The study of the polymorphic system of 2-chloro-4-nitrobenzoic acid

The solid-state behaviour of 2-chloro-4-nitrobenzoic (2c4n) acid has been reinvestigated. Infrared spectra, differential scanning calorimetry (DSC), and thermomicroscopy investigations as well as X-ray powder patterns for two modifications of the substance are described. Modification II is the thermodynamically stable crystal form from absolute zero to its transition point which occurs at approximately 97 °C according to the DSC analysis. Above that temperature, the material transforms to modification I which undergoes dramatic evolution of its lattice parameters with increasing temperature. Synchrotron powder diffraction measurements confirm the existence of two polymorphic forms. Both modifications have been structurally characterized by single-crystal X-ray diffraction. The acid molecules in both crystal structures are organized into R22(8) dimer units at the first graph set level in modification I and at the second level in modification II.