Crystal structures of tolfenamic acid polymorphic forms I and II with precise hydrogen-atom positions for nuclear magnetic resonance studies.

The structures of tolfenamic acid [TFA; 2-(3-chloro-2-methyl­anilino)benzoic acid, C14H12ClNO2] polymorph forms I and II have been redetermined [compare Andersen et al. (1989). J. Chem. Soc., Perkin Trans. 2, pp. 1443–1447] with improved precision using high-resolution X-ray diffraction data and Hirshfield atom refinement in order to better define both hydrogen-atom locations and their associated bond lengths. Covalent bond lengths to hydrogen were found to be significantly longer throughout both structures, especially for the anilino H atom, which is involved in an important intra­molecular N—H...O hydrogen bond to the carb­oxy­lic acid group. This hydrogen bond is shown to clearly perturb the electron density around both oxygen atoms in the latter group. The extended structures of both polymorphs feature carb­oxy­lic acid inversion dimers. These structures provide an improved foundation for nuclear magnetic resonance studies in both solution and the solid state. [ABSTRACT FROM AUTHOR]