Conformational polymorphism

Co-crystals, Salts or Mixtures of Both? The Case of Tenofovir Alafenamide Fumarates

Tenofovir alafenamide fumarate (TAF) is the newest prodrug of tenofovir that constitutes several drug products used for the treatment of HIV/AIDS. Although the solid-state properties of its predecessor tenofovir disoproxil fumarate have been investigated and described in the literature, there are no data in the scientific literature on the solid state properties of TAF. In our report, we describe the preparation of two novel polymorphs II and III of tenofovir alafenamide monofumarate (TA MF2 and TA MF3). The solid-state structure of these compounds was investigated in parallel to the previously known tenofovir alafenamide monofumarate form I (TA MF1) and tenofovir alafenamide hemifumarate (TA HF). Interestingly, the single-crystal X-ray diffraction of TA HF revealed that this derivative exists as a co-crystal form. In addition, we prepared a crystalline tenofovir alafenamide free base (TA) and its hydrochloride salt (TA HCl), which enabled us to determine the structure of TA MF derivatives using ¹⁵N-ssNMR (¹⁵N-solid state nuclear magnetic resonance). Surprisingly, we observed that TA MF1 exists as a mixed ionization state complex or pure salt, while TA MF2 and TA MF3 can be obtained as pure co-crystal forms.

Conformational equilibria of pharmaceuticals in supercritical CO2, IR spectroscopy and quantum chemical calculations

In this work we demonstrate a self-consistent effective technique of analyzing the conformational equilibria of active pharmaceutical ingredient (API) molecules dissolved in supercritical carbon dioxide in a wide range of thermodynamic parameters of state. This approach can be useful for pharmaceutics when the crystalline forms of pharmaceuticals with a high purity degree and desirable polymorphism are produced using CO₂-based supercritical fluids technologies. Within this approach we use a combination of quantum chemical calculations and in situ IR spectroscopy. Quantum chemical calculations allow us to perform the initial conformational search and to determine the energy characteristics of the most stable conformers of API and the energy barriers of transitions between them. IR spectroscopy gives the information on the equilibrium of the most stable conformers of pharmaceuticals dissolved in scCO₂ in the thermodynamic parameter range of interest. Finally we validate our approach by applying it to the study of carbamazepine dissolved in scCO₂ being in permanent contact with an excess of crystalline carbamazepine as an example. The conformational search for carbamazepine molecules in scCO₂ was also performed using molecular dynamics simulation for comparison with the results obtained by the technique presented in this paper.

Color polymorphism in organic crystals

Color polymorphism is an interesting property of chemical systems which present crystal polymorphs of different colors. It is a rare phenomenon, with only a few examples reported in the literature hitherto. Nevertheless, systems exhibiting color polymorphism have many potential applications in different domains, such as pigment, sensor, and technology industries. Here, known representative chemical systems showing color polymorphism are reviewed, and the reasons for them to present such property discussed. Also, since some of the concepts related to color polymorphism have been frequently used imprecisely in the scientific literature, this article provides concise, systematic definitions for these concepts.

Crystallization and single molecule magnetic behavior of quadruple-stranded helicates: tuning the anisotropic axes

It has been successfully proven that the bis-β-diketone ligands with proper lengths and flexibility are essential for the construction of multiple-stranded helicates, where two Dy3+ centers in subtly different environments allow the tuning of the anisotropic axes. Based on our previous work, we have designed a flexible bis-β-diketone ligand BTT (BTT = 3,3''-bis(4,4,4-trifluoro-1,3-dioxobutyl)-m-terphenyl), which is successfully utilized to construct quadruple-stranded helicates with the formula of [C6H16N]2[Ln2(BTT)4]·2CH2Cl2·4CH3OH [Ln = Ce (1), Dy (2)]. Structural analysis indicates that they crystallize in the tetragonal space group P4/n, and each Ln3+ center is chelated by four diketonate moieties from four ligands, giving rise to a dinuclear quadruple-stranded helicate. Magnetic measurements show that 2 displays single molecular magnet behavior under an applied DC field of 2000 Oe. Further investigations indicate that the anisotropic axes of the Dy3+ centers can be tuned depending on the bis-β-diketonate ligands used to assemble the Dy helicates.

