Amino group stretching vibrations in primary aliphatic amines

Primary aliphatic amines can be characterized by infrared spectroscopy as to the nature and degree of branching of the alkyl chain, using the fundamental antisymmetric and symmetric NH2 stretching frequencies measured in dilute C2Cl4 solution. Where steric and direct conjugation effects are absent both frequencies and the integrated intensity of the antisymmetric band increase linearly with decreasing inductive electron-donating power of the alkyl chain. The integrated intensity of the symmetric band, which is smaller than that for the antisymmetric band in all the 26 compounds studied, is independent of the inductive nature of the alkyl chain as a first approximation. These results indicate that the N-atom lone-pair orbital in RNH2 plays a negligibly small role in modifying the inductive effect of R on NH2 frequencies and intensities as long as the N-atom hybridization remains sp3.