X-ray diffraction study of the phase transition of K₂Mn₂(BeF₄)₃: a new type of low-temperature structure for langbeinites

The twinned low-temperature monoclinic (P112₁) structure of K₂Mn₂(BeF₄)₃ has been determined. Unlike the known structures of langbeinite, the low-temperature unit cell is doubled along the crystallographic b axis with respect to the parent phase. In the monoclinic phase the BeF₄ tetrahedra occupying the same position in the cubic phase along two consecutive unit cells rotate differently.

Structural basis for the phase transitions of Cs₂HgCl₄

The low-temperature 2c₀ superstructure of Cs₂HgCl₄ has been determined by X-ray scattering. The comparison of the parent structure with the 2c₀, 3c₀ and 5a₀ superstructures shows that the phase transitions are driven towards increased bonding of the underbonded Cs(1)-type atoms.

Neutron diffraction study of φ-Bi₈Pb₅O₁₇: structure refinement and analysis of cationic ordering

The φ-Bi₈Pb₅O₁₇ triclinic structure, recently solved by the synergic use of electron and powder X-ray diffraction, has been refined on neutron diffraction data. The occurrence of cationic ordering, indicated by valence bond and charge distribution analysis, has been confirmed by site occupancy refinements. The structure was found to be ordered to a large extent, with Bi and Pb atoms occupying preferentially different lattice sites.

Electron density in the sodium vanadium oxide bronze β-Na_xV₂O₅ at 9 K

The crystal structure and the electron density in β-Na_xV₂O₅ [x = 0.282 (3)] have been studied at 9.6 K.

General method for the description, visualization and comparison of metal coordination spheres: geometrical preferences, deformations and interconversion pathways

A Euclidean dissimilarity index, R_c(x), based on L—M—L valence angles is defined which permits the intercomparison and mapping of the coordination sphere geometries of ML_n species in crystal structures of metal complexes. Taking account of atomic permutational symmetry, the technique identifies clusters of observations that represent geometrical preferences and local distortions, together with pathways that map geometrical interconversions of coordination sphere geometries.

Study of rare-earth π-complexes by means of Voronoi–Dirichlet polyhedra

An investigation of 135 π-complexes of rare-earth atoms was carried out with Voronoi–Dirichlet polyhedra. A novel method for the evaluation of the sizes of polyatomic ligands and steric effects in the structure of organometallic compounds was developed.

Commensurate structure of Ca₂CoSi₂O₇, a new twinned orthorhombic structure

The structure of the commensurate phase of Ca₂CoSi₂O₇ has been derived from the modulated structure described in (3 + 2)-dimensional space and refined by a least-squares method in three-dimensional space. The structure is composed of twins of orthorhombic individuals.

Relationship between bond valence and bond softness of alkali halides and chalcogenides

Detailed studies on the significance of higher coordination shells and a novel empirical correlation of the bond-valence parameter b to the softness of a bond permit the systematic determination of softness-sensitive bond-valence parameters.

Di­pyrimidine–copper(II) dinitrate complexes showing magnetic interactions

The crystal structures of [Cu(NO₃)₂(pm)₂]_n (pm = pyrimidine), [Cu(NO₃)₂(H₂O)₂(pm)]_n and Cu(NO₃)₂(H₂O)₂(pm)₂ were determined and relations between chain structures and magnetic interactions were discussed.

Structure of the X-phase of 38% brominated betaine calcium chloride dihydrate

The structures of the Pnma and P2₁2₁2₁ phases of 38% brominated betaine (trimethylammonioacetate) calcium chloride dihydrate have been determined by single-crystal neutron diffraction at 295 and 20 K, respectively. The distortion with respect to the high-temperature Pnma phase is characterized by anti-symmetric displacements of the betaine molecules as well as of the Ca octahedra. The direct phase transition that occurs around 80 K between these two phases is interpreted in the scope of a symmetry-mode analysis.

Hydrogen-bonded hexagonal and pseudo-hexagonal grid motifs in supramolecular cobalt(II) and nickel(II) cupferronato complexes incorporating neutral N-donors with intermolecular NH₂ connectors and solvent molecules

The self-assembly of cobalt(II) and nickel(II) octahedra formed by cupferronato and neutral N-donor ligands and their NH₂ functions leads to extended two-dimensional hexagonal and pseudo-hexagonal grid-type networks. The solvent molecules play a key role in such systems.

Asymmetric hydrogen bonds in centrosymmetric environment: neutron study of very short hydrogen bonds in potassium hydrogen di­chloromaleate

The very short hydrogen bonds in potassium hydrogen dichloromaleate have been studied by neutron diffraction at 30 and 295 K. The hydrogen bonds are asymmetric, although the environment is centrosymmetric.

Hydrated metal complexes of N-(6-amino-3,4-di­hydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)glycinate: interplay of molecular, molecular–electronic and supramolecular structures

The title anion forms finite molecular aggregates with Li⁺ and Mn²⁺, simple aquated salts with Mg²⁺ and Zn²⁺, but organic–inorganic hybrid sheets with Na⁺ and K⁺.

Hydrated salts of 3,5-di­hydroxybenzoic acid with organic di­amines: hydrogen-bonded supramolecular structures in two and three dimensions

3,5-Dihydroxybenzoic acid forms hydrated salt-type adducts with a range of organic diamines. The molecular components are linked by an extensive series of N—H⋯O, O—H⋯O and O—H⋯N hydrogen bonds into continuous arrays in two or three dimensions.

