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Twenty seven N-(methoxysalicylidene)-haloaniline (halo = F, Cl or Br) compounds were synthesized. The crystal structures of all 27 compounds have been determined at low temperature and are reported herein, along with a variable-temperature neutron diffraction study on two of the compounds. New polymorphs were identified for two of the compounds along with a temperature-induced phase transition for one of the other compounds. Visual observations on the thermochromism of the 27 compounds are also reported. The interplanar angle between the two aromatic rings and the intermolecular interactions in the structures are examined and linked to the visual observations on the thermochromism.

Three {[(isoxazol-3-yl)imino]­meth­yl}phenols were synthesized and structurally characterized. All three structures contain an intra­molecular O—H⋯N hydrogen bond and none were found to be strongly thermochromic., The synthesis and structures of three isoxazole-containing Schiff bases are reported, namely, (E)-2-{[(isoxazol-3-yl)imino]­meth­yl}phenol, C10H8N2O2, (E)-2-{[(5-methyl­isoxazol-3-yl)imino]­meth­yl}phenol, C11H10N2O2, and (E)-2,4-di-tert-butyl-6-{[(isoxazol-3-yl)imino]­meth­yl}phenol, C18H24N2O2. All three structures contain an intra­molecular O—H⋯N hydrogen bond, alongside weaker inter­molecular C—H⋯N and C—H⋯O contacts. The C—O(H) and imine C=N bond lengths were consistent with structures existing in the enol rather than the keto form. Despite having dihedral angles

X-ray diffraction experiments at very low temperatures require samples to be isolated from atmospheric conditions and held under vacuum. These conditions are usually maintainedviathe use of beryllium chambers, which also scatter X-rays, causing unwanted contamination of the sample's diffraction pattern. The removal of this contamination requires novel data-collection and processing procedures to be employed. Herein a new approach is described, which utilizes the differences in origin of scattering vectors from the sample and the beryllium to eliminate non-sample scattering. The programMasqueradehas been written to remove contaminated regions of the diffraction data from the processing programs. Coupled with experiments at different detector distances, it allows for the acquisition of decontaminated data. Studies of several single crystals have shown that this approach increases data quality, highlighted by the improvement in internal agreement factor with the test case of cytidine presented herein.

Crystals of molecular complexes of dimethyl sulfoxide with trichloromethane (chloroform), (CH3)2SO·2CHCl3, (I), and dichloromethane, (CH3)2SO·CH2Cl2, (II), have been grownin situ. In both compounds, the components are linked together by (Cl)C—H...O interactions. The dimethyl sulfoxide molecules in (I) are bound into chains by C—H...O interactions. In (II), pairs of the components form centrosymmetric rings, linked into a three-dimensional network by C—H...O contacts and dipole–dipole interactions between dimethyl sulfoxide molecules.

The synthesis, chemical and structural characterization of a series of pentamethylcyclopentadienyl (Cp*) tantalum imido complexes and aryloxide derivatives are presented. Specifically, the imido complexes Cp*Ta(N t Bu)(CH2 R)2, where R = Ph [dibenzyl(tert-butylamido) (η5-pentamethylcyclopentadienyl)tantalum(IV) (1)], Me2Ph [tert-butylamido)bis(2-methyl-2-phenylpropyl) (η5-pentamethylcyclopentadienyl)tantalum(IV) (2)], CMe3 [(tert-butylamido)bis(2,2-dimethylpropyl) (η5-pentamethylcyclopentadienyl)tantalum(IV) (3)], are reported. The crystal structure of (3) reveals α-agostic interactions with the Ta atom. The resulting increase in the tantalum core coordination improves electronic stability. As such it does not react with pentafluorophenol, in contrast to the other two reported imido complexes [(1) and (2)]. Addition of C6F5OH to (1) yields a dimeric aryl-oxide derivative, [Cp*Ta(CH2Ph)(OC6H5)(μ-O)]2 [di-μ-oxido-bis[benzyl(pentafluorophenolato) (η5-pentamethylcyclopentadienyl)tantalum(V)] (4)]. Its crystal structure reveals long Ta—O(C6H5) bonds but short oxo-bridging Ta—O bonds. This is explained by accounting for the fierce electronic competition for the vacant d π orbitals of the electrophilic TaV centre. Steric congestion around each metal is alleviated by a large twist angle (77.1°) between the benzyl and pentafluorophenyl ligands and the ordering of each of these groups into stacked pairs. The imido complex (2) reacts with C6F5OH to produce a mixture of Cp*Ta(OC6F5)4 [tetrakis(pentafluorophenolato)(η5-pentamethylcyclopentadienyl)tantalum(V) (5)] and [Cp*Ta(OC6F5)2(μ-O)]2 [di-μ-oxido-bis[bis(pentafluorophenolato)(η5-pentamethylcyclopentadienyl)tantalum(V)] (6)]. Steric congestion is offset in both cases by the twisting of its pentafluorophenyl ligands. Particularly strong electronic competition for the empty d π metal orbitals in (6) is reflected in its bond geometry, and owes itself to the more numerous electron-withdrawing pentafluorophenyl ligands. The balance of steric and electronic factors affecting the reactivity of Cp* tantalum imido based complexes with pentafluorophenol is therefore addressed.

