The real crystal structure of the (NbSe₄)_10/3I charge density wave (CDW) compound is studied by simulation of the X-ray diffuse scattering. The average structure of the low-temperature twinned phase is determined and the phase transition is attributed to the formation of a CDW. The diffuse streaking, present in X-ray diffraction patterns above and below the transition at T = 282 K, is shown to be a projection of diffuse concentric rings perpendicular to the c* direction. The simulated patterns, based on a mismatch model between infinite NbSe₄ chains, correlated by I atoms, are in good accordance with the experimental patterns. In addition to the experiments, the electronic properties of the high- and the low-temperature phases are calculated with the extended Hückel tight-binding method. The Fermi surfaces of the average structures above and below the phase transition appear very similar. Their shapes support a nesting instability and a CDW formation. The weak incommensurate CDW satellites, present below the phase transition, are at 100 K properly described by a modulation wavevector q = [0.06 (1), 0, 0.55 (1)].

The structure of hP386-Al_57.4Cu_3.6Ta_39.0 was determined by single-crystal X-ray diffraction analysis. It can be described as a hexagonal close-packing of two types of endohedral fullerene-like clusters with different Frank–Kasper polyhedra filling the gaps. The description of the structure as a superstructure and as a layered structure illustrates other characteristic structural building principles. The diffuse scattering, which can be observed in some of the crystals, is qualitatively well reproduced by a disorder model. A comparison with the structures of the other complex intermetallics in the system Al–Cu–Ta indicates the decisive role that Cu plays in the constitution and packing of the clusters.

Several methods for absolute structure refinement were tested using single-crystal X-ray diffraction data collected using Cu Kα radiation for 23 crystals with no element heavier than oxygen: conventional refinement using an inversion twin model, estimation using intensity quotients in SHELXL2012, estimation using Bayesian methods in PLATON, estimation using restraints consisting of numerical intensity differences in CRYSTALS and estimation using differences and quotients in TOPAS-Academic where both quantities were coded in terms of other structural parameters and implemented as restraints. The conventional refinement approach yielded accurate values of the Flack parameter, but with standard uncertainties ranging from 0.15 to 0.77. The other methods also yielded accurate values of the Flack parameter, but with much higher precision. Absolute structure was established in all cases, even for a hydrocarbon. The procedures in which restraints are coded explicitly in terms of other structural parameters enable the Flack parameter to correlate with these other parameters, so that it is determined along with those parameters during refinement.

Hirshfeld surfaces and two-dimensional fingerprint plots are used to visualize and analyze intermolecular interactions in six new phosphoramidate structures, [2,6-F₂—C₆H₃C(O)NH]P(O)[X]₂ {X = N(C₂H₅)₂ (1), [X]₂ = NHCH₂C(CH₃)₂CH₂NH and with one CH₃OH solvated molecule (2)}, [C₆H₅O]₂P(O)Y [Y = NC₄H₈O (3), NHC₆H₄(3-Br) (4)] and [Z]₂P(O)OP(O)[Z]₂ [Z = N(CH₃)(CH₂C₆H₅) (5), NHC₆H₄(4-CH₃) (6)]. Study of the short intermolecular contacts in structures (1)–(6) by Hirshfeld surfaces demonstrate that the O atom of P=O is a better H-atom acceptor than the O atom of C=O for (1) and (2), and also relative to the O atom of the C₆H₅O group for (3) and (4), and relative to the bridge O atom of the P(O)OP(O) segment for (5) and (6). The results confirm that the crystal packing is related to the kind of substituent linked to the P atom. Compounds (1), (2), (4) and (6), with characteristic N—H...O hydrogen bonds, show a pair of intense spikes (including the intermolecular H...O contacts) in the fingerprint plots, summarizing the major features of each structure in the related two-dimensional plot. For (3) and (5), without any N—H unit, the two short spikes are observed for (3) but are absent for (5). The upper d_e and d_i values (distances to the Hirshfeld surfaces for the nearest atoms outside and inside) in the fingerprint plots are more compact in (3) than in (4), and in (5) than in (6), reflecting the more efficient packing in (3) and (5). The tertiary N atoms of (3) and (5) do not take part in any intermolecular contacts involving H atoms. Moreover, structures (3)–(6) show greater contribution from C...H contacts relative to O...H contacts. Finally, Hirshfeld surfaces and fingerprint plots are employed for a comparison of the two independent molecules in the asymmetric unit of (1) and also, for a comparison of (6), in the orthorhombic crystal system, with the previously reported monoclinic polymorph (Pourayoubi, Fadaei et al., 2012).

