The NH Stretching Vibration and NH–N Hydrogen Bonding in Several Aromatic Compounds

The three-micron infrared absorption bands assigned to the NH stretching vibration have been studied in pyrrole, indole, carbazole, diphenylamine, and aniline. Spectra have been obtained in solution in carbon tetrachloride, carbon disulfide, chloroform, and benzene over a wide range of concentrations. In pyrrole and indole with increasing concentration the sharp NH band fades out and a broader band to lower frequencies appears. While the results are not as clearcut for diphenylamine and aniline, for these four compounds hydrogen bonding satisfactorily explains the observed data. This conclusion, for pyrrole and indole, sets aside the infrared spectroscopic basis for Pauling's hypothesis of two species of molecules. Carbazole was not soluble enough to make association studies possible. The relation of both the solvent and the solute molecular structure to the position of the associated and unassociated frequencies is discussed.