Reversible order-disorder phase transition and interaction topology in 4-carboxyanilinium nitrate

Abstract

A reversible order-disorder phase transition in 4-carboxyanilinium nitrate (4-CAN), a 1:1 adduct formed between p-amino benzoic acid (PABA) and nitric acid has been studied via variable temperature single crystal X-ray diffraction and differential scanning calorimetry (DSC). DSC heating scans of 4-CAN shows presence of a reversible first-order phase transition at 309.5 K. At temperatures below the phase transition temperature the crystal structure was found to be monoclinic, space group P21/c, Z = 4 and at temperatures above the phase transition temperature the crystal structure was found to be orthorhombic, space group Pbcm, Z = 4. In both high and low temperature phases of 4-CAN crystal, the 4-carboxyanilinium cations and nitrate anions are connected through a three-dimensional network of strong charge-assisted NH…O and COH…O hydrogen bonds. The intermolecular interaction topology has been quantitatively analyzed using the computational tool of energy framework analysis and very high values of interaction energies are found for the hydrogen bonded anion-cation pairs in the crystal structure of 4-CAN.