Acta Crystallographica Section B-structural Science最新文献

The various superstructure phases that occur with the anion-deficient compositions of binary oxides MO(2-x) with the fluorite structure as parent are explored here in terms of the original 'coordination defect' (or CD) concept in which each vacant oxygen site, □, is 'coordinated' by six O atoms thereby creating the octahedral 'structure-determining' entity [M(3.5)□O(6)]. It emerges that the structure and composition of each anion-deficient (polymorph) phase can be described in terms of crystallographic `motifs' which comprise sets of parallel coplanar polygons based on ½<210>(F) and ½<111>(F) CD linkages.

The synthesis and characterization of a series of halogen-substituted pseudoterpyridine Zn(II) homoleptic mononuclear complexes, based on ligands L(11)-L(44) [2,6-pyridinedicarboxaldehydebis(p-R-phenylimines), R = F, Cl, Br, I] are reported. Neither of the structures contain relatively strong classical hydrogen bonds (OH···O, NH···O, OH···N, NH···N) and the structure packing is thus determined by a subtle interplay of weaker interactions. Isostructurality of the four halogen analogues is very rare, and in this study -Br, -Cl and -F are found to be isostructural in different degrees, whereas -I is not. Interestingly, although it is closely isostructural to the -Cl and -Br compounds, the F analogue is shown not to form F···O bonds, while the Cl and the Br analogues do form Hal···O bonds. This raises an important question on the role of Hal···O bonds in the structuration of the crystal packing, particularly the stabilization effect. Similarly, while the CH···Hal interaction seems to give one-dimensional cohesion in the -Cl and -Br analogues, this feature is absent in the -F analogue, despite its close isostructurality. CH···O interactions appear to dominate to a first degree the cohesion between the anionic trifluoromethanesulfonate network and the cationic Zn-pyridinedicarboxaldehydebis(p-R-phenylimines) network. The analysis of these interactions is corroborated by reduced density gradient calculations based on promolecular densities.

The structure of low-temperature grown GaAs with equidistant δ-layers of Sb and P was studied by analysis of the X-ray curves, which was supported by optical absorption measurements and transmission electron microscopy. The simultaneous fitting of the X-ray reflectivity curve and diffraction ones for GaAs (004) and GaAs (115) crystallographic planes provided reliable information about the period of δ-layer superlattice, thickness of the Sb and P δ-layers, and amount of excess As. Variation of these parameters was documented when excess As precipitated into As nanoinclusions upon annealing. The Sb and P δ-layers impact differently on the As precipitation processes in low-temperature grown GaAs. The combination of Sb and P δ-layers appears to be an effective tool for spatial patterning of the nanoinclusion array and prevention of the defect formation under annealing.

A range of single-crystal structures of the type [Pd(cod)(LL'-Bid)]A, where LL'-Bid = acetylacetonato (acac), thenoyltrifluoroactetonato (thtfac) and hexafluoroacetylacetonato (hfacac), and A = tetrafluoroborate (BF(4)(-)) and hexafluorophosphate (PF(6)(-)), are reported. The complexes [Pd(cod)(acac)]PF(6) (I), [Pd(cod)(thtfac)]PF(6) (III), [Pd(cod)(thtfac)]BF(4) (IV) and [Pd(cod)(hfacac)]PF(6) (V) are isostructural in the monoclinic space group P2(1)/c. The influence of the variation of the β-diketonato-type ligands on the coordination geometry of cis,cis-1,5-cycloocta-1,5-diene (cod) was investigated and found that no significant changes to the Pd-C and C=C bond distances were observed. The `Venus fly trap' parameters vary by 7.8° for the 'jaw' angle (ψ), while the `bite' angle (χ) remains virtually constant.

The crystal structure of the high-pressure polymorph (α') of an antidiabetic drug, chlorpropamide [4-chloro-N-(propylaminocarbonyl)benzenesulfonamide, C(10)H(13)ClN(2)O(3)S], which is formed at ~2.8 GPa from the α-polymorph (P2(1)2(1)2(1)) on hydrostatic compression in saturated ethanol solution, has been determined. As a result of the phase transition, the a, c and α parameters change jumpwise, whereas the changes in b parameter are continuous through the phase transition point. The high-pressure form is monoclinic (P2(1)11) and has Z' equal to 2, the two independent molecules differing in their conformations. The hydrogen bonds expand slightly in the high-pressure polymorph after the transition, and this expansion is interrelated with the changes in molecular conformations enabling a denser packing. The transition is reversible, but the crystal quality deteriorates as a result of multiple compression-decompression cycles, and a pseudomerohedral twinning accompanies the transformation.

