Amino group stretching vibrations in primary acid amides

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Abstract
Most primary alkyl, primary α-substituted alkyl, and primary aromatic acid amides exhibit three sharp concentration and temperature dependent absorption bands at 3 518 ± 3, 3 504 ± 4, and 3 486 ± 8 cm−1, in dilute (0.003 M or less) CCl4solution, just below the fundamental antisymmetric NH2stretching vibration. These are tentatively assigned to the shifted antisymmetric NH2stretching vibration in cyclic dimers, cyclic trimers, and cyclic tetramers respectively, with increasing NH … O hydrogen bond strength. In all alkyl amides the antisymmetric NH2band intensity exceeds the symmetric band intensity. Conjugation of the CONH2group with π-electron systems reverses this intensity relationship, except where the acidic NH2group can form intramolecular hydrogen bonds. These observations substantiate the previously published views on the nature of the vibrational mechanism of the NH2group. No evidence is found for enolization of amides in CCl4solution.