Raman spectra of crystalline secondary amides.

We present a Raman-spectroscopic study of secondary amides (acetanilide, methacetin, phenacetine, orthorhombic and monoclinic polymorphs of paracetamol) as well as simple amides formanilide and benzanilide. The study was carried out on single crystals and in the temperature range of 5-300K. The series of compounds with the same molecular fragment - acetamide group - can serve as a model system to study the interrelation between this group and the properties of the intermolecular "peptide-type" NH⋯OC hydrogen bonds. For all of the "acetamide family" of the compounds, similar changes in the Raman spectra were observed upon cooling of the samples: emergence of new Amide I(-) and Amide I(+) bands, which are red and blue shifted, respectively, from the conventional Amide-I band by around of 5-10cm-1 . Corresponding changes in the same temperature range were observed for the NH out-of-plane bending (Amide V) and NH stretching vibrations of the NH⋯OC hydrogen bond. All of the spectral changes observed upon cooling of the samples can be presumed to result from a delocalization of the Amide-I and NH modes and appearance of dynamical (Davydov's) splitting at low temperature.