Amino group stretching vibrations in primary acid amides

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.