Vibrational Spectra of Primary and Secondary Aliphatic Amines

Abstract

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.