Infrared spectral and X-ray studies of polymorphic forms of 2-(2-methyl-3-chloroanilino)nicotinic acid

Abstract

The polymorphic forms of 2-(2-methyl-3-chloroanilino) nicotinic acid, C₁₃H₁₁ClO₂N₂, were classified as (I)–(IV) by their i.r. and u.v. spectra, thermograms, and X-ray powder diffraction patterns. Form (I) is monoclinic, space group P2₁/c, a= 7.625(1), b= 14.201(1), c= 11.672(1)Å, β= 101.65(1)°, and Z= 4. Form (II) is orthorhombic, space group Pca2₁a= 23.597(6), b= 4.042(1), c= 12.127(3)Å, and Z= 4. Form (III) is triclinic, space group P, a= 13.810(1), b= 3.858(1), c= 10.984(2)Å, α= 94.98(1), β= 94.42(1), γ= 95.57(1)°, and Z= 2. Form (IV) is triclinic, space group P, a= 7.670(1), b= 7.254(1), c= 10.882(1)Å, α= 100.66(1), β= 102.02(1), γ= 86.97(1)°, and Z= 2. Assignments of the i.r. vibration bands to the intra- and inter-molecular hydrogen bonds have been performed taking into consideration the deuterium isotopic effects, the spectral changes observed at low temperature and the spectra of related compounds. Three types of intermolecular hydrogen bonds (A)–(C) were respectively allocated to forms (I), (II), and (III) and (IV). The intramolecular hydrogen bonds found in all forms were identified as NH ⋯ O. The crystal structures were determined by X-ray analysis. The results confirmed the hydrogen bonds indicated by the i.r. spectral study. Relationships were established between the AH stretching frequencies and the A ⋯ B distances for the intra- and inter-molecular AH ⋯ B hydrogen bonds detected in all the forms. Shifts of the longest wavelength band in the u.v. spectra were correlated with the dihedral angles between the pyridine and benzene rings.