Vibrational Spectroscopic Studies of Cocrystals and Salts. 1. The Benzamide-Benzoic Acid System

X-ray powder diffraction, differential scanning calorimetry, infrared absorption spectroscopy, and Raman spectroscopy have been used to study the phenomenon of cocrystal formation in stoichiometric and nonstoichiometric mixtures of benzamide and benzoic acid. Although Raman spectroscopy was not useful in characterization of the products, trends in the infrared absorption bands of the various products were used to demonstrate a number of important aspects regarding the nature of the interactions in the cocrystal. Little change in the vibrational modes associated with the phenyl groups of the respective reactants took place upon cocrystal formation, but changes in intensities of the various vibrational modes associated with the amide group of benzamide and the carboxylic acid group of benzoic acid were observed upon formation of the cocrystal. A phase diagram of the system was deduced from thermal analyses of the nonstoichiometric mixtures, and the infrared absorption spectra of these materials were used to obtain additional insight into the nature of these solids.