Calculation of Diamagnetic Susceptibility Tensors of Organic Crystals: From Coronene to Pharmaceutical Polymorphs

Understanding why crystallization in strong magnetic fields can lead to new polymorphs requires methods to calculate the diamagnetic response of organic molecular crystals. We develop the calculation of the macroscopic diamagnetic susceptibility tensor, χ^cryst, for organic molecular crystals using periodic density functional methods. The crystal magnetic susceptibility tensor, χ^cryst, for all experimentally known polymorphs, and its molecular counterpart, χ^mol, are calculated for flexible pharmaceuticals such as carbamazepine, flufenamic acid, and chalcones, and rigid molecules, such as benzene, pyridine, acridine, anthracene, and coronene, whose molecular magnetic properties have been traditionally studied. A tensor addition method is developed to approximate the crystal diamagnetic susceptibility tensor, χ^cryst, from the molecular one, χ^mol, giving good agreement with those calculated directly using the more costly periodic density functional method for χ^cryst. The response of pharmaceutical molecules and crystals to magnetic fields, as embodied by χ^cryst, is largely determined by the packing in the crystal, as well as the molecular conformation. The anisotropy of χ^cryst can vary considerably between polymorphs though the isotropic terms are fairly constant. The implications for developing a computational method for predicting whether crystallization in a magnetic field could produce a novel or different polymorph are discussed.

Manipulation of Pharmaceutical Polymorphic Transformation Process Using Excipients

BACKGROUND

In the pharmaceutical field, it is vital to ensure a consistent product containing a single solid-state form of the active pharmaceutical ingredient (API) in the drug product. However, some APIs are suffering from the risk of transformation of their target forms during processing, formulation and storage.

METHODS

The purpose of this review is to summarize the relevant category of excipients and demonstrate the availability and importance of using excipients as a key strategy to manipulate pharmaceutical polymorphic transformation.

RESULTS

The excipient effects on solvent-mediated phase transformations, solid-state transitions and amorphous crystallization are significant. Common pharmaceutical excipients including amino acids and derivatives, surfactants, and various polymers and their different manipulation effects were summarized and discussed.

CONCLUSION

Appropriate use of excipients plays a role in manipulating polymorphic transformation process of corresponding APIs, with a promising application of guaranteeing the stability and effectiveness of drug dosage forms.

Conformational Heterogeneity and Self-Assembly of α,β,γ-Hybrid Peptides Containing Fenamic Acid: Multistimuli-Responsive Phase-Selective Gelation

The effect of fenamic acid-α-aminoisobutyric acid corner motif in α,β,γ-hybrid peptides has been reported. From X-ray single-crystal diffraction studies, it is observed that Phe-containing peptide 1 has an "S"-shaped conformation that is stabilized by two consecutive intramolecular N-H···N hydrogen bonds. However, the tyrosine analogue peptide 2 has an "S"-shaped conformation, which is stabilized by consecutive intramolecular six-member N-H···N and seven-member N-H···O hydrogen bonds. The asymmetric unit of peptide 3 containing m-aminobenzoic acid has two molecules which are stabilized by multiple intermolecular hydrogen-bonding interactions. There are also π-π stacking interactions between the aromatic rings of fenamic acid. The peptides 1 and 2 have a polydisperse microsphere morphology, but peptide 3 has an entangled fiber-like morphology. Peptides 1-3 do not form organogels. However, in the presence of water, the peptide 3 forms a phase-selective instant gel in xylene. The gel exhibits high stability and thermal reversibility. The phase-selective gel of peptide 3 is highly responsive to H₂SO₄.

