Symmetry-independent Molecules Research Articles

Conformation of the hydrazo bond in new 2-methyl-3,5-dinitro-6-(2-phenylhydrazinyl)pyridine and its influence on the structural and optic properties – Quantum chemical DFT calculations

A new methyl-dinitro-phenylhydrazinyl-pyridine derivative [2-methyl-3,5-dinitro-6-(2-phenylhydrazinyl)pyridine] was synthesised and characterised by means of structural and spectroscopic measurements. The X-ray diffraction studies revealed that the compound crystallises in the centrosymmetric monoclinic space group P21/n, with two symmetry-independent molecules in the asymmetric unit with Z = 8. Hydrazo bridge C-NH-NH-C links two fragments composed of phenyl ring and pyridine unit substituted with methyl and nitro groups. Such a structure was confirmed by 1H and 13C NMR studies as well as IR, Raman, UV–Vis, and emission spectra. The results were analysed using the quantum-chemical DFT calculations. The paper reports the vibrational characteristics and discusses dynamical properties of this moiety. The full set of the normal modes typical of the hydrazo bridge was identified and assigned to respective IR and Raman bands. The results of structural and spectroscopic studies were used to find the dependence between the conformation of the θ–NH-NH-ϕ system and its optic properties. The experimental UV–Vis and emission spectra were discussed in terms of the calculated singlet and triplet states that allowed assigning the unique spectral pattern originating from the electrons of the hydrazo-bridge system.

In-silico investigation of 4-nitro-N-1H-pyrazol-3-ylbenzamide towards its potential use against SARS-CoV-2: a DFT, molecular docking and molecular dynamics study

In the present research work, we report the synthesis and characterization of novel pyrazole derivative obtained by the condensation reaction of 4-nitro benzaldehyde group with one equivalent of the 2-amino pyrazole yielding 4-nitro-N-1H-pyrazol-3-ylbenzamide with high yield. The two symmetry-independent molecules (molecule A and molecule B) differ about the central C-N bond, with the dihedral angles between the pyrazole ring system and the nitrobenzene ring being 13.90° and 18.64°, respectively. By optimizing the symmetry-independent dimer molecules, the rotational barrier between the conformers is found to be within the 2.5–5.5 kcal/mol range. QTAIM and RDG based NCI isosurface revealed the presence of strong N-H…N and C-H…O hydrogen bonds which stabilize the two independent centrosymmetric inversion-related dimers. Further, weak and short directional interactions such as C-H…N, H…H and C-H…π were also analyzed systematically using various topological parameters. The compound is found to adhere to the Lipinski’s rule of five and exhibit good pharmacokinetic properties. The results of molecular docking studies performed against SARS-CoV-2 virus main protease (PDB IDs: 6LU7, 6W9C and 6WQF) revealed that the compound showed better docking scores. Molecular docking studies verified the inhibition activity of the synthesized novel compound. Finally, the binding free energy and contributed energies were calculated using MM-GBSA method. The 6LU7-ligand complex showed highest binding free energy and among all other interactions, the contributions of the covalent binding and van der Waals energy are found to be significant. Communicated by Ramaswamy H. Sarma

Two polymorphs of fluoren-9-ylidene malononitrile

Fluoren-9-ylidene malononitrile (FM), a compound with two different crystal structures, has been explored to better understand its polymorphism. Two polymorphs were obtained from different solvents used in crystallization. The form 1, obtained from chloroform crystallizes in triclinic P1¯ space group. On the other hand, the form 2 obtained from acetonitrile crystallizes in orthorhombic Sohncke space group P212121. Two symmetry independent molecules (Z′ = 2) were observed in the asymmetric unit of form 1 representing it to be a rare type of crystals for which Z′ > 1. Distinctly different crystal packing was observed. While in form 1, the molecules stack infinitely in head-to-tail manner, in form 2, the molecules stack in head-to-head manner. The form 1 showed stronger intermolecular interaction compared to form 2, as evidenced by Hirshfeld surface analysis. In addition to PXRD, Raman, FT-IR spectroscopy measurements, analyses of shape index, curvedness, and 2-D finger print plots are discussed, which showed distinct features of these two polymorphs.

