine N‐oxides and mono‐substituted benzenes, which supports the suggestion that electronic factors other than Δq are more important for the ipso carbon.