Hydrated polymorphs of 2,3,4,5,6-pentachlorobenzoic acid: Crystallographical and computational analyses of disordered hydrogen-bonding networks

Abstract

The title compound, 2,3,4,5,6-pentachlorobenzoic acid (PCBAH), was found to have two crystal polymorphs, I and II. In this work, novel two hydrated polymorphs, polymorph III and IV, were reported. An asymmetric unit of the polymorph III is comprised of one PCBAH and a water molecule. They align alternatively along the a axis, forming C22(6) hydrogen-bonding. The water molecule makes another hydrogen bond with a water molecule in an inversion symmetry to give two-legged ladder hydrogen-bonding network. In polymorph IV, there are two PCBAHs, 2,3,4,5,6-pentachlorobenzate (PCBA) and H3O+ in an asymmetric unit. H3O+ molecule makes hydrogen-bonding with two PCBA anions, forming C22(6) hydrogen-bonding along the 21 axis. Two PCBAH molecule make hydrogen bonds with both H3O+ and PCBA. In both hydrated polymorphs, hydrogen atoms of water molecules are disordered. The positions of hydrogen atoms of the water molecules were determined with assistance of computational calculations under the crystalline environment.