Design, synthesis and structural study of two symmetry-independent pyridone analogue in the asymmetric unit

In this research, we have reported the synthesis of Methyl-4-(4-chlorophenyl)-5-cyano-2-methyl-6-oxo-1,6-dihydropyridine-3-carboxylate which is a hybrid of two different biologically active structures. The obtained product was characterized by FT-IR, 1HNMR, 13C NMR, SCXRD and HR-MS. The obtained compound was recrystallized in ethanol giving colourless pure single crystals. SCXRD analysis showed two symmetry independent asymmetric molecules with interesting structural features. Multiple graph sets were also observed which supported the formation of a supramolecular network. DFT studies using B3LYP and M06–2X functions revealed two stable conformers which is in support of the crystal structures obtained. Furthermore, in silico molecular docking analysis in Eg5 kinesin and survivin protein revealed binding affinities of -6.9 kcal/mol and -6.1 kcal/mol respectively. All the results obtained suggests the need of further exploration to discover the complete pharmaceutical potential.

Synthesis and crystal structure studies of 5-(tri-fluoro-meth-yl)-1,3,4-thia-diazol-2(3H)-one at 180 K.

The synthesis and crystal structure of C3HF3N2OS, systematic name 5-(tri-fluoro-meth-yl)-1,3,4-thia-diazol-2(3H)-one (5-TMD-2-one), a compound containing the pharmacologically important heterocycle 1,3,4-thia-diazole, is presented. The asymmetric unit comprises six independent mol-ecules (Z' = 6), all of which are planar. The r.m.s. deviations from each mean plane range from 0.0063 to 0.0381 Å, not including the CF3 fluorine atoms. Within the crystal, two of the mol-ecules form hydrogen-bonded dimers that in turn combine with inversion-related copies to form tetra-meric constructs. Similar tetra-mers, but lacking inversion symmetry, are formed by the remaining four mol-ecules. The tetra-mers are linked into tape-like motifs by S⋯O and O⋯O close contacts. The environments of each symmetry-independent mol-ecule were compared via a Hirshfeld surface analysis. The most abundant atom-atom contacts are between fluorine atoms, while the strongest result from N-H⋯O hydrogen bonds.

Low-density preference of the ambient and high-pressure polymorphs of DL-menthol.

Lower-density polymorphs of DL-menthol were nucleated and crystallized in their high-pressure stability regions. Up to 0.30 GPa, the triclinic DL-menthol polymorph α, which is stable at atmospheric pressure, is less dense than a new β polymorph, which becomes stable above 0.40 GPa, but is less dense than the α polymorph at this pressure. The compression of polymorph α to at least 3.37 GPa is monotonic, with no signs of phase transitions. However, recrystallizations of DL-menthol above 0.40 GPa yield the β polymorph, which is less compressible and becomes less dense than α-DL-menthol. At 0.10 MPa, the melting point of the β polymorph is 14°C, much lower compared with those of α-DL-menthol (42-43°C) and L-menthol (36-38°C). The structures of both DL-menthol polymorphs α and β are very similar with respect to the lattice dimensions, the aggregation of OH...O molecules bonded into Ci symmetric chains, the presence of three symmetry-independent molecules (Z' = 3), their sequence ABCC'B'A', the disorder of the hydroxyl protons and the parallel arrangement of the chains. However, the different symmetries relating the chains constitute a high kinetic barrier for the solid-solid transition between polymorphs α and β, hence their crystallizations below or above 0.40 GPa, respectively, are required. In the structure of polymorph α, the directional OH...O bonds are shorter and the voids are larger compared with those in polymorph β, which leads to the reverse density relation of the polymorphs in their stability regions. This low-density preference reduces the Gibbs free-energy difference between the polymorphs: when polymorph α is compressed to above 0.40 GPa, the work component pΔV counteracts the transition to the less dense polymorph β, and on reducing the pressure of polymorph β to below 0.40 GPa, its transition to the less dense polymorph α is also hampered by the work contribution.