XIPHOS has been developed to expand home laboratory facilities closer to those found at central facilities, offering extremes of sample environment and flux densities far greater than standard laboratory sources. The system has a minimum operating temperature of 1.9 K, and has a direct-drive molybdenum rotating-anode generator coupled with the latest multilayer optics. XIPHOS has been specifically designed to accommodate various sample environments and is now operational. Furthermore, it has been calibrated with structural phase transitions from 14 to 148 K. Results are also presented from a full low-temperature data collection ofm-nitroaniline to demonstrate the quality of results attainable from XIPHOS.

In order to gain a deeper understanding into the bonding situation in rhodium complexes containing rhodium–carbon interactions, the experimental charge-density analysis for [Rh(C7H8)(PPh3)Cl] (1) is reported. Accurate, high-resolution (sin θ/λ = 1.08 Å−1), single-crystal data were obtained at 100 K. The results from the investigation were interesting in relation to the interactions between the rhodium metal centre and the norbornadiene fragment and illustrate the importance of such analyses in studying bonding in organometallic complexes.

The crystal structures of two cocrystals composed of 4,4′-(fluorene-9,9-di­yl)diphenol (C25H18O2) and 6-methyl-2H-pyridone (C6H7NO) are reported, namely 4,4′-(fluorene-9,9-di­yl)diphenol–6-methyl-2H-pyridone (1/2), C25H18O2·2C6H7NO, (I), and 4,4′-(fluorene-9,9-di­yl)diphenol–6-methyl-2H-pyridone–water (1/3/3), C25H18O2·3C6H7NO·3H2O, (II). In both cocrystals, the mutual orientation between two 6-methyl-2H-pyridone mol­ecules in principle enables photodimerization, yet in both cases no photodimerization occurs. In cocrystal (I) this is probably due to poor orbital overlap, while in the case of cocrystal (II) it is suggested that the lack of reaction is due to the highly complex hydrogen-bonding network that exists in the structure.

Pyridine-3,5-dicarboxylic acid has been studied by single-crystal neutron diffraction at 15 and 296 K. Pyridine-3,5-dicarboxylic acid, in which the carboxylic acid protons have been replaced by deuterons, has also been studied at 15, 150 and 296 K. The protonated structure contains a short N...H...O hydrogen bond [N...O 2.523 (2) Angstrom at 15 K]. Temperature-dependent proton migration occurs where the N--H distance in the hydrogen bond changes from 1.213 (4) Angstrom at 15 K to 1.308 (6) Angstrom at 300 K. In the deuterated structure the overall hydrogen-bond length increased [N...O 2.538 (3) Angstrom at 15 K] and the magnitude of the migration increased so that the N--D distance changes from 1.151 (3) Angstrom at 15 K to 1.457 (4) Angstrom at 300 K.

A simple pressure cell for single-crystal X-ray crystallography at pressures up to 1 kbar (105 kPa) has been developed. A special attachment for mounting the cell onto a goniometer and a new data collection procedure are described.