Mechanistic Insight toward Understanding the Role of Charge in Thiourea Organocatalysis

Pyranylation and glycosylation are pivotal for accessing a myriad of natural products, pharmaceuticals, and drug candidates. Catalytic approaches for enabling these transformations are of utmost importance and integral to advancing this area of synthesis. In exploring this chemical space, a combined experimental and computational mechanistic study of pyranylation and 2-deoxygalactosylation catalyzed by a cationic thiourea organocatalyst is reported. To this end, a thiourea-cyclopropenium organocatalyst was employed as a model system in combination with an arsenal of mechanistic techniques, including ¹³C kinetic isotope effect experiments, deuterated labeling studies, variable-temperature ¹H NMR spectroscopy, and density functional theory calculations. From these studies, two distinct reaction pathways were identified for this transformation corresponding to either dual hydrogen bond (H-bond) activation or Brønsted acid catalysis. The former involving thiourea orchestrated bifurcated hydrogen bonding proceeded in an asynchronous concerted fashion. In contrast, the latter stepwise mechanism involving Brønsted acid catalysis hinged upon the formation of an oxocarbenium intermediate accompanied by subsequent alcohol addition.

Polymorphism in natural alkamides from Aniba riparia (Nees) Mez (Lauraceae)

The elucidation of crystal structures of polymorphs of riparins I, II, and III combined with thermal analysis studies allowed the determination of the thermodynamic relationships between polymorphic pairs.

Is it usual to be unusual? An investigation into molecular conformations in organic crystals

Molecular conformations can influence the structure and properties of crystalline solids.

Reply to the ‘Comment on “Polymorphism of levofloxacin: structure, properties and phase transformation”’ by Tejender S. Thakur, CrystEngComm, 2020, , DOI: 10.1039/C9CE01400D

Minor structural differences in molecular conformation were found where the levofloxacin molecule takes a less stable conformer in energy rather than the stable conformer as previously expected.

Polymorphs of 2,4,6-tris(4-pyridyl)-1,3,5-triazine and their mechanical properties

The second polymorph of the compound 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT) is reported, TPT-II, which crystallizes in space group I2/a. Its higher density and more efficient space filling indicate the lower entropy of TPT-II, while its slightly lower melting point indicates its weaker intermolecular interactions. The conditions of the crystallization experiments for TPT-I and TPT-II are the dominant factors that determine the final crystalline products. The crystals of TPT-II are long needles. They exhibit bending behaviour along the crystallographic b direction when a mechanical force is imposed perpendicular to it, and regain their original shape after the external stress is removed. The elasticity of the single crystals is interpreted in terms of intermolecular interactions and an energy framework analysis.

Polymorph selectivity of an AIE luminogen under nano-confinement to visualize polymer microstructures

Despite the huge progress of luminescent molecular assemblies over the past decade, it is still challenging to understand their confined behavior in semi-crystalline polymers for constrained space recognition. Here, we report a polymorphic luminogen with aggregation-induced emission (AIE), capable of selective growth in polymer amorphous and crystalline phases with distinct color. The polymorphic behaviors of the AIE luminogen embedded within the polymer network are dependent on the size of nano-confinement: a thermodynamically stable polymorph of the AIE luminogen with green emission is stabilized in the amorphous phase, while a metastable polymorph with yellow emission is confined in the crystalline phase. The information on polymer crystalline and amorphous phases is transformed into distinct fluorescence colors, allowing a single AIE luminogen as a fluorescent marker for visualization of polymer microstructures in terms of amorphous and crystalline phase distribution, quantitative polymer crystallinity measurement, and spatial morphological arrangement. Our findings demonstrate that confinement of the AIE luminogen in the polymer network can achieve free space recognition and also provide a correlation between microscopic morphologies and macroscopic optical signals. We envision that our strategy will inspire the development of other materials with spatial confinement to incorporate AIE luminogens for various applications.

A polymorphic AIEgen is capable of selective growth in amorphous and crystalline polymer phases with distinct color for microstructure visualization.

