ИСТИНА

Polymorphism is an important phenomenon for molecular crystals. Many organic compounds exist in two or more crystalline forms, which differ in conformation of the molecule and/or crystal packing. The crystallization of a certain form is usually defined by crystallization conditions, including the interactions with solvent molecules during crystallization. Polymorphism is known not only for pure compounds, but also for multi-component structures such as co-crystals of solvates. It is important that in case of crystal solvates, the crystal packing is mainly defined by interaction of the parent compound, as solvent molecules are usually much smaller. For a flexible solvent, it is possible that differences in its conformation cause polymorphism; however, such cases are unusual and require detailed consideration. Compound (1) 8H-6-methyl-13a-carboxy-quinazolino[3,4-b]quinazolin-5-ium crystallizes in two polymorphic forms of acid crystal solvate with a stoichiometric ratio of 1:1, 1a (sp. gr. P21/c, Z’=1) and 1b (sp. gr. P21/c, Z’=3). Despite certain similarity of the unit cell parameters, molecular geometry and supramolecular organization of the main compound, the structures are very different in conformation and crystal packing of the solvent molecules. Thus, in 1a the acetic acid molecule in trans-conformation (which is itself not very typical), while in 1b two symmetry independent molecules are in trans- and one is in cis-conformation. As a result, instead of infinite H-bonded chains in 1a, in 1b the solvent molecules form only a trimeric associate that is additionally bonded to the main compound. The features of the supramolecular organization, the results of the quantum-chemical calculations of the associates and crystal structures are reported; and possible reasons leading to the observed polymorphism are discussed.