Pressure effects on H-ordering in hydrogen bonds and interactions in benzoic acid

Abstract

High-pressure structures of benzoic acid, C₆H₅COOH, reveals the interplay of proton disorder in O–H⋯O hydrogen bonds, molecular orientation and O⋯O distance. The benzoic acid structures have been determined by single crystal X-ray diffraction up to 2.21 GPa at 296 K. The initial ordering of H atoms in the hydrogen bonds of benzoic acid dimers is reversed above 0.30 GPa. These opposite effects are due to the H-stabilization by enhanced interactions of the molecule in the asymmetric crystal environment, and to the compressed O⋯O distance, lowering the potential energy barrier for the H-hopping, respectively. Pressure facilitates the esterification of benzoic acid in methanol and ethanol solutions, but no symmetry, structure or crystal volume changes indicative of phase transition postulated for this compound have been observed.