Schiff Base in Ketoamine Form and Rh(η4-cod)-Schiff Base Complex with Z′ = 2 Structure from Pairwise C-H···Metallochelate-π Contacts

Condensation of 2-hydroxybenzaldehyde (salicylaldehyde) or 2-hydroxy-1-naphthaldehyde with 2-ethylaniline yields the Schiff base compound of (E)-2-(((2-ethylphenyl)imino)methyl)phenol (HL1) or (E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-ol (HL2), which in turn react with the dinuclear complex of [Rh(η4-cod)(µ-O2CCH3)]2 (cod = cycloocta-1,5-diene) to afford the mononuclear (η4-cod){(E)-2-(((2-ethylphenyl)imino)methyl)phenolato-κ2N,O}rhodium(I), [Rh(η4-cod)(L1)] (1) or (η4-cod){(E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-olato-κ2N,O}rhodium(I), [Rh(η4-cod)(L2)] (2) (L1 or L2 = deprotonated Schiff base ligand). The X-ray structure determination revealed that the HL2 exists in the solid state not as the usual (imine)N···H-O(phenol) form (enolamine form) but as the zwitterionic (imine)N-H+···-O(phenol) form (ketoamine form). 1H NMR spectra for HL2 in different solvents demonstrated the existence of keto-enol tautomerism (i.e., keto ⇆ enol equilibrium) in solution. The structure for 1 and 2 showed that the deprotonated Schiff base ligand coordinates to the Rh(η4-cod)-fragment as a six-membered N^O-chelate around the rhodium atom with a close-to-square-planar geometry. Two symmetry-independent molecules (with Rh1 and Rh2) were found in the asymmetric unit in 1 in a structure with Z' = 2. The supramolecular packing in HL2 was organized by π-π and C-H···π contacts, while only two recognized C-H···π contacts were revealed in 1 and 2. Remarkably, there were reciprocal or pairwise C-H···π contacts between a pair of each of the symmetry-independent molecules in 1. This pairwise C-H contact to the Rh-N^O chelate (metalloaromatic) ring may be a reason for the two symmetry-independent molecules in 1. Differential scanning calorimetry (DSC) analyses revealed an irreversible phase transformation from the crystalline-solid to the isotropic-liquid phase and subsequently confirmed the thermal stability of the compounds. Absorption spectra in solution were explained by excited state properties from DFT/TD-DFT calculations.

Synthesis, crystal structure and Hirshfeld analysis of triphenylphosphine–(4-bromophenyl)borane

The title compound, alternatively called (4-bromophenyl)dihydrido(triphenylphosphanyl)boron, C24H21BBrP, crystallizes in the space group P-1 with one symmetry-independent molecule in the asymmetric unit. The B and P atoms both adopt the expected tetrahedral geometry. A possible intramolecular B-Hδ-...δ+H-C dihydrogen bond exists between an arene group on phosphorus and the borane center. In the solid state, the title molecule is connected to its neighbors by intermolecular C-H...π interactions, with the arene rings on the P atom acting as hydrogen-bond donors and the arene ring of the borane acting as the acceptor. Hirshfeld analysis supports the existence of these interactions, as well as weak Br...Br interactions between neighboring molecules.

Synthesis of a Pyridone-Based Phthalimide Fleximer and Its Characterization and Supramolecular Property Evaluation.