Co-crystals of the yl­ide (benzoyl­methyl­ene)­tri­phenyl­phospho­rane (BPPY) with either fumaric acid, viz. (benzoyl­methyl­ene)­tri­phenyl­phospho­rane–fumaric acid (2/1), C26H21OP·0.5C4H4O4, or terephthalic acid, viz. (benzoyl­methyl­ene)­tri­phenyl­phospho­rane–terephthalic acid (2/1), C26H21OP·0.5C8H6O4, have a stoichiometric ratio of 2:1 between the yl­ide and the corresponding di­carboxyl­ic acid. In both adducts, the acid component lies across a centre of inversion. In neither case is the yl­ide protonated by the organic acid; instead the H atoms of the non-ionized di­carboxyl­ic acid mol­ecules participate in the formation of strong O—H⋯O hydrogen bonds with the benzoyl O atom of the yl­ide species. These structures are the first reported examples of co-crystals containing non-protonated BPPY.

The 1:2 adduct of benzene-1,2,4,5-tetracarboxylic acid and 4,4′-bipyridyl at 100 K has been studied by single-crystal neutron diffraction at 20, 200 and 296 K. The structure contains two short, strong N...O hydrogen bonds: one O—H...N hydrogen bond [O...N 2.6104 (17) Å at 20 K] and one short N—H...O hydrogen bond [N...O 2.5220 (17) Å at 20 K]. The N—H distance in the N—H...O hydrogen bond changes from 1.207 (3) Å at 20 K to 1.302 (4) Å at 296 K and the N...O distance increases to 2.5315 (16) Å at 296 K. At 200 K the H atom lies in an intermediate position 1.251 (6) Å from the N atom with an N...O separation of 2.520 (4) Å. The O—H...N hydrogen bond, on the other hand, does not change with temperature.

In the title mol­ecule, C15H10N2O5S, there are weak C—H⋯O and C—H⋯N interactions which generate rings of motifs S(5), S(6), S(8), R{_2^1}(5), R{_2^1}(6), R{_1^2}(4), R{_2^2}(6), R{_2^2}(7), R{_2^2}(8) and R{_2^2}(13). The supramolecular aggregation is completed by the presence of π–π interactions.

Within and between mol­ecules of the title compound, C7H11N2+·C6H3N2O5−, there are N—H⋯O and C—H⋯O interactions which generate rings of motifs S(5), D, D{_1^2}(4), R{_2^1}(7), R{_2^1}(5), R{_1^2}(6), R{_2^2}(10) and R{_2^2}(14). The supramolecular aggregation is completed by the presence of C—H⋯π and π–π interactions.

Within and between mol­ecules of the title compound, C7H11N2+·C6H5NO3−·C6H4NO3, there are C—H⋯O, C—H⋯N, O—H⋯O and N—H⋯O interactions, which generate rings of graph-set motifs S(5), R12(5), R12(6), R12(7) and R21(4). The supramolecular aggregation is completed by the presence of three different π–π interactions and short van der Waals contacts.

In the title mol­ecule, C14H11NO7S, (I), there are weak C—H⋯O interactions which generate rings of motifs S(5), S(6), R12(5) and R22(7). The supramolecular aggregation is completed by the presence of C—H⋯π and π–π interactions.

Two subsets of data, corresponding to different crystalline modifications of the title compound, 5-oxatricyclo[5.1.0.01,3]octane-4-one (C7H8O2), have been obtained from the same experiment. Both structures were successfully solved and refined. The packing of identical layers of molecules is different for monoclinic and orthorhombic forms.

Copper(II) hypophosphite has been shown to exist as several polymorphs. The crystal structures of monoclinic α-, ortho­rhombic β- and ortho­rhombic γ-Cu(H2PO2)2 have been determined at different temperatures. The geometry of the hypophosphite anion in all three polymorphs is very close to the idealized one, with point symmetry mm2. Despite having different space groups, the structures of the α- and β-polymorphs are very similar. The polymeric layers formed by the Cu atoms and the hypophosphite ions, which are identical in the α- and β-polymorphs, stack in the third dimension in different ways. Each hypophosphite anion is coordinated to three Cu atoms. On cooling, a minimum amount of contraction was observed in the direction normal to the layers. The structure of the polymeric layers in the γ-­polymorph is quite different. There are two symmetry-independent hypophosphite anions; the first is coordinated to two Cu atoms, while the second is coordinated to four Cu atoms. In all three polymorphs, the Cu atoms are coordinated by six O atoms of six hypophosphite anions, forming tetragonal bipyramids; in the α- and β-polymorphs, there are four short and two long Cu—O distances, while in the γ-polymorph, there are four long and two short Cu—O distances.

