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, C14H12CINO2] 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 center dot center dot center dot 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.