Vibrational Spectroscopic Studies of Cocrystals and Salts. 2. The Benzylamine−Benzoic Acid System

Abstract

X-ray powder diffraction, differential scanning calorimetry, infrared absorption spectroscopy, and Raman spectroscopy have been used to study the phenomenon of salt formation in the 1:1 stoichiometric product formed by benzylamine and benzoic acid. Full understanding of the effects of salt formation on the energies of the vibrational modes of motion was obtained by the complete assignment of the spectra of the starting materials and of the benzylammonium benzoate product. In addition, benzoic acid was found to form a 1:1 cocrystal with the benzylammonium benzoate salt, and this product was also characterized using the full range of techniques. Both infrared absorption and Raman spectroscopies were found to be useful vibrational spectroscopic techniques for characterization of the products, with the phenomenon of salt formation leading to the strongest perturbations in the vibrational modes associated with the carboxylic acid group of benzoic acid and the amine group of benzylamine. Little change in the energies of the vibrational modes associated with the carbon−carbon and carbon−hydrogen portions of the compounds was observed, demonstrating that the structural changes associated with the salt formation only affected the energies of the vibrational modes directly involved in the proton transfer and ion association. However, perturbations in the carbon−carbon and carbon−hydrogen vibrational modes were observed in the benzoic acid cocrystal with benzylammonium benzoate, demonstrating the importance of the phenyl rings in the cocrystal formation.