Structural rationalisation of co-crystals formed between trithiocyanuric acid and molecules containing hydrogen bonding functionality

Crystallisation of trithiocyanuric acid (TTCA) from various organic solvents that have hydrogen bonding capability (acetone, 2-butanone, dimethylformamide, dimethyl sulfoxide, methanol and acetonitrile) leads to the formation of co-crystals in which the solvent molecules are incorporated together with TTCA in the crystal structure. Structure determination by single-crystal X-ray diffraction reveals that these co-crystals can be classified into different groups depending upon the topological arrangement of the TTCA molecules in the crystal structure. Thus, three different types of single-tape arrangements of TTCA molecules and one type of double-tape arrangement of TTCA molecules are identified. In all co-crystals, hydrogen-bonding interactions are formed through the involvement of N-H bonds of TTCA molecules in these tapes and the other molecule in the co-crystal. Detailed rationalisation of the structural properties of these co-crystals is presented.     

Conformational polymorphism in a cobalt(II) dithiocarbamate complex

Two conformational polymorphs of (N,N‐dibutyldithiocarbamato‐κ²S,S′)[tris(3,5‐diphenylpyrazol‐1‐yl‐κN²)hydroborato]cobalt(II), [Co(C₄₅H₃₄BN₆)(C₉H₁₈NS₂)] or [Tp^Ph2Co(S₂CNBu₂)], 1 , are accessible by recrystallization from dichloromethane–methanol to give orthorhombic polymorph 1a , while slow evaporation from acetonitrile produces triclinic polymorph 1b . The two polymorphs have been characterized by IR spectroscopy and single‐crystal X‐ray crystallography at 150 K. Polymorphs 1a and 1b crystallize in the orthorhombic space group Pbca and the triclinic space group P, respectively. The polymorphs have a trans ( 1a ) and cis ( 1b ) orientation of the butyl groups with respect to the S₂CN plane of the dithiocarbamate ligand, which results in an intermediate five‐coordinate geometry for 1a and a square‐pyramidal geometry for 1b . Hirshfeld surface analysis reveals minor differences between the two polymorphs, with 1a exhibiting stronger C—H…S interactions and 1b favouring C—H…π interactions.     

Vibrational Circular Dichroism Spectroscopy of Three Multidentate Nitrogen Donor Ligands: Conformational Flexibility and Solvent Effects

A series of multidentate nitrogen donor ligands have been synthesized and characterized and their conformational distributions in solution have been investigated. Vibrational absorption (VA) and vibrational circular dichroism (VCD) spectroscopy, complemented with DFT calculations, have been used to probe the conformations of these important ligands in solution directly. These three ligands demonstrate very different conformational flexibility; the pyridine subunits and amine groups may adopt a number of different conformations. Experimental VA and VCD data measured in CDCl₃ have been compared to the theoretical spectra of all possible most stable conformers. Solvent effects have been taken into account by using the implicit polarizable continuum model and explicit solvation model. The explicit hydrogen‐bonding solvation model is important for explaining the VCD sign‐reverse phenomenon in the amide I region. Good agreement has been achieved between experimental and predicted spectra for all three ligands; thus allowing detailed examination of the related conformational structures and distributions in solution.     

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.     

Detection of Significant Aprotic Solvent Effects on the Conformational Distribution of Methyl 4‐Nitrophenyl Sulfoxide: From Gas‐Phase Rotational to Liquid‐Crystal NMR Spectroscopy

The conformational equilibrium of methyl 4‐nitrophenyl sulfoxide (MNPSO) was experimentally investigated in the gas phase by using microwave spectroscopy and in isotropic and nematic liquid‐crystal solutions, in which the solvents are nonaqueous and aprotic, by using NMR spectroscopy; moreover, it was theoretically studied in vacuo and in solution at different levels of theory. The overall set of results indicates a significant dependence of the solute conformational distribution on the solvent dielectric permittivity constant: when dissolved in low‐polarity media, the most stable conformation of MNPSO proved to be strongly twisted with respect to that in more polar solvents, in which the conformational distribution maximum essentially coincides with that obtained in the gas phase. We discuss a possible explanation of this behavior, which rests on electrostatic solute–solvent interactions and is supported by calculations of the solute electric dipole moment as a function of the torsional angle. This function shows that the least polar conformation of MNPSO is located at a twist angle close to that of the conformational distribution maximum found in less‐polar solvents. This fact, associated with a relatively flat torsional potential, can justify the stabilization of the twisted conformation by the less‐polar solvents.     