The structure of the title compound has been determined using low-temperature (150 K) single-crystal X-ray and neutron diffraction data. Crystals adopt the uncommon space group P42/ncm and display a complex set of intermolecular interactions in which the water molecules play the crucial role: the water O-atom [O2(w)] accepts two hydrogen bonds and both water H atoms act as bifurcated donors. A set of O—H...O hydrogen bonds is formed around the 42 axis comprising (a) a cyclic tetrameric synthon involving four donor-H from two water molecules and two O(hydroxy) acceptors from two parent molecules, and (b) short discrete O(hydroxy)—H...O2(w) hydrogen bonds which link these tetramers along the c axis. Four Br...Br interactions [3.708 (1) Å] form cyclic Br4 tetramers around the \bar 4 axis and are linked to the O—H...O system via O2(w)—H...Br bonds with H...Br = 2.995 (2) Å. Finally, the O—H...O system is further linked to the parent molecules via C≡C...H...O2(w) bonds of 2.354 (3) Å. The supramolecular structure of the title hydrate is compared with that of the non-hydrated parent molecule, which also forms cyclic O—H...O bonded tetrameric synthons, and with its (non-hydrated) tetrachloro analogue, which forms cyclic tetrameric Cl4 synthons [Madhavi, Desiraju et al. (2000b). Acta Cryst. B56, 1063–1070].

In the title compound, [PdCl2(C26H24P2)]·3CDCl3, the [PdCl2(dppe)] mol­ecule adopts a distorted square-planar metal coordination and an asymmetrical twist conformation of the five-membered metallocycle. The structure contains infinite channels filled with deutero­chloro­form of crystallization and displays an (a, b/2, c) subcell.

A neutron diffraction study of the non-linear optical (NLO) material 4-nitro-4′-methylbenzylidene aniline (NMBA) is presented. NMBA exhibits a large macroscopic second-order NLO susceptibility, χ(2), and this study shows that hydrogen bonding is, in part, responsible for this. No hydrogen bonding was reported in the X-ray study [Ponomarev et al. (1977). Sov. Phys. Crystallogr. 22, 223–225], whereas the present work shows that C—H...X hydrogen bonds (where X = N, O or π) direct the nature of the three-dimensional lattice. C—H...X (X = N or O) hydrogen bonds are common; however, C—H...π hydrogen-bond motifs are relatively rare. Such intermolecular interactions help extend the molecular charge transfer into the supramolecular realm, the charge transfer originating as a consequence of the high level of molecular planarity and strong donor-to-acceptor interactions. Molecular planarity, coupled with the favourable nature of the hydrogen bonds, results in parallel stacking of molecules in both the a and c crystallographic directions with extremely close interplanar spacings. Such a combination of influential hydrogen-bonding characteristics accounts, in part, for the large second-order NLO output of the material since the phenomenon is so critically dependent upon the nature of the charge transfer.

Structures of the title compounds are all mediated by strong cooperative arrangements of O—H...O hydrogen bonds, supported by a variety of weaker interactions which affect the type of O—H...O synthon that is formed. The tetrafluoro compound contains hexameric O—H...O synthons in a supramolecular chair conformation, together with C≡C—H...F interactions. However, the tetrachloro and tetrabromo compounds both form tetrameric O—H...O synthons. This dominant pattern is supported by halogen...halogen interactions having one C—Cl[Br]...Cl[Br] angle close to 180° and the other close to 90°, and by C≡C—H...Cl[Br] interactions.

