Studies on the Infrared Spectra of Heterocyclic Compounds. VII. Hydrogen-Bond Effect on Pyridine 1-Oxide and its Derivatives

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Infrared investigations were made on the hydrogen-bond effect of pyridine 1-oxide and its derivatives with some proton donors. The assignment for N-O stretching frequency (strong absorption in the region of 1190 and 1320 cm^-1) was further confirmed by observing the spectral change of N-oxide compounds following hydrogen-bond formation with methanol in non-polar solvent. The hydrogen-bonding ability of various derivatives was estimated in terms of the magnitude of shift in bonded OH frequency of methanol and an excellent linear relation was found by plotting these frequencies against the σ-values of the substituents. Anomalous behavior in 2-methyl derivatives was also pointed out and discussed. From a comparison of the frequency shift of the carbonyl stretching absorption in methanol and water, it was found that the electron-releasing ability of the N-oxide function towards 4-position of the ring is largely decreased in polar solvent with active hydrogen.