Raman spectra of crystalline secondary amides

TL;DR

This study investigates the temperature-dependent Raman spectra of crystalline secondary amides, revealing spectral shifts and mode delocalization linked to hydrogen bonding and molecular dynamics.

Contribution

It provides detailed analysis of Raman spectral changes in secondary amides across temperatures, highlighting the effects of hydrogen bonds and molecular delocalization.

Findings

Appearance of new Amide I(+) and I(-) bands on cooling

Spectral shifts linked to hydrogen bond dynamics

Mode delocalization and dynamical splitting at low temperatures

Abstract

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.