Multicomponent solid forms of the uric acid reabsorption inhibitor lesinurad and cocrystal polymorphs with urea: DFT simulation and solubility study

Lesinurad (systematic name: 2-{[5-bromo-4-(4-cyclopropylnaphthalen-1-yl)-4H-1,2,4-triazol-3-yl]sulfanyl}acetic acid, C₁₇H₁₄BrN₃O₂S) is a selective uric acid reabsorption inhibitor related to gout, which exhibits poor aqueous solubility. High-throughput solid-form screening was performed to screen for new solid forms with improved pharmaceutically relevant properties. During polymorph screening, we obtained two solvates with methanol (CH₃OH) and ethanol (C₂H₅OH). Binary systems with caffeine (systematic name: 3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione, C₈H₁₀N₄O₂) and nicotinamide (C₆H₆N₂O), polymorphs with urea (CH₄N₂O) and eutectics with similar drugs, like allopurinol and febuxostat, were prepared using the crystal engineering approach. All these novel solid forms were confirmed by XRD, DSC and FT-IR. The crystal structures were solved by single-crystal and powder X-ray diffraction. The crystal structures indicate that the lesinurad molecule is highly flexible and the triazole moiety, along with the rotatable thioacetic acid (side chain) and cyclopropane ring, is almost perpendicular to the planar naphthalene moiety. The carboxylic acid-triazole heterosynthon in the drug is interrupted by the presence of methanol and ethanol molecules in their crystal structures and forms intermolecular macrocyclic rings. The caffeine cocrystal maintains the consistency of the acid-triazole heterosynthons as in the drug and, in addition, they are bound by several auxiliary interactions. In the binary system of nicotinamide and urea, the acid-triazole heterosynthon is replaced by an acid-amide synthon. Among the urea cocrystal polymorphs, Form I (P-1, 1:1) consists of an acid-amide (urea) heterodimer, whereas in Form II (P2₁/c, 2:2), both acid-amide heterosynthons and urea-urea dimers co-exist. Density functional theory (DFT) calculations further support the experimentally observed synthon hierarchies in the cocrystals. Aqueous solubility experiments of lesinurad and its binary solids in pH 5 acetate buffer medium indicate the apparent solubility order lesinurad-urea Form I (43-fold) > lesinurad-caffeine (20-fold) > lesinurad-allopurinol (12-fold) ≃ lesinurad-nicotinamide (11-fold) > lesinurad, and this order is correlated with the crystal structures.

Crystal structure analysis of the biologically active drug mol-ecule riluzole and riluzolium chloride

This study is an investigation into the crystal structure of the biologically active drug mol-ecule riluzole [RZ, 6-(tri-fluoro-meth-oxy)-1,3-benzo-thia-zol-2-amine], C8H5F3N2OS, and its derivative, the riluzolium chloride salt [RZHCl, 2-amino-6-(tri-fluoro-meth-oxy)-1,3-benzo-thia-zol-3-ium chloride], C8H6F3N2OS+·Cl-. In spite of repeated efforts to crystallize the drug, its crystal structure has not been reported to date, hence the current study provides a method for obtaining crystals of both riluzole and its corresponding salt, riluzolium hydro-chloride. The salt was obtained by grinding HCl with the drug and crystallizing the obtained solid from di-chloro-methane. The crystals of riluzole were obtained in the presence of l-glutamic acid and d-glutamic acid in separate experiments. In the crystal structure of RZHCl, the -OCF3 moiety is perpendicular to the mol-ecular plane containing the riluzolium ion, as can be seen by the torsion angle of 107.4 (3)°. In the case of riluzole, the torsion angles of the four different mol-ecules in the asymmetric unit show that in three cases the tri-fluoro-meth-oxy group is perpendicular to the riluzole mol-ecular plane and only in one mol-ecule does the -OCF3 group lie in the same mol-ecular plane. The crystal structure of riluzole primarily consists of strong N-H⋯N hydrogen bonds along with weak C-H⋯F, C-H⋯S, F⋯F, C⋯C and C⋯S inter-actions, while that of its salt is stabilized by strong [N-H]+⋯Cl- and weak C-H⋯Cl-, N-H⋯S, C-H⋯F, C⋯C, S⋯N and S⋯Cl- inter-actions.

