Syn and anti polymorphs of 2,6-dimethoxy benzoic acid and its molecular and ionic cocrystals: Structural analysis and energetic perspective

The conformational polymorphism in 2,6-dimethoxy benzoic acid, in which the acidic hydrogen atomcan be either in anantior asynconformation with respect to the carbonyl oxygen atom, has beenstudied. The compound crystallizes in two conformational polymorphic forms,antiandsyn. The COOHgroup lies out of the phenyl ring plane due to steric repulsion with the ortho-methoxy substituents. Theacidic OeH bond in the COOH carboxyl group inantiappears to be more polar/ionic compared to thesynform. Both polymorphs were characterized by single-crystal X-ray diffraction and for theantiform aneutron diffraction measurement was carried out. In addition, a new molecular co-crystal (MCC) and anew ionic cocrystal (ICC) form were also obtained and characterized by single crystal X-ray diffraction inwhich the protonated 2,6-dimethoxy benzoic acid units adoptsynconfiguration. The crystal packing,hydrogen bonding and intermolecular interactions are discussed in terms of Hirshfeld surface analysisand derived properties such as the contact enrichment ratio and energy frameworks. Cohesive energiesbased on fully periodic DFT calculations as well as based on the sum of dimeric interaction energies wereestimated for bothsynandantiforms. They reveal the higher stability of the crystal structure in theantiform while thesynform is more stable in the isolated state. Room temperature Raman spectra werecollected on the single crystals of theantiand ionic cocrystal forms and compared with both isolated-molecule and periodic calculations