The structures of four para-substituted derivatives of diphenylethynylmethanol have been determined [ditolylethynylmethanol, di(4-chlorophenyl)ethynylmethanol, di(4-bromophenyl)ethynylmethanol and bis(4,4′-biphenylyl)ethynylmethanol]. The dimethyl, dichloro, dibromo and diphenyl compounds have been analysed using X-ray diffraction at 150 K, and the dichloro compound has also been studied using neutron diffraction at 150 K. In common with the parent diphenylethynylmethanol [Garcia, Ramos, Rodriguez & Fronczek (1995). Acta Cryst. C51, 2674–2676], all four derivatives fail to form the expected strong O—H...O hydrogen bonds due to steric hindrance. Instead, the supramolecular structural organization in this family of gem-alkynols is mediated by a variety of weaker interactions. The two most acidic protons, O—H and C[triple-bond]C—H, participate in weak hydrogen bonds to π-acceptors, forming synthons that stabilize all five structures. These primary interactions are reinforced by a variety of other weak hydrogen bonds involving C—H donors and the hydroxy-O as an acceptor, and by halogen...halogen interactions in the dichloro and dibromo compounds.

A systematic survey of the Cambridge Structural Database (CSD) has identified all intramolecular hydrogen-bonded ring motifs comprising less than 20 atoms with N and O donors and acceptors. The probabilities of formation Pm of the 50 most common motifs, which chiefly comprise five- and six-membered rings, have been derived by considering the number of intramolecular motifs which could possibly form. The most probable motifs (Pm > 85%) are planar conjugated six-membered rings with a propensity for resonance-assisted hydrogen bonding and these form the shortest contacts, whilst saturated six-membered rings typically have Pm < 10%. The influence of intramolecular-motif formation on intermolecular hydrogen-bond formation has been assessed for a planar conjugated model substructure, showing that a donor-H is considerably less likely to form an intermolecular bond if it forms an intramolecular one. On the other hand, the involvement of a carbonyl acceptor in an intramolecular bond does not significantly affect its ability to act as an intermolecular acceptor and thus carbonyl acceptors display a substantially higher inclination for bifurcation if one hydrogen bond is intramolecular.

The title saccharinate complexes, aqua[1,2-benzisothiazol-3(2H)-onato 1,1-dioxide-N]bis(1,10-phenanthroline-N,N′)man­ganese(II) 1,2-benz­isothia­zol-3(2H)-onate 1,1-dioxide,[Mn(C7H4NO3S)(C12H8N2)2(H2O)](C7H4NO3S), and aqua[1,2-benz­iso­thiazol-3(2H)-onato 1,1-dioxide-N]­bis­(2,2′-bi­pyri­dine-N,N′)­cobalt(II) 1,2-benz­iso­thia­zol-3(2H)-onate 1,1-di­oxide, [Co­(C7H4NO3S)­(C10H8N2)2­(H2O)]­(C7H4NO3S), have been prepared and their crystal structures determined at 150 K. The structure of the manganese complex consists of repeated alternating [Mn(phen)2(sac)(H2O)]+ cations and non-coordinated saccharinate anions. The water molecule, bound to manganese as part of a slightly distorted octahedral arrangement, is hydrogen bonded to an O atom of the SO2 group in the saccharinate counter-ion. In contrast, the cobalt complex has one pseudo-octahedral [Co(bipy)2(sac)(H2O)]+ cation, with the cobalt-bound water molecule hydrogen bonded to the N atom of the accompanying free saccharinate anion.

New software,OLEX2, has been developed for the determination, visualization and analysis of molecular crystal structures. The software has a portable mouse-driven workflow-oriented and fully comprehensive graphical user interface for structure solution, refinement and report generation, as well as novel tools for structure analysis.OLEX2seamlessly links all aspects of the structure solution, refinement and publication process and presents them in a single workflow-driven package, with the ultimate goal of producing an application which will be useful to both chemists and crystallographers.

X-ray diffraction data sets collected on both Nonius and Siemens (Bruker) goniometers equipped with charge-coupled device (CCD) area detectors have been tested for the electron-density determination of the aluminosilicate mineral compound α-spodumene LiAl(SiO3)2, aluminium lithium silicon oxide. Data collection strategies, reflection intensity peak integration methods and experimental error estimates are different for the two instruments. Therefore, the consistency and quality of the two types of CCD measurements have been carefully compared to each other and to high-resolution data collected on a conventional CAD-4 point-detector diffractometer. Multipole density model refinements were carried out against the CCD data and the statistical factors analysed in terms of experimental weighting schemes based on the standard uncertainties of the diffraction intensities derived by the Nonius and Siemens software programs. Consistent experimental electron-density features in the Si–O–Si and Si–O–Al bridges were found from both CCD data sets. The net atomic charges obtained from the kappa refinements against each CCD data set are also in good agreement and quite comparable with the results of the conventional CAD-4 experiment.