Polymorph Selection of ROY by Flow-Driven Crystallization

The selection of polymorphs of the organic compound 5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile, ROY, is studied experimentally in the confined space between two horizontal glass plates when an acetone solution of ROY of variable concentration is injected at a variable flow rate into water. Depending on the local concentration within the radial flow, a polymorph selection is observed such that red prisms are favored close to the injection center while yellow needles are the preferred polymorph close to the edge of the injected ROY domain. At larger flow rates, a buoyancy-driven instability induces stripes at the outer edge of the displacement pattern, in which specific polymorphs are seen to crystallize. Our results evidence the possibility of a selection of ROY polymorph structures in out-of-equilibrium flow conditions.

Supramolecular Similarity in Polymorphs: Use of Similarity Indices (IX)

A systematic investigation to assess the degree of similarity between polymorphs was carried out. A similarity indices (I^X) approach was applied in ten series of polymorphs with different characteristics and number of molecules in the asymmetric unit. Geometric (I^D), contact area (I^C), and stabilization energy (I^G) parameters were used. It was possible to situate each comparison in different regions of similarity within the polymorphism phenomenon and determine the boundaries between quasi-isostructural polymorphs and polymorphs of low similarity. The multiparameter I^DCG index was used as a robust tool to determine the total similarity within the polymorphism phenomenon. The highest contribution of the stabilization energy parameter (45%) toward the final value of similarity (I^DCG) was observed, followed by the contact area index (32%). The geometric index contributed approximately 23% to the final value of I^DCG. This information reinforces the importance of the contact area and stabilization energy in assessing the degree of similarity between crystalline structures. A new descriptor (I^Q) based on the comparison of the energetic contribution of intermolecular interaction types present in each crystal structure is presented. I^Q can be a versatile tool and applicable even for systems that do not share any similarity.

A Million Crystal Structures: The Whole Is Greater than the Sum of Its Parts

The founding in 1965 of what is now called the Cambridge Structural Database (CSD) has reaped dividends in numerous and diverse areas of chemical research. Each of the million or so crystal structures in the database was solved for its own particular reason, but collected together, the structures can be reused to address a multitude of new problems. In this Review, which is focused mainly on the last 10 years, we chronicle the contribution of the CSD to research into molecular geometries, molecular interactions, and molecular assemblies and demonstrate its value in the design of biologically active molecules and the solid forms in which they are delivered. Its potential in other commercially relevant areas is described, including gas storage and delivery, thin films, and (opto)electronics. The CSD also aids the solution of new crystal structures. Because no scientific instrument is without shortcomings, the limitations of CSD research are assessed. We emphasize the importance of maintaining database quality: notwithstanding the arrival of big data and machine learning, it remains perilous to ignore the principle of garbage in, garbage out. Finally, we explain why the CSD must evolve with the world around it to ensure it remains fit for purpose in the years ahead.

A theoretical NMR study of polymorphism in crystal structures of azoles and benzazoles

The NMR chemical shifts of two azoles and one benzazole whose crystal structures present polymorphism have been computed using the GIPAW approach. ¹⁵ N and ¹³ C nuclei have been studied. Statistical analysis of the computed ¹³ C and ¹⁵ N chemical shifts indicates that the GIPAW chemical shifts reproduce with a high degree of accuracy those experimentally reported. This methodology can be used to identify other polymorphic crystal structures.

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.

