Vibrational Spectra of Primary and Secondary Aliphatic Amines

The infrared absorption spectra of twenty primary and thirteen secondary amines have been studied. A strong band, assigned to the asymmetric C–N stretching mode of secondary amines, is found at 1139±7 cm—1 in compounds having a primary alpha carbon and at 1181±10 cm—1 in compounds having a secondary alpha carbon. The primary amines have a corresponding C–N band at 1079±11, 1040±3, and 1030±8 cm—1 for compounds with primary, secondary, and tertiary alpha carbons, respectively. A strong broad band is found at 739±11 and 718±18 cm—1 in the spectra of secondary amines having primary and secondary alpha carbons. This absorption is assigned to the N–H bending mode. In liquid primary amines, pairs of strong bands at 778±15 and 825±15, 794±16 and 840±11, and 776±8 and 833±18 cm—1 are assigned to the symmetric and asymmetric NH2 bending modes in compounds having primary, secondary and tertiary alpha carbon atoms, respectively. Shifts in frequency of some of these bands are noted in the gas phase or in solution. A very strong, broad band centered at less than 290 cm—1 is observed in the spectra of the primary amines, but not the secondary amines. This band disappears in the gas phase and is assigned to the torsional oscillation of the —NH2 group. A simplified normal coordinate treatment of the R—NH2 and R2—NH vibrations is given in support of the assignments.