Polymorphism of 6-methyluracil, which affects the regulation of lipid peroxidation and wound healing, has been studied by experimental and quantum chemical methods. Two known polymorphic modifications and two new crystalline forms were crystallized and characterized by single crystal and powder X-ray diffraction (XRD) methods as well as by the differential scanning calorimetry (DSC) method and infrared (IR) spectroscopy. The calculations of pairwise interaction energies between molecules and lattice energies in periodic boundary conditions have shown that the polymorphic form 6MU_I used in the pharmaceutical industry and two new forms 6MU_III and 6MU_IV, which can be formed due to temperature violations, may be considered as metastable. The centrosymmetric dimer bound by two N–H···O hydrogen bonds was recognized as a dimeric building unit in all of the polymorphic forms of 6-methyluracil. Four polymorphic forms have a layered structure from the viewpoint of interaction energies between dimeric building units. The layers parallel to the (100) crystallographic plane were recognized as a basic structural motif in the 6MU_I, 6MU_III, and 6MU_IV crystals. In the 6MU_II structure, a basic structural motif is a layer parallel to the (001) crystallographic plane. The ratio between the interaction energies within the basic structural motif and between neighboring layers correlates with the relative stability of the studied polymorphic forms. The most stable polymorphic form 6MU_II has the most anisotropic “energetic” structure, while the interaction energies in the least stable form 6MU_IV are very close in various directions. The modeling of shear deformations of layers in the metastable polymorphic structures has not revealed any possibility of these crystals to be deformed under external mechanical stress or pressure influence. These results allow the use of metastable polymorphic forms of 6-methyluracil in the pharmaceutical industry without any limitations.

The hemihydrate of 4-[(benzylamino)carbonyl]-1-methylpyridinium bromide, C14H15N2O+·Br−·0.5H2O, was studied by single-crystal and powder X-ray diffraction methods. In the asymmetric unit, two organic cations of similar conformation, two bromide anions and one water molecule are present. In the crystal, N—H...Br hydrogen bonds link the cations and anions. The formation of a set of intermolecular C—H...Br and C—H...π interactions result in double chains extending parallel to [011]. A Hirshfeld surface analysis showed high contributions of H...H and C...H/H...C short contacts to the total Hirshfeld surfaces of the cations.

Two salts of 4-[(benzylamino)carbonyl]-1-methylpyridinium (Am) with chloride (C14H15N2O+·Cl−) and bromide (C14H15N2O+·Br−) anions were studied and compared with the iodide salt. AmCl crystallizes in the centrosymmetric space group P21/n while AmBr and AmI form crystals in the Sohncke space group P212121. Crystals of AmBr are isostructural to those of AmI. The cation and anion are bound by an N–H...Hal hydrogen bond. Hirshfeld surface analysis was used to compare different types of intermolecular interactions in the three structures under study.