The structure of the title compound, μ-azido-1:2κ 2 N 1 -[μ-4-(3-methyl-2-pyridyl-2κN-amino)-1-(3-methyl-2-pyridyl-1κN-imino)-1,2-dihydrophthalazin-2-yl-1:2κ 2 N 2 :N 3 ]bis[(azido-κN 1 )copper(II)], [Cu 2 (C 20 H 17 N 6 )(N 3 ) 3 ], (I), determined from data recorded at 40 (1) K using a new open-flow He gas cryostat, is presented here as part of an extended study program on dicopper(II) azide-bridged compounds. Each pair of Cu II centres in (I) is equatorially bridged by a μ 2 -1,1-azide, with a bridge angle of 107.2 (1)°. The dinuclear centres form tetranuclear clusters through two μ 2 -1,3-azide bridges, which in turn form chains along the a axis. Two C-H...N intermolecular interactions interlink these chains along the [110] and [110] directions.

The crystal structure of [Co(NO 2 )(NH 3 ) 5 ](SiF 6 ) has been determined at 293 and 150 K. The complex cations in the structure are linked by N-H...F hydrogen bonds to the (SiF 6 ) 2- anions and by N-H...O hydrogen bonds to other cations. The role of these hydrogen bonds and of the repulsive O...F interactions in the anisotropy of the structural distortion on cooling is discussed.

An appreciable degree of pyramidalization of the amine N atom is observed in the title compound. The existence of polar chains, induced by N—H...O synthons, is confirmed. C—H...O interactions, not noted in a previous X-ray study, were found to stabilize further the known head-to-tail assembling of the chains. The structure can be described as non-polar (101) layers, embodying chains interlinked by centrosymmetric dimers, connected by C(aryl)—H...π interactions. The latter are not present in m-nitroaniline, 2-methyl-4-nitroaniline and other related compounds with chains built from similar N—H...O synthons and assembled head-to-head. This finding implies that an obvious relationship between molecular recognition patterns and crystal structures should not be assumed.

The title compound, C12H12NO2P, is the first structurally studied phospho­rus hydroxy­lamine derivative. The N atom is pyramidal. In the crystal structure, hydrogen bonds link mol­ecules into double ribbons.

The title compound, C17H20NO4P, contains pyramidal N atoms and adopts similar conformations in its three independent mol­ecules (A, B and C). Mol­ecules A and B form a dimer in the crystal structure by way of a pair of N—H⋯O hydrogen bonds, as does C with its inversion-generated partner.

The mol­ecules of the title complex, C12H23N·0.5H2O2, are linked together by O—H⋯N and N—H⋯O hydrogen bonds to give ten-membered rings, which form flat ribbons parallel to the a axis. Centrosymmetric H2O2 mol­ecules, as well as amino groups, act as both donors and acceptors of hydrogen bonds.

The reaction of pyrazine with hydrogen peroxide resulted in piperazine-2,3,5,6-tetrayl tetra­hydro­peroxide, crystallizing as its pyrazine disolvate dihydrate, C4H10N2O8·2C4H4N2·2H2O. In the crystal structure, the tetra­peroxo mol­ecules, which possess a crystallographically imposed centre of symmetry, are linked into a three-dimensional network by hydrogen-bonding inter­actions involving the pyrazine and water mol­ecules.

The title compound, [Mg2Br2(C5H4N)2(C4H8O)3]·0.5C4H8O, contains dimeric associations of Mg atoms bridged by tetrahydrofuran (THF) molecules. The coordination polyhedron of the Mg atom is a slightly distorted MgCNO2Br trigonal bipyramid with two THF molecules in the axial positions. One O atom occupies a site with symmetry 2.

The molecules of the title complex, C12H23N·C6H11N, are linked together in chains by O—H...N and N—H...O hydrogen bonds.