Charge Distributions of Nitro Groups Within Organic Explosive Crystals: Effects on Sensitivity and Modeling

The charge distribution of NO₂ groups within the crystalline polymorphs of energetic materials strongly affects their explosive properties. We use the recently introduced basis-space iterated stockholder atom partitioning of high-quality charge distributions to examine the approximations that can be made in modeling polymorphs and their physical properties, using 1,3,5-trinitroperhydro-1,3,5-triazine, trinitrotoluene, 1-3-5-trinitrobenzene, and hexanitrobenzene as exemplars. The NO₂ charge distribution is strongly affected by the neighboring atoms, the rest of the molecules, and also significantly by the NO₂ torsion angle within the possible variations found in observed crystal structures. Thus, the proposed correlations between the molecular electrostatic properties, such as trigger-bond potential or maxima in the electrostatic potential, and impact sensitivity will be affected by the changes in conformation that occur on crystallization. We establish the relationship between the NO₂ torsion angle and the likelihood of occurrence in observed crystal structures, the conformational energy, and the charge and dipole magnitude on each atom, and how this varies with the neighboring groups. We examine the effect of analytically rotating the atomic multipole moments to model changes in torsion angle and establish that this is a viable approach for crystal structures but is not accurate enough to model the relative lattice energies. This establishes the basis of transferability of the NO₂ charge distribution for realistic nonempirical model intermolecular potentials for simulating energetic materials.

Exfoliating a CdII -Purine Framework: Conversion of Nanosheets-to-Nanofibers and Studies of Elastic and Capacitive Properties

Layered bulk crystals are amenable to exfoliation to yield 2D nanosheets through isolation and intercalation processes, which could be further converted to 1D nanoscale structures. The latter inherit gross morphological and physical properties associated with the precursor structures. Herein, we report three purine-based crystal structures 1, 2, and 3, where 3 is obtained by a single-crystal-to-single-crystal transformation from 2 and is a conformational polymorph of 1. Next, we describe the sonication-assisted liquid exfoliation of 1, a Cd^II -purine coordination framework, into nanosheets and nanofibers in a solvent-dependent process. The exfoliation was carefully studied at low temperatures to ascertain this unique conversion. This work also features the determination of the Young's modulus and surface potential of the bioinspired Cd^II -based nanostructures by using amplitude modulation-frequency modulation atomic force microscopy and Kelvin probe force microscopy, respectively, revealing their interesting elastic and capacitive properties for their possible use in electronics and energy devices. Electron impedance spectroscopy measurements further established a higher value of capacitance for the exfoliated Cd^II framework as compared to the ligand alone.

Crystal Structure Optimization and Gibbs Free Energy Comparison of Five Sulfathiazole Polymorphs by the Embedded Fragment QM Method at the DFT Level

Molecular crystal plays an important role in many fields of science and technology, but it often crystallizes in different polymorphs with different physical properties. To guide the experimental synthesis of candidate materials, the atomic-scale model is frequently used to predict the most stable polymorph and its structural properties. Here, we show how an ab initio method can be used to achieve a rapid and accurate prediction of sulfathiazole crystal polymorphs (an antibiotic drug), based on the Gibbs free energy calculation and Raman spectra analysis. At the atmospheric pressure and the temperature of 300 K, we demonstrate that form III (FIII) is the most stable structure of sulfathiazole. The agreement between the predicted and experimental crystal structures corresponds to the order of stability for five sulfathiazole polymorphs as FI < FV < FIV < FII < FIII, which is achieved by employing the density functional theory (DFT) calculations.

Structural, Magnetic, and Optical Studies of the Polymorphic 9'-Anthracenyl Dithiadiazolyl Radical