The title compound exists as polymorph (I), Fdd2 with Z = 8 [Pérez-Folch et al. (1997). J. Chem. Cryst. 27, 367-369; Marsh (2004). Acta Cryst. B60, 252-253], and as polymorph (II), P2(1)2(1)2 with Z = 2 [Martins et al. (2009). J. Phys. Chem. A, 113, 5998-6003]. We have redetermined both structures at somewhat lower temperatures [(I) at 180 K rather than room temperature; (II) at 100 K rather than 150 K]. For polymorph (I) the space group Fdd2 is confirmed rather than the original choice of Cc. The molecular structures of both polymorphs are essentially identical, with exact crystallographic twofold symmetry, approximate C(2v) symmetry, and a trans orientation of the H-N-C=O moiety. In both polymorphs the molecules associate into chains of rings with graph set C(4)[R(2)(1)(6)] via bifurcated hydrogen-bond systems C(N-H)(2)···O=C. In the polar structure (I) the chains are necessarily all parallel, whereas in (II) equal numbers of parallel and antiparallel chains are present. Further physical investigations [differential scanning calorimetry (DSC), powder investigations, solvent-induced phase conversions] were undertaken: these showed: (i) that the commercially available compound consists predominantly of polymorph (II), which on heating transforms into polymorph (I) by an endothermic reaction, so that both polymorphs are related by enantiotropism; (ii) that polymorph (I) represents the more stable modification at room temperature, where polymorph (II) is metastable, with the thermodynamic transition temperature lying somewhere between 253 K and room temperature. An apparent third polymorph, consisting of fibrous needles, was shown by powder diffraction to consist of a mixture of polymorphs (I) and (II).

For the crystal structures of oxoborates, the value of ~1.2 Å can be regarded as the lower physical limit for B(III)-O bond lengths. The oxoborate crystal structures with B(III)-O bond lengths shorter than ~1.2 Å have been found to be either doubtful or clearly erroneous.

The bisphosphoramidate (C(6)H(5)O)(2)P(O)NH(CH(2))(4)NHP(O)(OC(6)H(5))(2) crystallizes in two polymorphs, one (ndl) with a needle habit from tetrahydrofuran (THF)/ethanol and another (prm) which forms prisms from H(2)O/ethanol. The molecules in the two forms differ from each other in some torsion angles and the orientation of the diaminobutane bridge, although the differences between the similar bond lengths are not significant for the two polymorphs. The geometry optimizations at the B3LYP/6-31+G* level for isolated molecules show that the two conformers which exist in the crystalline state also represent local gas-phase energy minima. The decrease in the N-H distance from the optimized to the crystal structures has been described in terms of the decrease in electron density (ρ) at the bond-critical point (b.c.p.) of the N-H bond path when the molecule participates in hydrogen bonding, comparing the results of atoms-in-molecules (AIM) and natural bond orbital (NBO) analyses for fully optimized structures ndl and prm with their hydrogen-bonded model clusters.

The one-step reaction of isophthaloyl dichloride with the 2-aminopyridine derivative (methyl 6-aminonicotinate) yields (i) a trimer-based macrocycle (EsIO)(3) and (ii) a tetramer-based macrocycle (EsIO)(4) in modest isolated synthetic yields (total of 25%), together with (iii) longer open-chain oligomers. The macrocyclization relies on the semi-flexible imide hinge formed by reaction of the 2-amino(pyridine) functional group with two acyl chloride functional groups. The determining factors in macrocycle synthesis are: (a) imide formation using the heteroaromatic ortho-N functionality; (b) the inherent ability of the imide to twist by 85-115° from planarity (as measured by the CO···CO imide torsion angles), thereby providing a hinge for macrocyclic ring closure or potentially (non)helical assembly in oligomer/polymer formation; (c) the conformational flexibility of the isophthaloyl group with meta-related carbonyl groups to twist and adopt either syn or anti conformations, although the syn conformation is observed structurally for all isophthaloyl groups in both (EsIO)(3) (trezimide) and (EsIO)(4) (tennimide) macrocycles.

The charge-flipping algorithm (CFA) is a member of the diverse family of dual-space iterative phasing algorithms. These algorithms use alternating modifications in direct and reciprocal space to find a solution to the phase problem. The current state-of-the-art CFA is reviewed and it is put in the context of related dual-space algorithms with relevance for crystallography. The CFA has found applications in many crystallographic problems. The principal applications in various fields are described with sections devoted to routine structure solution, the solution of complex structures from powder diffraction data, the solution of incommensurately modulated crystals and quasicrystals, macromolecular crystallography and single-particle imaging.