Two New Cocrystals of the Dipicolinic Acid. Hirshfeld Atom Refinement of Crystal Structures and Quantum Theory of Atoms in Molecules Analysis of Molecular Complexes

Pyridine-2,6-dicarboxylic acid (dipicolinic acid; DPA) is a very interesting compound. In addition to its biological significance, the molecule has some interesting structural features. Due to the free rotation of two carboxylic groups, it can very easily change conformation, and thus noncovalently interact as a proton donor and/or acceptor in different molecular systems. In this work, we present two new bicomponent cocrystals of dipicolinic acid. Cocrystallizing molecules are zwitterions of nicotinic acid and glycine, and the hydrogen bonds formed are strong, charge-assisted ones. To obtain a proper crystal structure model and correct hydrogen atom position, we applied Hirshfeld Atom Refinement (HAR). It allowed us to overcome optimization calculation problems and complete the analysis of the particular molecular complexes that occur within the crystal structures as well as in the gas phase environment.