The fluorescent 9'-anthracenyl-functionalized dithiadiazolyl radical (3) exhibits four structurally determined crystalline phases, all of which are monomeric in the solid state. Polymorph 3α (monoclinic P2₁/ c, Z' = 2) is isolated when the radical is condensed onto a cold substrate (enthalpically favored polymorph), whereas 3β (orthorhombic P2₁ 2₁ 2₁, Z' = 3) is collected on a warm substrate (entropically favored polymorph). The α and β polymorphs exhibit chemically distinct structures with 3α exhibiting face-to-face π-π interactions between anthracenyl groups, while 3β exhibits edge-to-face π-π interactions. 3α undergoes an irreversible conversion to 3β on warming to 120 °C (393 K). The β-phase undergoes a series of reversible solid-state transformations on cooling; below 300 K a phase transition occurs to form 3γ (monoclinic P2₁/ c, Z' = 1), and on further cooling below 165 K, a further transition is observed to 3δ (monoclinic P2₁/ n, Z' = 2). Both 3β → 3γ and 3γ → 3δ transitions are reversible (single-crystal X-ray diffraction), and the 3γ → 3δ process exhibits thermal hysteresis with a clear feature observed by heat capacity measurements. Heating 3β above 160 °C generates a fifth polymorph (3ε) which is distinct from 3α-3δ based on powder X-ray diffraction data. The magnetic behavior of both 3α and the 3β/3γ/3δ system reflect an S = 1/2 paramagnet with weak antiferromagnetic coupling. The reversible 3δ ↔ 3γ phase transition exhibits thermal hysteresis of 20 K. Below 50 K, the value of χ_m T for 3δ approaches 0 emu·K·mol^-1 consistent with formation of a gapped state with an S = 0 ground-state configuration. In solution, both paramagnetic 3 and diamagnetic [3][GaCl₄] exhibit similar absorption and emission profiles reflecting similar absorption and emission mechanisms for paramagnetic and diamagnetic forms. Both emit in the deep-blue region of the visible spectrum (λ_em ∼ 440 nm) upon excitation at 255 nm with quantum yields of 4% (3) and 30% ([3][GaCl₄]) affording a switching ratio [Φ_F(3⁺)/Φ_F(3)] of 7.5 in quantum efficiency with oxidation state. Solid-state films of both 3 and [3][GaCl₄] exhibit emission bands at a longer wavelength (490 nm) attributed to excimer emission.

Exploring concomitant/conformational dimorphism in a difluoro-substituted phosphoramidate derivative

The concomitant occurrence of dimorphs of diphenyl (3,4-difluorophenyl)phosphoramidate, C₁₈H₁₄F₂NO₃P, was observed via a solution-mediated crystallization process with variation in the symmetry-free molecules (Z'). The existence of two forms, i.e. Form I (block, Z' = 1) and Form II (needle, Z' = 2), was characterized by single-crystal X-ray diffraction, differential scanning calorimetry and powder X-ray diffraction. Furthermore, a quantitative analysis of the energetics of the different intermolecular interactions was carried out via the energy decomposition method (PIXEL), which corroborates with inputs from the energy framework and looks at the topology of the various intermolecular interactions present in both forms. The unequivocally distinguished contribution of strong N-H...O hydrogen bonds along with other interactions, such as C-H...O, C-H...F, π-π and C-H...π, mapped on the Hirshfeld surface is depicted by two-dimensional fingerprint plots. Apart from the major electrostatic contribution from N-H...O hydrogen bonds, the crystal structures are stabilized by contributions from the dispersion energy. The closely related melting points and opposite trends in the calculated lattice energies are interesting to investigate with respect to the thermodynamic stability of the observed dimorphs. The significant variation in the torsion angles in both forms helps in classifying them in the category of conformational polymorphs.

Acridine form IX

A new polymorph of acridine was obtained during a study of the polymorphism of that mol­ecule. This structure was previously predicted in a computational search.

We report a new polymorph of acridine, C₁₃H₉N, denoted form IX, obtained as thin needles by slow evaporation of a toluene solution. The structure was solved and refined from powder X-ray data. The structures of five unsolvated forms were previously known, but this is only the second with one mol­ecule in the asymmetric unit. The melting point [differential scanning calorimetry (DSC) onset] and heat of fusion are 108.8 (3) °C and 19.2 (4) kJ mol⁻¹, respectively.