The title compound, K+·C6H4NO5S−·H2O, forms a three-dimensional polymeric structure with an O8 coordination environment of the K+ cation.

In the title compound, C9H13N3O2, the asymmetric unit contains two almost identical but crystallographically independent molecules. The molecules are linked together by pairs of weak C—H...O interactions into zigzag chains, which, in turn, form corrugated layers perpendicular to the a axis.

Crystallographic data for 309 C—X...O(nitro)—C, X = Cl, Br, I, interactions, involving 245 symmetry-independent X atoms, have been investigated out to 0.2 Å above van der Waals radii limits [\nu(O) + \nu(X)]. A total of 138 (45%) of these interactions are shorter than \nu(O) + \nu(X), with the degree of interpenetration of the X and O atoms increasing in the order Cl < Br < I. The analysis also shows that: (a) the C—X...O angle tends to linearity as the X...O distance shortens, (b) the angle of approach of X to the C—NO2 plane is preferentially less than 45° for Cl, with an increasing tendency to in-plane X approach in the order Cl < Br < I, and (c) the halogen (X) forms either (i) mono-coordinate interactions with one nitro O atom, with X...O in a cis relationship to the nitro C substituent about the N—O bond, or (ii) approaches both nitro O atoms in a bifurcated manner; the tendency to form such bifurcated motifs increases in the order Cl < Br < I. Only iodine consistently forms short interactions with both nitro O atoms. Ab-initio-based molecular orbital calculations, using intermolecular perturbation theory (IMPT) applied to a nitromethane–1-chloro-2-methylacetylene model dimer, agree with the analysis of experimental crystal structure geometries. The IMPT calculations yield an attractive interaction energy of ca −6 kJ mol−1 for Cl...O at the 6-31G* basis set level. Calculations for Br...O at the (only available) [6s4p1d] basis set level indicate that this interaction is more attractive than Cl...O.

The crystal structures of {4-[bis(diethylamino)methylium]phenyl}dicyanomethanide (I), {4-[cyano(4-methylpiperidyl)methylium]phenyl } dicyanomethanide (II), {4-[cyano(1-pyrrolidinyl)methylium]phenyl}dicyanomethanide (IV) and (Z)-{4-[1,3-dicyano-3-(diethylamino)-2-propenylium]phenyl} dicyanomethanide have been determined by X-ray crystallography. Together with (Z)-{4-[1-cyano-3-(diethylamino)-2-propenylium]phenyl}dicyanomethanide [(III): Cole, J. C., Howard, J. A. K., Cross, G. H. & Szablewski, M. (1995). Acta Cryst. C51, 715–718], these compounds exhibit varying degrees of quinoidal characteristics and yet have predominantly zwitterionic ground state character. The presence of high dipole moments in the crystal can thus be inferred and are substantiated by measurements of high solution-state dipole moments. Negative solvatochromism (hypsochromism on increasing solvent polarity) confirms the zwitterionic character in at least two of the series.

The Fddd four-circle diffractometer system has been developed to perform X-ray diffraction experiments at temperatures as low as 9 K. The diffractometer consists of: (i) a Siemens rotating-anode generator; (ii) a Huber goniometer with offset X circle; (iii) a Siemens Fast Scintillation Detector; and (iv) an APD `202' Displex cryogenic refrigerator. The belt-driven rotating anode gives an X-ray flux that is far in excess of that obtained from a sealed X-ray tube and leads to faster data collections and the opportunity to study smaller samples. X-ray beam alignment requires precise movements of the 300 kg goniometer and this is achieved using compressed air pads. Steel-braided gas lines that allow the transfer of helium gas between the Displex and the compressor are supported by a counterbalance system. The stress on these lines has been reduced by attaching them to the Displex via high-pressure rotating joints and by passing them through a metal ring that is attached to the χ circle. A compact vacuum gauge has been mounted through one of the four ports on the top of the cryostat and gives valuable information on the pressure within the Displex sample chamber during an experiment. Crystals are mounted on sharpened 0.3 mm graphite pencil leads that are held in a newly designed sample mount. The temperature reported from the second stage of the Displex has been assessed by consideration of temperature standards that include the solid-state phase transitions of benzil (1,2-diphenylethanedione) and terbium vanadate.

