Insight to the thermodynamic stability of molecular crystals through crystallographic studies of a multipolymorph system

Is it possible to predict the stability of a crystal structure under the influence of pressure? Quantum chemical study of ibuprofen crystals

Quantum chemical modeling was used to analyze the crystalline structure of ibuprofen under atmospheric pressure to determine the structural features, providing its stability under pressure.

3D Electron Diffraction for Chemical Analysis: Instrumentation Developments and Innovative Applications

In the past few years, many exciting papers reported results based on crystal structure determination by electron diffraction. The aim of this review is to provide general and practical information to structural chemists interested in stepping into this emerging field. We discuss technical characteristics of electron microscopes for research units that would like to acquire their own instrumentation, as well as those practical aspects that appear different between X-ray and electron crystallography. We also include a discussion about applications where electron crystallography provides information that is different, and possibly complementary, with respect to what is available from X-ray crystallography.

Raman imaging of amorphous-amorphous phase separation in small molecule co-amorphous systems

Many new active pharmaceutical ingredients (API) undergoing development have low permeabilities or low aqueous solubilities. However, the amorphous state is usually more soluble than its crystalline counterpart. The amorphous state has a higher Gibb's free energy, which can improve the apparent solubility but decrease the stability since the amorphous state tends to transform to the more stable crystalline form. Before recrystallization, a co-amorphous binary mixture's ingredients have to undergo a phase separation. The aim of this study was to obtain a better understanding of the amorphous-amorphous phase separation in co-amorphous binary mixtures and test the suitability of imaging Raman spectroscopy for detecting this phenomenon. To study the phase separation, we prepared three different 50:50 mass ratio binary mixtures of APIs: paracetamol-terfenadine, (PAR-TRF), paracetamol-indomethacin (PAR-IMC) and terfenadine-indomethacin (TRF-IMC). The binary mixtures were amorphized with melt-quenching and stored above their glass transition temperature (T_g) to monitor their phase separation. Thermal degradation was determined with a high performance liquid chromatography (HPLC) method to ensure that melt-quenching did not cause any thermal degradation of the molecules. Thermodynamic attributes (crystallization tendency, melting point (T_m) and T_g) were measured with differential scanning calorimetry (DSC) to ensure that the co-amorphous systems transformed to the amorphous state and remained amorphous after cooling and reheating. Phase separation was studied from the surface and cross-section (CS) with Raman imaging to examine if it occurred more on the surface than in the bulk. The Raman spectra were analyzed with principal component analysis (PCA) and Contour plots were produced from the PCA-score values to visualize concentration differences in the mixtures. The results showed that API vs API concentrations increased as a function of time in both surface and CS images before crystallization. This suggests that Raman imaging is a suitable technique to detect the phase separation phenomena in small molecule co-amorphous binary mixtures.

Three differently coloured polymorphs of 3,6-bis(4-chlorophenyl)-2,5-dipropyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione

In this paper, the conformational polymorphism of a chlorinated diketopyrrolopyrrole (DPP) dye having flexible substituents in a non-hydrogen-bonding system is reported. The propyl-substituted DPP derivative (PR3C) has three polymorphic forms, each showing a different colour (red, orange and yellow). All polymorphs could be obtained concomitantly under various crystallization conditions. The results of the crystal structure analysis indicate that PR3C adopts different conformations in each polymorph. The packing effect caused by the difference in the arrangement of neighbouring molecules was found to play an important role in the occurrence of the observed polymorphism. The thermodynamic stability relationship between the three polymorphs was identified by thermal analysis and indicates that the yellow polymorph is the thermally stable form. The results indicate that the yellow form and orange form are enantiotropically related, and the other polymorph is monotropically related to the others.

Triclinic and monoclinic polymorphs of meso-(E,E)-1,1'-[1,2-bis(4-chlorophenyl)ethane-1,2-diyl]bis(phenyldiazene): the high-yield synthesis of an unexpected product, concomitant polymorphism and configurational disorder

Pyrazolidine-3,5-diones and their derivatives exhibit a wide range of biological activities. Seeking to explore the effect of combining a hydrocarbyl ring substituent, as present in sulfinpyrazone (used to treat gout), with a chlorinated aryl ring, as present in muzolimine (a diuretic), we explored the reaction between 1-phenylpyrazolidine-3,5-dione and 4-chlorobenzaldehyde under mildly basic conditions in the expectation of producing the simple condensation product 4-(4-chlorobenzylidene)-1-phenylpyrazolidine-3,5-dione. However, the reaction product proved to be meso-(E,E)-1,1'-[1,2-bis(4-chlorophenyl)ethane-1,2-diyl]bis(phenyldiazene), C26H20Cl2N4, and a tentative mechanism is proposed. Crystallization from ethanol produces two concomitant polymorphs, i.e. a triclinic form, (I), in the space group P-1, and a monoclinic form, (II), in the space group C2/c. In both polymorphs, the molecules lie across centres of inversion, but in (II), the molecules are subject to whole-molecule disorder equivalent to configurational disorder with occupancies of 0.6021 (19) and 0.3979 (19). There are no hydrogen bonds in the crystal structure of polymorph (I), but the molecules of polymorph (II) are linked by C-H...π(arene) hydrogen bonds into complex chains, which are further linked into sheets by C-H...N interactions.

4,4'-Bipyridine-1,1'-diium acetylenedicarboxylate: a new member of the (H2bipy)[Cu(ox)2] (bipy is 4,4'-bipyridine and ox is oxalate) family

4,4'-Bipyridine-1,1'-diium (H2bipy) acetylenedicarboxylate, C10H12N2(2+)·C4O4(2-), (1), is a new member of a family of related structures with similar unit-cell parameters. The structures in this family reported previously [Chen et al. (2012). CrystEngComm, 14, 6400-6403] are (H2bipy)[Cu(ox)2] (ox is oxalate), (2), (H2bipy)[NaH(ox)2], (3), and (H2bipy)[H2(ox)2], (4). Compound (1) has a one-dimensional structure, in which H2bipy(2+) cations and acetylenedicarboxylate (ADC(2-)) anions are linked through a typical supramolecular synthon, i.e. R2(2)(7), and form linear `-cation-anion-' ribbons. Through an array of nonclassical C-H...O hydrogen bonds, adjacent ribbons interact to give two-dimensional sheets. These sheets stack to form a layered structure via π-π interactions between the H2bipy(2+) cations of neighbouring layers. The supramolecular isostructurality of compounds (1)-(4) is ascribed to the synergistic effect of multiple interactions in these structures. The balanced strong and weak intermolecular interactions stabilizing this structure type include strong charge-assisted N-H...O hydrogen bonds, C-H...O contacts and π-π interactions.