In this study, a novel pyridone-based phthalimide fleximer, that is, ethyl 5-cyano-6-(3-(1,3-dioxoisoindolin-2-yl)propoxy)-4-(3-methoxyphenyl)-2-methylnicotinate, was synthesized, and its structure was established by the single-crystal X-ray diffraction method. The supramolecular self-assembly of the titled compound through noncovalent interactions was then investigated thoroughly. The titled compound crystallized with two symmetry-independent molecules (A and B, Z′ = 2). In agreement with experimental observations, our density functional theory calculations also showed that the titled compound has a flexible motif and can occur in various conformations, including molecules A and B. The investigation of the supramolecular framework revealed that the molecules are notably bound by the nonclassical C–H···O and C–H···N hydrogen bonds and C–H···π interactions. Hirshfeld surface analysis was carried out to quantify the various intermolecular interactions. The dual anti-inflammatory activity of the tilted compound was also explored by molecular docking in the active sites of 5-LOX and COX-2 receptors, which revealed good binding affinities of −9.0 and −8.6 kcal/mol, respectively.

Packing Preferences of Chalcones: A Model Conjugated Pharmaceutical Scaffold.

We sought the crystal packing preferences of the chalcone scaffold by analyzing 232 single-component crystal structures of chalcones with a small (six or fewer non-hydrogen atoms) substituent on either or both rings, including the unsubstituted molecule. This covers 216 molecules, as some are polymorphic, and 277 independent molecular conformations, as 16% of the crystal structures have more than one symmetry independent molecule. Quantum mechanical conformational profiles of the unsubstituted molecule and the almost 5000 crystal structures within 20 kJ mol–1 of the global minimum generated in a crystal structure prediction (CSP) study have been used to complement this analysis. Although π conjugation would be expected to favor a planar molecule, there are a significant number of crystal structures containing nonplanar molecules with an approximately 50° angle between the aromatic rings. The relative orientations of the molecules in the inversion-related dimers and translation-related dimers in the experimental crystal structures show the same trends as in the CSP-generated structures for the unsubstituted molecule, allowing for the substituent making the side-to-side distances larger. There is no type of dimer geometry associated with particularly favorable lattice energies for the chalcone core. Less than a third of the experimental structures show a face-to-face contact associated with π···π stacking. Analysis of the experimental crystal structures with XPac and Mercury finds various pairs of isostructural crystals, but the largest isostructural set had only 15 structures, with all substituents (mainly halogens) in the para position. The most common one-dimensional motif, found in half of the experimental crystal structures, is a translation-related side-to-side packing, which can be adopted by all the observed conformations. This close-packed motif can be adopted by chalcones with a particularly wide variety of substituents as the substituents are at the periphery. Thus, although the crystal structures of the substituted chalcones show thermodynamically plausible packings of the chalcone scaffold, there is little evidence for any crystal engineering principle of preferred chalcone scaffold packing beyond close packing of the specific molecule.

Structural and catalytic properties of the [Ni(BIPHEP)X2] complexes, BIPHEP = 2,2-diphenylphosphino-1,1-biphenyl; X = Cl, Br

The synthesis and catalytic properties in Kumada CC coupling of the [Ni(BIPHEP)X2] complexes, X = Cl (1), Br (2), are described. The crystal structures of the BIPHEP ligand and 2 are also presented and compared with previously reported crystal structures of atropisomeric bidentate phosphine ligands (P,P) and related [M(P,P)X2] complexes (M = Ni, Pd, Pt). BIPHEP crystallizes in the C2/c space group, with both enantiomers present in the unit cell. This is consistent with BIPHEP being a “tropos” ligand. Complex 2 crystallizes in the P21/a space group. There are two symmetry-independent molecules in the asymmetric unit, namely 2a and 2b, in which the BIPHEP ligand adopts the S or the R configuration, respectively. Complexes 2a and 2b exhibit a severely tetrahedrally-distorted square planar NiP2Br2 coordination sphere, with a PNiP bite angle of 93.3° and 94.7°, respectively. The observed catalytic behavior of complexes 1 and 2 in the Kumada coupling between p-tert-butyl-halogenobenzene and p-tolylmagnesium chloride is benchmarked against that of [Ni(dppp)Cl2], dppp = 1,3-bis(diphenylphoshpino)propane. However, all three complexes are catalytically inactive in the Suzuki-Miyaura coupling reaction.

