Effect of Substituent Size and Isomerization on the Polymorphism of 2-(Naphthalenylamino)-benzoic Acids

To probe the effect of substituent size and isomerization on the polymorphism of fenamic acid (FA) derivatives, we synthesized two FA analogues, namely, 2-(naphthalen-1-ylamino)-benzoic acid and 2-(naphthalen-2-ylamino)-benzoic acid (NBAs), and investigated their polymorphism. Contrary to the lack of polymorphism in FA, we discovered two crystal forms (1-I, 1-II, 2-I, and 2-II) for each of the two compounds. We characterized the two polymorphic systems with single-crystal X-ray diffraction, powder X-ray diffraction, and infrared and Raman spectroscopy, and observed certain structural similarities between 1-I and 2-I. The phase behavior of these two systems were studied with differential scanning calorimetry. Computational studies such as the search for stable conformers, lattice-energy calculations, and Hirshfeld surface analysis were performed to shed light on the polymorphism, relative stability of the forms in each system, and contribution of intermolecular interactions to the overall stability of the systems. The study confirmed the effect of substituent size on the polymorphism of the compounds.