Infrared Spectra of Acyclic Imides. I. Two Modifications of Diacetamide in the Crystalline State

Two crystal forms have been found for diacetamide (the form A and the form B). Infrared spectra of these two forms of diacetamide and diacetamide-d have been measured in the region 3600 to 400 cm−1 and many differences of absorption bands have been observed. It has been shown that these differences of absorption bands can be explained to result from rotational isomerism. The trans-trans planar structure of the –CONHCO–group has been proposed to correspond approximately to the molecular structure of the unstable form (the form B) of diacetamide. The frequency shifts of absorption bands on deuteration in the potassium bromide region has been discussed to give support to this interpretation. An assignment has been given to each absorption band in the potassium bromide region for the form A and the form B of diacetamide and diacetamide-d.