Hydrogen bonding features of amide functionalities in four novel salts of an antiemetic drug domperidone

Domperidone, 5-chloro-1-(1-[3-(2-oxo-2,3-dihydro-1H-benzo-[d] imidazol-1-yl)propyl]-piperidin-4-yl)-1H-benzo[d]imidazol-2(3H)-one, C22H24ClN5O2, is an antiemetic drug, used to relieve the patients from nausea and vomiting sickness. In the present study, four novel domperidone salts viz., domperidone chloride hydrate methanol solvate, (I), bis(domperidone nitrate nitric acid) hydrate, (II), domperidone nitrate 1.046 hydrate (III) and domperidone methylsulfate hemihydrate, (IV) are reported. Structure (I) crystallized in the monoclinic space group P21/n, while, structures of (II) - (IV) crystallized in the triclinic space group P1¯. In all four structures the N atom of piperidine ring is protonated and further confirmed by IR and 1H NMR analyses. In (I), (II) and molecule B of (III), the orientation of chlorine-substituted oxobenzimidazolyl and non-substituted oxobenzimidazolyl rings are in anti-clinal conformations, while molecule A of (III) and (IV), are in syn-clinal conformations. The protonated piperidine ring adopts a chair conformation in all four structures. In (I), inversion-related domperidone molecules are involved in forming a R22(28) dimer, while in (II), symmetry independent domperidone molecules A and B are involved in constructing the R22(28) dimer. In (III) and (IV), two sets of amide R22(8) dimer are observed. In all four structures, the anions and solvent molecules play a crucial role in aggregating the domperidone dimers into three-dimensional hydrogen bonded networks. It is interesting to note that the characteristic feature of domperidone, amide R22(8) dimer is observed in (III) and (IV), while, in (I) and (II), a very different and unique R22(28) dimer is observed between the domperidone molecules. The formation of this R22(28) dimer appears to be influenced by the conformations of the chlorine-substituted oxobenzimidazolyl and non-substituted oxobenzimidazolyl groups.

New cobalt(II) Schiff base complex: Synthesis, characterization, DFT calculation and antimicrobial activity

A new cobalt(II) Schiff-base complex [Co(L)2] was prepared by using equivalent molar of Schiff-base ligand [HL=(E)-N'-(2-hydroxy-3-methoxybenzylidene)isonicotinohydrazide] and CoCl2·6H2O. The structure of the complex was characterized by single-crystal X-ray diffraction, IR, ESI-MS, elemental analysis and DFT method. X-ray crystal structure analysis showed that the complex crystallized in space group Pbca, with two symmetry-independent molecules. The experimental vibrational bands(IR) have been discussed and assigned based on DFT calculations. The antibacterial activity of Schiff base and its complex against S. pombe were assessed by bio-microcalorimetry at 32 °C. Some thermokinetic parameters (k, Pmax,tG and Qtotal) were derived from the metabolic power–time curves, and their quantitative relationship with the concentration were analyzed. The half inhibition concentration (IC50) of Schiff base and its complex were calculated to be 1.18 × 10−2 mol L−1 and 1.07 × 10−3 mol L−1, respectively. The inhibition effect of the complex showed much stronger than that of Schiff base.

A cocrystal of L-ascorbic acid with picolinic acid: the role of O-H...O, N-H...O and C-H...O hydrogen bonds and L-ascorbic acid conformation in structure stabilization.