Challenges in the use of atomistic simulations to predict solubilities of drug-like molecules

Background: Solubility is a physical property of high importance to the pharmaceutical industry, the prediction of which for potential drugs has so far been a hard task. We attempted to predict the solubility of acetylsalicylic acid (ASA) by estimating the absolute chemical potentials of its most stable polymorph and of solutions with different concentrations of the drug molecule. Methods: Chemical potentials were estimated from all-atom molecular dynamics simulations.  We used the Einstein molecule method (EMM) to predict the absolute chemical potential of the solid and solvation free energy calculations to predict the excess chemical potentials of the liquid-phase systems. Results: Reliable estimations of the chemical potentials for the solid and for a single ASA molecule using the EMM required an extremely large number of intermediate states for the free energy calculations, meaning that the calculations were extremely demanding computationally. Despite the computational cost, however, the computed value did not agree well with the experimental value, potentially due to limitations with the underlying energy model. Perhaps better values could be obtained with a better energy model; however, it seems likely computational cost may remain a limiting factor for use of this particular approach to solubility estimation.    Conclusions: Solubility prediction of drug-like solids remains computationally challenging, and it appears that both the underlying energy model and the computational approach applied may need improvement before the approach is suitable for routine use.

Orientation-dependent conformational polymorphs in two similar pyridine/pyrazine phenolic esters

The ring orientations in the conformational polymorphs of two similar pyridine/pyrazine phenolic esters were investigated to explore the effect of supramolecular assemblies in the solid state.

Separation or combination of non-covalently linked partners provides polymorphs of N-(aryl)-2-(propan-2-ylidene)hydrazine carbothioamides

Polymorphs of N-(2-methoxyphenyl)-2-(propan-2ylidene)hydrazine carbothioamide and N-(4-nitrophenyl)-2-(propan-2-ylidene)hydrazine carbothioamide differing in homomeric assemblies are described.

Coordination-induced conformation diversity for pharmaceutical polymorph control

Template-induced heteronucleation can dramatically influence crystal polymorphism.

Solid state photodimerization of 9-tert-butyl anthracene ester produces an exceptionally metastable polymorph according to first-principles calculations

Molecular crystal engineering seeks to tune the material properties by controlling the crystal packing.

Photo-physical properties of salts of a di-topic imidazole-tethered anthracene derivative in solid and solution

Combined effects of Dexter-quenching and protonation to change the photoluminescence of a di-topic anthracene–amine conjugate by nitrophenols and aromatic carboxylic acids in solid and solution are presented.

High pressure: a complementary tool for probing solid-state processes

High pressure offers insight into the mechanisms of a wide range of solid-state phenomena occurring under atmospheric pressure conditions.

Structural behaviour of OP-ROY at extreme conditions

A highly-flexible organic molecule, with numerous polymorphs at ambient conditions, shows remarkable metastability, resisting the effects of extreme pressure and temperature.

Conformational variation of ligands in mercury halide complexes; high and low Z′ structures

Small changes in the ligand resulted in a conformational variation of L^Py to L^Pz which led to high and low Z′ structures in the corresponding metal complexes.

Polymorphism and molecular conformations of nicosulfuron: structure, properties and desolvation process

Nine solid forms of nicosulfuron were found for the first time and their structures and properties were studied in detail.

Polymorphism in some new bis-hydrazone compounds

We describe the polymorphism of four new bis-hydrazone compounds, namely butane-2,3-dione 2,3-bis{[bis(4-fluorophenyl)methylidene]hydrazone}, C₃₀H₂₂F₄N₄ (1), butane-2,3-dione 2,3-bis{[bis(4-chlorophenyl)methylidene]hydrazone}, C₃₀H₂₂Cl₄N₄ (2), butane-2,3-dione 2,3-bis{[bis(4-methylphenyl)methylidene]hydrazone}, C₃₄H₃₄N₄ (3), and butane-2,3-dione 2,3-bis({bis[4-(dimethylamino)phenyl]methylidene}hydrazone), C₃₈H₄₆N₈ (4), derived by the condensation reaction between substituted benzophenone hydrazone and butane-2,3-dione. Concomitant polymorphism has been observed in 1, 2 and 3. Overlays of molecules of the different polymorphs indicate that there is conformational adjustment in the crystal structures of the polymorphs of 1 and 2, i.e. packing polymorphism, which was confirmed by a computational study. On the other hand, conformational change was observed in the cases of the polymorphs of compounds 3 and 4, i.e. conformational polymorphism.