For two solid complexes, [Co(NH3)5NO2]X2 (X = Cl−, Br−), pentaamminenitrocobalt(III) dichloride/dibromide, the anisotropy of structural distortion on cooling the samples from 290 to 150 K was studied by variable-temperature single-crystal X-ray diffraction. The changes in lattice parameters were used to calculate the strain tensors and to scan linear strain in various crystallographic directions. The changes in different intra- and intermolecular distances in the structure were followed. Lattice strain on cooling was compared with lattice strain in the same structures under high hydrostatic pressure and with structural distortion in the course of homogeneous nitro–nitrito linkage isomerization. The effect of isomorphous substitution (Br− for Cl−) with respect to structural strain was discussed. Structural strain in the monoclinic chloride and bromide was compared also with the previously obtained data [Boldyreva, Kivikoski & Howard (1997). Acta Cryst. B53, 394–404] on strain in the orthorhombic structure of another compound of the same CoIII nitropentaammine series, [Co(NH3)5NO2]CI(NO3).

The structures of [Co(NH3)5NO2]Cl2 and [Co(NH3)5NO2]Br2, were refined at 290 K and at 150 K. For both compounds the same crystal was used for data collection at different temperatures. The data at 290 K are in good agreement with previously published results [Bortin (1968). Acta Chem. Scand. 22, 2890–2898; Cotton & Edwards (1968). Acta Cryst. B24, 474–477; Kubota & Ohba (1992). Acta Cryst. B48, 627–632]. The structures at 150 K are anisotropically distorted as compared with those at 290 K, but the space group and the general structural pattern remain the same.

The title compound, C16H6F12, has been obtained as a by-product of the reaction between 2,5-bis­(trifluoro­meth­yl)phenyllithium and zinc(II) chloride. The asymmetric unit contains two independent mol­ecules with a similar almost perpendic­ular conformation of the biphenyl fragments.

Crystallographic observations of eight-membered ring conformations, retrieved from the Cambridge Structural Database, have been mapped and classified using symmetry-adapted deformation coordinates, principal component analysis and cluster analysis. Seven subsets of eight-membered rings, containing 11–32 conformational observations, have been analysed: cyclooctane (dataset 8C1), cyclooctene (8C2), cycloocta-1,3-diene (8C3), mono-exo-unsaturated carbocycles (8C4), monohetero (8A1), 1,5-dihetero (8A2) and 1,3,5,7-tetrahetero rings (8A3). The energetically preferred (by ~7 kJ mol−1) boat-chair form is adopted by 26 of the 32 examples of 8C1, although varying degrees of twist are induced by fusion to rings of sizes three, four and five. Crystallographic results for other subsets also populate the lower-energy areas of the appropriate potential energy hypersurface, but the analyses are complicated by the effects of ring fusion and by the small numbers of relevant crystal structures in some cases.

The crystal structure of isogentisin, C14H10O5, a natural product isolated from Gentiana lutea, has been determined. The phenolic ring system is essentially planar and the displacement of the methoxy substituent from the mean molecular plane is very small. The structure is stabilized by a one-dimensional chain of intermolecular hydrogen bonds.

The title molecular complex, 4-ammonio­cyclo­hexanol (4-hydroxy­cyclo­hexyl)­carbamate, C6H14NO+·C7H12NO3−, forms an ionic column with N—H⋯O, O—H⋯O and C—H⋯O interactions. There are two different cyclic supramol­ecular synthons of note. The crystal structures of ionic amino acids also have similar structural patterns.

The relative configuration of the title compound, C18H27NO4, was determined as being R,S,S,S. There are three crystallographically independent mol­ecules in the asymmetric unit, which show only slight conformational differences. Molecules in the crystal structure are connected by hydrogen bonds in ribbons along the a axis.

The molecule of the title salt, C26H22OP+.C4H3O4−, consists of a benzoyl­methyl­tri­phenyl­phospho­nium cation (HBPPY) and a hydrogen maleate anion. Owing to a strong O—H⋯O intraionic hydrogen bond, each hydrogen maleate ion forms a ring, described by the graph-set notation S11(7). Several C—H⋯O interactions are present between the cations and anions and contribute to the stabilization of the crystal structure.