A new 1:1 cocrystal (L-Asc-Pic) of L-ascorbic acid (vitamin C) with picolinic acid was prepared as a powder and as single crystals. The crystal structure was solved and refined from single-crystal X-ray diffraction (SCXRD) data collected at 293 (2) and 100 (2) K. The samples of the L-Asc-Pic cocrystal were characterized by elemental (HCNS) analysis and titrimetric methods, TG/DTG/DSC, and IR and Raman spectroscopy. The asymmetric unit comprises a picolinic acid zwitterion and an L-ascorbic acid molecule. The stabilization energy of intermolecular interactions involving hydrogen bonds, the vibrational spectrum and the energies of the frontier molecular orbitals were calculated using the GAUSSIAN09 and the CrystalExplorer17 programs. The charge distribution on the atoms of the L-Asc-Pic cocrystal, L-ascorbic acid itself and its 12 known cocrystals (structures from Version 5.40 of the Cambridge Structural Database) were calculated by the methods of Mulliken, Voronoi and Hirshfeld charge analyses (ADF) at the bp86/TZ2P+ level of theory. The total effective charges and conformations of the L-ascorbic acid molecules in the new and previously reported cocrystals were compared with those of the two symmetry-independent molecules in the crystals of L-ascorbic acid. A correlation between molecular conformation and its effective charge is discussed.

Structure and Physical Properties of Cardamonin: A Spectroscopic and Computational Approach.

This is the first study of the crystal structure of cardamonin (CA) confirmed using single-crystal XRD analysis. In the crystal lattice of CA, two symmetry independent molecules are linked by hydrogen bonds within the layers and by the π···π stacking interactions in the columns which lead to the occurrence of two types of conformations among the CA molecules in the crystal structure. To better understand the stability of these arrangements in both crystals and the gaseous phase, seven different CA dimers were theoretically calculated. The molecular structures were optimized using density functional theory (DFT) at the B3LYP/6–311G+(d,p) level and the spectroscopic results were compared. It was found that the calculated configurations of dimer I and III were almost identical to the ones found in the CA crystal lattice. The calculated UV-Vis spectra for the CA monomer and dimer I were perfectly consistent with the experimental spectroscopic data. Furthermore, enhanced emissions induced by aggregated CA molecules were registered in the aqueous solution with the increase of water fractions. The obtained results will help to further understand the relation between a variety of conformations and the biological properties of CA, and the results are also promising in terms of the applicability of CA as a bioimaging probe to monitor biological processes.

Theoretical Study of the Structures of 4-(2,3,5,6-Tetrafluoropyridyl)Diphenylphosphine Oxide and Tris(Pentafluorophenyl)Phosphine Oxide: Why Does the Crystal Structure of (Tetrafluoropyridyl)Diphenylphosphine Oxide Have Two Different P=O Bond Lengths?

The crystal structure of 4-(2,3,5,6-tetrafluoropyridyl)diphenylphosphine oxide (1) contains two independent molecules in the asymmetric unit. Although the molecules are virtually identical in all other aspects, the P=O bond distances differ by ca. 0.02 Å. In contrast, although tris(pentafluorophenyl)phosphine oxide (2) has a similar crystal structure, the P=O bond distances of the two independent molecules are identical. To investigate the reason for the difference, a density functional theory study was undertaken. Both structures comprise chains of molecules. The attraction between molecules of 1, which comprises lone pair–π, weak hydrogen bonding and C–H∙∙∙arene interactions, has energies of 70 and 71 kJ mol−1. The attraction between molecules of 2 comprises two lone pair–π interactions, and has energies of 99 and 100 kJ mol−1. There is weak hydrogen bonding between molecules of adjacent chains involving the oxygen atom of 1. For one molecule, this interaction is with a symmetry independent molecule, whereas for the other, it also occurs with a symmetry related molecule. This provides a reason for the difference in P=O distance. This interaction is not possible for 2, and so there is no difference between the P=O distances of 2.

Crystal structures of two furazidin polymorphs revealed by a joint effort of crystal structure prediction and NMR crystallography.

