Fundamental NH₂ Stretching Frequencies in Amines and Amides

Abstract

IN molecules with functional groups of the type –AB₂, two stretching vibrations corresponding to the symmetric (ν_s) and asymmetric (ν_{a s}) modes of vibration of the B atoms are observed. Bellamy and Williams¹ have demonstrated that so long as the two A–B bonds are equivalent, and mass and coupling effects are negligible, polar effects acting on the group induce proportional changes in ν_s and ν_{a s}. For sixty-two primary amines covering a wide range of molecular type in solution and in the pure liquid state, they found: with a standard deviation of 4.8 cm.⁻¹. Where neighbouring groups destroyed the bond equivalence, as in hydrogen bond interactions with only one hydrogen atom of the amino-group, this relationship no longer held. Stewart² has shown that equation (1) can be predicted from the force field treatment of Linnett³, assuming identical NH bond stretching force constants.