Kitaigorodsky Revisited: Polymorphism and Mixed Crystals of Acridine/Phenazine

Non‐stoichiometric molecular mixed crystals have potential as functional materials, the properties of which can be tailored by rationally changing their composition. The guidelines for their preparation were summarized over thirty years ago by Alexander Kitaigorodsky. Here those principles are revised in light of new studies on the acridine/phenazine system, and solvent‐assisted grinding is presented as a convenient synthetic procedure to afford a more homogeneous product than traditional solvent‐evaporation methods. Finally, the proposed prerequisite of crystal isostructurality/isomorphicity for the pure compounds, which seems to be violated in the present case, is discussed.     

Conformational identity change of 3-methylbutanone from gaseous phase to solutions: An IR vibrational spectroscopy study

Infrared vibrational spectroscopy of 3-methylbutanone [Me(CO)iPr] leads to two conclusions: (1) The conformational identity is different in the gas phase and in various solvents. (2) In the gas phase, type B rovibrational structures are observed. Thus, the molecular symmetry isC ₅. The following interpretation is based upon a model which implicitly takes the solvent into account in the framework of an empirical calculation. The solvent increases the interconversion barrier between two enantiomers. As a consequence, the molecule changes conformation, moving from the stable conformation with bisected carbonyl seen in the gas phase, to a conformation with an eclipsed carbonyl in solutions.     

Conformational analysis: a new approach by means of chemometrics

In conformational analysis, the systematic search method completely maps the space but suffers from the combinatorial explosion problem because the number of conformations increases exponentially with the number of free rotation angles. This study introduces a new methodology of conformational analysis that controls the combinatorial explosion. It is based on a dimensional reduction of the system through the use of principal component analysis. The results are exactly the same as those obtained for the complete search but, in this case, the number of conformations increases only quadratically with the number of free rotation angles. The method is applied to a series of three drugs: omeprazole, pantoprazole, lansoprazole-benzimidazoles that suppress gastric-acid secretion by means of H+, K+-ATPase enzyme inhibition.     

Hirshfeld surface analysis of sulfameter (polymorph III), sulfameter dioxane monosolvate and sulfameter tetrahydrofuran monosolvate,all at 296 K

The ability of the antibacterial agent sulfameter (SMT) to form solvates is investigated. The X‐ray crystal structures of sulfameter solvates have been determined to be conformational polymorphs. Both 1,4‐dioxane and tetrahydrofuran form solvates with sulfameter in a 1:1 molar ratio. 4‐Amino‐N‐(5‐methoxypyrimidin‐2‐yl)benzenesulfonamide (polymorph III), C₁₁H₁₂N₄O₃S, (1), has two molecules of sulfameter in the asymmetric unit cell. 4‐Amino‐N‐(5‐methoxypyrimidin‐2‐yl)benzenesulfonamide 1,4‐dioxane monosolvate, C₁₁H₁₂N₄O₃S·C₄H₈O₂, (2), and 4‐amino‐N‐(5‐methoxypyrimidin‐2‐yl)benzenesulfonamide tetrahydrofuran monosolvate, C₁₁H₁₂N₄O₃S·C₄H₈O, (3), crystallize in the imide form. Hirshfeld surface analyses and fingerprint analyses were performed to study the nature of the interactions and their quantitative contributions towards the crystal packing. Finally, Hirshfeld surfaces, fingerprint plots and structural overlays were employed for a comparison of the two independent molecules in the asymmetric unit of (1), and also for a comparison of (2) and (3) in the monoclinic crystal system. A three‐dimensional hydrogen‐bonding network exists in all three structures, involving one of the sulfone O atoms and the aniline N atom. All three structures are stabilized by strong intermolecular N—H...N interactions. The tetrahydrofuran solvent molecule also takes part in forming significant intermolecular C—H...O interactions in the crystal structure of (3), contributing to the stability of the crystal packing.