This work presents the crystal structure determination of two elusive polymorphs of furazidin, an antibacterial agent, employing a combination of crystal structure prediction (CSP) calculations and an NMR crystallography approach. Two previously uncharacterized neat crystal forms, one of which has two symmetry-independent molecules (form I), whereas the other one is a Z' = 1 polymorph (form II), crystallize in P21/c and P1 space groups, respectively, and both are built by different conformers, displaying different intermolecular interactions. It is demonstrated that the usage of either CSP or NMR crystallography alone is insufficient to successfully elucidate the above-mentioned crystal structures, especially in the case of the Z' = 2 polymorph. In addition, cases of serendipitous agreement in terms of 1H or 13C NMR data obtained for the CSP-generated crystal structures different from the ones observed in the laboratory (false-positive matches) are analyzed and described. While for the majority of analyzed crystal structures the obtained agreement with the NMR experiment is indicative of some structural features in common with the experimental structure, the mentioned serendipity observed in exceptional cases points to the necessity of caution when using an NMR crystallography approach in crystal structure determination.

The influence of deuteration on the crystal structure of hybrid halide perovskites: a temperature-dependent neutron diffraction study of FAPbBr3.

This paper discusses the full structural solution of the hybrid perovskite formamidinium lead tribromide (FAPbBr3) and its temperature-dependent phase transitions in the range from 3 K to 300 K using neutron powder diffraction and synchrotron X-ray diffraction. Special emphasis is put on the influence of deuteration on formamidinium, its position in the unit cell and disordering in comparison to fully hydrogenated FAPbBr3. The temperature-dependent measurements show that deuteration critically influences the crystal structures, i.e. results in partially-ordered temperature-dependent structural modifications in which two symmetry-independent molecule positions with additional dislocation of the molecular centre atom and molecular angle inclinations are present.

Influence of fluorine substitution on the molecular conformation of 3'-deoxy-3'-fluoro-5-methyluridine.

Fluorine substitutions on the furanose ring of nucleosides are known to strongly influence the conformational properties of oligonucleotides. In order to assess the effect of fluorine on the conformation of 3'-deoxy-3'-fluoro-5-methyluridine (RTF), C10H13FN2O5, we studied its stereochemistry in the crystalline state using X-ray crystallography. The compound crystallizes in the chiral orthorhombic space group P212121 and contains two symmetry-independent molecules (A and B) in the asymmetric unit. The furanose ring in molecules A and B adopts conformations between envelope (2E, 2'-endo, P= 162°) and twisted (2T3, 2'-endo and 3'exo, P= 180°), with pseudorotation phase angles (P) of 164.3 and 170.2°, respectively. The maximum puckering amplitudes, νmax, for molecules A and B are 38.8 and 36.1°, respectively. In contrast, for 5-methyluridine (RTOH), the value of P is 21.2°, which is between the 3E (3'-endo, P= 18.0°) and 3T4 (3'-endo and 4'-exo, P= 36°) conformations. The value of νmax for RTOH is 41.29°. Molecules A and B of RTF generate respective helical assemblies across the crystallographic 21-screw axis through classical N-H...O aand O-H...O hydrogen bonds supplemented by C-H...O contacts. Adjacent parallel helices of both molecules are linked to each other via O-H...O and O...π interactions.

Solvent influence on the crystal packing of 6-aminothiocytosine.

The crystal structures of 6-aminothiocytosine (systematic name: 4,6-diamino-1,2-dihydropyrimidine-2-thione, DAPMT, C4H6N4S), its hemihydrate (0.5H2O) and its dimethylformamide (DMF, C3H7NO) monosolvate were compared, and the influence of the type of solvent on the supramolecular motifs was analysed. In all three crystal structures, there are two symmetry-independent molecules (A and B), and these molecules are connected by three relatively short and directional hydrogen bonds to form chains of alternating A and B molecules. A further organization of these chains is dependent on the nature of the solvent molecule. In the unsolvated form, two orientations of the neighbouring chains are observed, and similar motifs - but only one per structure - can be observed in the solvated structures. These two different motifs can be connected by two different kinds of contacts, i.e. either π-π (hemihydrate) or staple-supported S...S (DMF). In the crystal structures, the O atoms of the solvent molecules are double acceptors of the same type of hydrogen bonds and bind the chains of DAPMT molecules into different motifs (dimeric or infinite chains). A Hirshfeld fingerprint analysis was used for visualization and additional interpretation of these results.