Polymorphism of 2-nitroaniline studied by calorimetric (DSC), structural (X-ray diffraction) and spectroscopic (FT-IR, Raman, UV-Vis) methods

The separation and growth methods of three ortho-nitroaniline (o-NA) polymorphs were found. The irreversible character of the α → β and β → γ phase transitions was revealed by differential scanning calorimetry (DSC) measurements and microscopic hot stage observations. The X-ray structure of the β-form was determined and compared with the γ phase structure solved by Daneshwar et al. [N.N. Daneshwar et al. Acta Crystallogr., Sect. B 34 (1978) 2507]. Intramolecular hydrogen bonding (intra H-bond) interactions are dominant in both structures. The IR and Raman spectral features of the solutions and of three polycrystalline o-NA polymorphs are specific for intramolecular resonance assisted H-bonds (RAHB's). The DFT calculations facilitated the almost complete assignments of bands to normal vibrations and the analysis of the measured spectra. The manifestations of weak inter H-bonds in the β and γ crystals and in the vibrational spectra of all polymorphs are observed as well; the strongest inter H-bonds occur in the γ polymorph. The differences in lowest electronic transition energies of three α, β and γ layers explain their different colours: the yellowish-green of the α form and the orange ones of the β- and γ- phases. The least stable α form is probably an amorphous one with the weakest inter H-bonds. The differences in relative orientations of the -NH2, -NO2 groups and phenyl rings in the β- and γ-phases indicate that the o-NA polymorphism has conformational character.