  The study of single-crystal Raman spectra of a series of crystalline
secondary amides (acetanilide, methacetin, phenacetine, orthorhombic and
monoclinic polymorphs of paracetamol) as well as simple amides formanilide and
benzanilide in the temperature range 5-300 K was carried out. The series of
compounds with the same molecular fragment, i.e. acetamide group, can serve as
a model system to study the interrelation between the latter and the properties
of the intermolecular "peptide-type" NH...O=C hydrogen bonds. For all the
"acetamide family" of compounds, similar changes in the Raman spectra were
observed on cooling the samples: an appearance of new Amide I(-) and Amide I(+)
bands that are red and blue shifted respectively from the conventional Amide I
band by around of 5-10 inverse centimeters. An appropriated changes in the same
temperature range were observed for the N-H out-of-plane bending (Amide V) and
N-H stretching vibrations of the N-H...O=C hydrogen bond. All the spectral
changes on cooling the samples can be supposed to result from delocalization of
the Amide I and N-H modes and appearance of dynamical splitting at low
temperature.
