a. Department of Physical Chemistry, University of Latvia, Jelgavas 1, Riga 1004, Latvia

b. Thomas Young Centre and Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K.

Self-association of four benzoic acid derivatives 2-chloro-4-nitrobenzoic acid, 2-methyl-4-nitrobenzoic acid, 3-hydroxybenzoic acid, and 2,6-dimethoxybenzoic acid in solution was investigated using spectroscopic measurements (Fourier transform infrared and 1H and 13C NMR spectroscopy) and molecular simulation methods. Based on the formation of hydrogen bonds, solvents can be divided into two groups: apolar solvents or solvents with a low hydrogen bond acceptor propensity, in which the benzoic acid derivatives form hydrogen-bonded dimers, and solvents with hydrogen bond acceptor propensity β > 0.3, interacting with the carboxylic group of benzoic acid, thus screening its interaction in the formation of self-associates. The formation propensity and structure of self-associates stabilized by weak interactions, such as π···π stacking and CH3···π interactions, however, are determined by the substituents in the benzene ring. Despite all the studied compounds being polymorphic, in none of the cases, an unequivocal structural link between self-associates present in the solution and the crystal form was observed.

a. Department of Physical Chemistry, University of Latvia, Jelgavas 1, Riga 1004, Latvia

b. Latvian Institute of Organic Synthesis, Aizkraukles 21, Riga 1006, Latvia

The arrangement of hydroxyl groups in the benzene ring has a significant effect on the propensity of dihydroxybenzoic acids (diOHBAs) to form different solid phases when crystallized from solution. All six diOHBAs were categorized into distinctive groups according to the solid phases obtained when crystallized from selected solvents. A combined study using crystal structure and molecule electrostatic potential surface analysis, as well as an exploration of molecular association in solution using spectroscopic methods and molecular dynamics simulations were used to determine the possible mechanism of how the location of the phenolic hydroxyl groups affect the diversity of solid phases formed by the diOHBAs. The crystal structure analysis showed that classical carboxylic acid homodimers and ring-like hydrogen bond motifs consisting of six diOHBA molecules are prominently present in almost all analyzed crystal structures. Both experimental spectroscopic investigations and molecular dynamics simulations indicated that the extent of intramolecular bonding between carboxyl and hydroxyl groups in solution has the most significant impact on the solid phases formed by the diOHBAs. Additionally, the extent of hydrogen bonding with solvent molecules and the mean lifetime of solute–solvent associates formed by diOHBAs and 2-propanol were also investigated.

a. Latvian Institute of Organic Synthesis, Aizkraukles 21, Riga 1006, Latvia

b. Department of Physical Chemistry, University of Latvia, Jelgavas 1, Riga 1004, Latvia

New pseudopolymorphs of ivermectin (IVM), a potential anti-COVID-19 drug, were prepared. The crystal structure for three pseudopolymorphic crystalline forms of IVM has been determined using single-crystal X-ray crystallographic analysis. The molecular conformation of IVM in crystals has been compared with the conformation of isolated molecules modeled by DFT calculations. In a solvent with relatively small molecules (ethanol), IVM forms monoclinic crystal structure (space group I2), which contains two types of voids. When crystallized from solvents with larger molecules, like γ-valerolactone (GVL) and methyl tert-butyl ether (MTBE), IVM forms orthorhombic crystal structure (space group P212121). Calculations of the lattice energy indicate that interactions between IVM and solvents play a minor role; the main contribution to energy is made by the interactions between the molecules of IVM itself, which form a framework in the crystal structure. Interactions between IVM and molecules of solvents were evaluated using Hirshfeld surface analysis. Thermal analysis of the new pseudopolymorphs of IVM was performed by differential scanning calorimetry and thermogravimetric analysis.

a. Department of Physical Chemistry, University of Latvia, Jelgavas 1, Riga 1004, Latvia

b. Latvian Institute of Organic Synthesis, Aizkraukles 21, Riga 1006, Latvia

c. Department of Physical Chemistry, Kazan Federal University, Kremlevskaya str. 18, 420008 Kazan, Russia

Dantrolene represents yet another interesting example of abundant molecular crystal polymorphism existing in at least six different neat polymorphs, three of which can be obtained via crystallization (I–III) and an additional three (IV– VI) via solid-state dehydration from three different monohydrates (MH-I–MH-III). The reasons for polymorph formation were rationalized by analyzing the crystal structures of the polymorphs and hydrates used in their preparation. The thermodynamic relations among the polymorphs were established from calorimetric data, solubility measurements, and lattice energy calculations.