A single-crystal to single-crystal transition in DL-alaninium semi-oxalate monohydrate at a pressure between 1.5 and 2.4 GPa was studied by single-crystal X-ray diffraction and Raman spectroscopy. This is the first example of a single-crystal diffraction study of a high-pressure phase transition in a crystalline amino acid salt hydrate. Selected hydrogen bonds switch over and become bifurcated, whereas the others are compressed continuously. The transition is accompanied by pronounced discontinuities in the cell parameters and volume versus pressure, although no radical changes in the molecular packing are induced. Although, in contrast to DL-alanine, in the crystal structure of the salt there are short O—H...O hydrogen bonds, the structure of the salt is more compressible. At the same time, the structure of DL-alanine does not undergo pressure-induced phase transitions, whereas the structure of DL-alaninium semi-oxalate monohydrate does, and at a relatively low pressure. The anisotropy of lattice strain for the low-pressure phase differs from that on cooling at ambient pressure; interestingly, the anisotropy of the pressure-induced compression of the high-pressure phase is quite similar to the lattice strain of the low-pressure phase on cooling.

It is well documented that the ethynyl group can act as a hydrogen-bond donor via its acidic C—H, and as a hydrogen-bond acceptor via the triple-bond π-density. Using the Cambridge Structural Database (CSD), it is shown that C—C[triple-bond]C—H forms hydrogen bonds to N, O, S or halogens in 74% of structures in which these bonds can form. Additionally, the ethynyl group forms C—H...π interactions with itself or with phenyl groups in 23% of structures and accepts hydrogen bonds from O—H, N—H or C(aromatic)—H in 47% of structures where such bonds are possible. Overall, C—C[triple-bond]C—H acts as a donor or acceptor in 87% of structures in which it occurs. These propensities for hydrogen-bond formation have been determined using quite tight geometrical constraints, and many more ethynyl groups form interactions with only slight relaxations of these constraints. We conclude that the ethynyl group makes crucial contributions to molecular aggregation in crystal structures, and this is exemplified by hydrogen-bond predictions for specific structures made using the statistical propensity tool now available in CSD system software.

The absolute structures of R-(−)-2-methylpiperazine (rmpip), S-(+)-2-methylpiperazine (smpip), R-(−)-2-methylpiperazinediium dibromide (rmpipBr) and S-(+)-2-methylpiperazinediium dibromide (smpipBr) have been determined by anomalous dispersion employing the Parsons' Q and Hooft methods. The studies were undertaken to determine the limitations of the absolute structure determination of light element structures (C, H, N) employing routine single-crystal X-ray diffraction laboratory conditions. The structures of the neutral methylpiperazines were known from a priori non-crystallographic methods and were confirmed by the absolute structure determination of their dibromide salts. By employing the full data collection of Bijvoet pairs and minimizing systematic errors, the absolute structure parameters 0.09 (8) (Hooft) for R-(−)-2-methylpiperazine and 0.05 (8) (Hooft) for S-(+)-2-methylpiperazine were determined.

N-phenyl-4-oxo-4H-2-chromone carboxamides were found to be inactive as MAO inhibitors in contrast with their N-phenyl-4-oxo-4H-3-chromone carboxamide isomers. In order to obtain a close insight into the docking mechanism for this family of compounds, the molecular and supramolecular structures of nine N-phenyl-4-oxo-4H-2-chromone carboxamides were determined. It was found that, in most of the secondary structures, the N(amido) and the O(carboxyl) of the carboxamide residue participate in strong intramolecular interactions, with the O atom of the chromene ring and with the H(ortho)—C (phenyl), respectively. When the phenyl ring had accessible acceptors as substituents a third intramolecular hydrogen bond was also observed. As a consequence, rotations of the chromone and phenyl rings around the N—C(alpha) and C(alpha′)—C=O are constrained and the compounds were found to be more planar than would otherwise be expected. The deviation from planarity of the whole molecule can be quantified by the dihedral angles between mean planes of the aromatic rings and it was found that they were mainly affected by the degree of torsion of the phenyl ring with respect to the amide residue. The molecular conformations assumed by the secondary amides clearly contrast with that of a related tertiary amide that was also determined in this study. The unavailability of the N in this compound as a donor strongly influences the molecular isomerism and conformation. This analysis demonstrates that the molecules can be classified into four groups depending on the types of interactions formed as described above. If the secondary N(amido) of the carboximide is involved in two intramolecular interactions then this atom does not form any intermolecular contacts. In all other cases it does and the supramolecular structure formed is in most cases supplemented by weak C—H...O interactions.

Equations (13)–(18) in the paper by Touaa & Sekkal [(2012), Acta Cryst. B68, 378388] are corrected.