Infrared Spectroscopy of Hydrogen Bonds in Benzoic Acid Derivatives

We have measured the Fourier transform IR spectra of CCl 4 solutions of benzoic acid and its biologically active derivatives. We investigated the proton-acceptor properties of the studied molecules theoretically by the molecular electrostatic potential method. The calculations are compared with experimental results. Based on an estimate of the proton-acceptor properties, we give an interpretation of the specific features of the IR spectra of benzoic acid and its derivatives in the region of the O–H and C = O vibrations. The mechanisms for interactions of the molecules are determined by the nature of substituents which are added to the benzene ring in positions para and meta to the carboxyl group. We identify the conditions for appearance of intermolecular hydrogen bonds of O–H · · · O = C, O–H · · · O–H types with formation of cyclic and linear dimers. We show that intramolecular hydrogen bonds of the type O–H · · · O–CH 3 prevent the hydroxyl groups from participating in intermolecular interactions.