X-ray and neutron structure of 1,8-(3,6,9-trioxaundecane-1,11-diyldi­oxy)-9,10-di­hydro-10,10-di­methylanthracene-9-ol (P326); some pitfalls of automatic data collection

The structure of the crown ether P326, the parent compound for a series of derivatives, is presented as determined by room-temperature X-ray diffraction and low-temperature neutron diffraction. Forming a very useful pedagogical example, a variety of situations arose through the two simultaneous and interdependent data collections and analyses, complicated by an extremely unusual combination of lattice geometry and pseudosymmetry.

Variable-temperature crystal structure studies of m-nitroaniline

The thermal evolution of the lattice parameters, expansion tensor coefficients, and rigid-body translation and libration tensors, as well as the torsional vibration of the nitro group, were studied in order to understand the molecular interactions and dynamics leading to thermodynamic transitions.

Experimental electron density of urea–phosphoric acid (1/1) at 100 K

The crystal formed by urea and phosphoric acid is stabilized by hydrogen bonds, including the very short d(O⋯O) = 2.410 (2) Å. Deformation maps obtained from multipole refinements show the hydrogen bonding effect on P—O bonds and the covalent character of the two O—H interactions of the short hydrogen bond.

Chemical bonding in energetic materials: β-NTO

The electron density and related properties of the quasi-stable β form of 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO; space group P2₁/c) have been determined from low-temperature X-ray diffraction.

X-ray studies on crystalline complexes involving amino acids and peptides. XXXVII. Novel aggregation patterns and effect of chirality in the complexes of DL- and L-lysine with glutaric acid

The complexes exhibit novel patterns of amino acid aggregation. They also illustrate the profound effect reversal of chirality can have on aggregation.

Structure of β-trimyristin and β-tris­tearin from high-resolution X-ray powder diffraction data

The crystal structures of β-1,2,3-tritetradecanoylglycerol (β-trimyristin or β-MMM) and β-1,2,3-trioctadecanoylglycerol (β-tristearin or β-SSS) have been determined from high-resolution synchrotron X-ray powder diffraction data using grid-search techniques and refined with Rietveld refinement to final R_p values of 0.053 and 0.041, respectively.

Superspace description of the structure of the composite crystal urea/n-octane at room temperature

The structure of the composite crystal urea/n-octane has been solved using the superspace formalism [superspace group P6₁22(00γ)0. A comparison of the proposed model with that of the urea/heptadecane crystal shows that the modulation of n-alkane molecules is provoked by the shape of the urea tunnels.

Average structure of the composite crystal urea/­octanedioic acid at room temperature within the superspace formalism

The average structure of the composite crystal urea/octanedioic (suberic) acid has been solved using the superspace formalism under the superspace group H′3₁21(00γ)00. The strong orientational disorder shown by the guest molecules has been modelled using 12 equiprobable orientations of suberic acid molecules within the urea tunnels.

Inclusion compounds of bi­naphthol with picolines: structures, selectivity and kinetics of desolvation

The structures of binaphthol with picoline isomers have been elucidated. The lattice energies are correlated with their thermal stabilities, selectivities and activation energies of desolvation.

Non-covalent interactions in the crystallization of the enantiomers of 1,7-dioxa­spiro[5.5]undecane (olive fly sex pheromone) by enantiospecific cyclodextrin hosts, hexa­kis­(2,3,6-tri-O-methyl)-α-cyclodextrin and hepta­kis­(2,3,6-tri-O-methyl)-β-cyclodextrin

Weak but numerous host–guest interactions and induced fit of the hosts are responsible for the observed enantioselectivity. The difference of the hosts to selectively bind one of the optical antipodes is reflected in very subtle differences in the C—H⋯O intermolecular distances between host and guest.

Influence of hydrogen bonding on the second harmonic generation effect: neutron diffraction study of 4-nitro-4′-methylbenzyl­idene aniline

4-Nitro-4′-methylbenzylidene aniline (NMBA) was studied by neutron diffraction in order to rationalize its large second-harmonic generation output via its structural characteristics: C—H⋯X hydrogen bonding and very close interplanar separations are shown to be important supramolecular features in this regard.

Interplay between dipolar, stacking and hydrogen-bond interactions in the crystal structures of unsymmetrically substituted esters, amides and nitriles of (R,R)-O,O′-di­benzoyltartaric acid

The results presented demonstrate the importance of dipole/dipole interactions between CO and βC*H groups in stabilizing the molecular conformation, and between carbonyl groups in stabilizing crystal packing of molecules that lack classical hydrogen-bond donor groups. An illustration is provided as to how a small change in mutual orientation of molecules arranged in a close-packed fashion causes a change in the character of intermolecular interactions from van der Waals to sandwich stacking and to dipolar. Hydrogen bonds with ester carbonyls as acceptors are formed by the amide but not by carboxyl donor groups.

Topochemically controlled solid-state methyl rearrangement in thiocyanurates

Some thiocyanurates undergo intermolecular methyl rearrangement in the solid state. The crystal structures and DSC thermographs of representatives of thiocyanurates are described and discussed. It is shown that this type of reaction in the solid state is topochemically controlled.

Migration of the proton in the strong O—H⋯O hydrogen bond in urea–phosphoric acid (1/1)

The position of the H atom in the strong O—H⋯O hydrogen bond in urea–phosphoric acid is monitored by neutron diffraction using a multiple single crystal sample. The proton position is found to change significantly as a function of temperature, migrating along the hydrogen bond to become more centred as the temperature increases.

Perspectives in structural biology

Crystal structure determination