Acta Crystallographica Section B-structural Science最新文献

The simple dialkyl oxalates are generally liquids at room temperature except for dimethyl and di-tert-butyl oxalate which melt at 327 and 343 K. The crystal structures of diethyl, di-iso-propyl, di-n-butyl, di-tert-butyl and methyl ethyl oxalates were determined. The liquid esters were crystallized using the cryocrystallization technique. A comparison of the intermolecular interactions and packing features in these crystal structures was carried out. The crystal structure of dimethyl oxalate was redetermined at various temperatures. The other compounds were also studied at several temperatures in order to assess the attractive nature of the hydrogen bonds therein. A number of moderate to well defined C-H···O interactions account for the higher melting points of the two solid esters. Additionally, a diminished entropic contribution ΔS(m) in di-tert-butyl oxalate possibly increases the melting point of this compound further.

N,N-Dimethylbiguanidinium bis(hydrogensquarate) features an impressive range of hydrogen bonds within the same crystal structure: neighbouring anions aggregate to a dianionic pair through two strong O-H···O interactions; one of these can be classified among the shortest hydrogen bonds ever studied. Cations and anions in this organic salt further interact via conventional N-H···O and nonclassical C-H···O contacts to an extended structure. As all these interactions occur in the same sample, the title compound is particularly suitable to monitor even subtle trends in hydrogen bonds. Neutron and high-resolution X-ray diffraction experiments have enabled us to determine the electron density precisely and to address its properties with an emphasis on the nature of the X-H···O interactions. Sensitive criteria such as the Laplacian of the electron density and energy densities in the bond-critical points reveal the incipient covalent character of the shortest O-H···O bond. These findings are in agreement with the precise geometry from neutron diffraction: the shortest hydrogen bond is also significantly more symmetric than the longer interactions.

Single crystals of Ca(4)Fe(2)Mn(0.5)Ti(0.5)O(9) have been synthesized using a flux method. The structural characterization using single-crystal X-ray diffraction revealed the space group Amma and unit-cell dimensions of a = 5.3510 (6), b = 26.669 (3), c = 5.4914 (6) Å. The structure is isotypic with Sr(3)NdFe(3)O(9) [Barrier et al. (2005). Chem. Mater. 17, 6619-6623] and exhibits separated brownmillerite-type layers. One-dimensional diffuse scattering shows that the unit cell is doubled along c by alternating the intra-layer order of tetrahedral chains, causing stacking faults along the b direction. A computer simulation was performed, proving that the observed intensity variations along the diffuse scattering rods originates from two different local structures depending on the configuration of the tetrahedral chains. Selected-area electron diffraction experiments exhibit well ordered regions characterized by satellite reflections corresponding to two different superstructures. Both superstructures can be described using the superspace group A2(1)/m(0βγ)0s, with γ = 0.5 and β ≃ 0.27 or β = 0.

Pharmaceutical compounds are mostly developed as solid dosage forms containing a single-crystal form. It means that the selection of a particular crystal state for a given molecule is an important step for further clinical outlooks. In this context, piracetam, a pharmaceutical molecule known since the sixties for its nootropic properties, is considered in the present work. This molecule is analyzed using several experimental and theoretical approaches. First, the conformational space of the molecule has been systematically explored by performing a quantum mechanics scan of the two most relevant dihedral angles of the lateral chain. The predicted stable conformations have been compared to all the reported experimental geometries retrieved from the Cambridge Structural Database (CSD) covering polymorphs and cocrystals structures. In parallel, different batches of powders have been recrystallized. Under specific conditions, single crystals of polymorph (III) of piracetam have been obtained, an outcome confirmed by crystallographic analysis.

A combined experimental and theoretical study of three isoindole derivatives was made on the basis of a topological analysis of their electron-density distributions. Experimental electron densities were determined from high-resolution X-ray diffraction data sets measured with synchrotron radiation at 100 K, whereas theoretical calculations were performed using DFT methods at the B3LYP6-311++G(3df,3pd) level of approximation. Both experimental and theoretical models are in good agreement with each other. Since the analysed structures possess a variety of hydrogen-bonding interactions, weak intermolecular contacts of C-H···C(π), C,N(π)···C,N(π) and H···H types were subject to our special interest and are discussed in detail. They were characterized quantitatively and qualitatively by topological properties using Bader's Atoms in Molecules theory and by mapping the electron-density distribution, electrostatic potential and a geometric function on the Hirshfeld surface. This way the forces and directions of intermolecular interactions as present on the molecular surfaces were depicted and described. These interactions not only guide crystal packing, but are likewise important for recognition processes involving (aza)isoindole fragments in a biological environment.

Structure analysis using single-crystal diffraction was carried out as a contribution to the dispute about the nature of the water channel structure of bassanite (CaSO(4)·0.5H(2)O). A recent result of Weiss & Bräu (2009) for the crystal structure of bassanite (monoclinic, space group C2) at ambient conditions of air humidity was confirmed. In the presence of high relative air humidity the crystal structure of bassanite transformed due to the incorporation of additional water of hydration. The crystal structure of CaSO(4)·0.625H(2)O was solved by single-crystal diffraction at 298 K and 75% relative air humidity. The experimental results provided an insight into both crystal structures. A model explaining the phase transition from CaSO(4)·0.625H(2)O to CaSO(4)·0.5H(2)O was derived. The monoclinic cell setting of CaSO(4)·0.5H(2)O and the trigonal cell setting of CaSO(4)·0.625H(2)O were confirmed by powder diffraction.

The molecular structures of three compounds, LD 425 (C(13)H(14)N(2)O(3)) (1), LD 489 (C(15)H(15)F(3)N(2)O(2)) (2) and LD 473 (C(17)H(19)F(3)N(2)O) (3), are determined by single-crystal X-ray diffraction (XRD) at 180 K. Azacoumarins (1) and (2) possess para-quinoidal bond-length patterns in their benzene rings due to intramolecular charge transfer (ICT) from these rings to the adjoining rings. In contrast, substitution of O with N within the coumarin heterocycle, to form a 2-quinolone, results in the suppression of this ICT effect. Instead, charge transfer within the heterocycle is shown to become more pronounced. Resonance theory is employed to discuss these bond pattern differences and characteristic spectral blue shifts in relation to their coumarin analogues. The application of this theory offers an intuitive understanding of the structure-property relationships in azacoumarins and 2-quinolones which is further supported by quantum chemical calculations. Such an understanding is important for recognizing ICT mechanisms in these compounds which can then be used to facilitate the molecular design of new laser dyes with the desired spectral shifts.

Following on from the success of the previous crystal structure prediction blind tests (CSP1999, CSP2001, CSP2004 and CSP2007), a fifth such collaborative project (CSP2010) was organized at the Cambridge Crystallographic Data Centre. A range of methodologies was used by the participating groups in order to evaluate the ability of the current computational methods to predict the crystal structures of the six organic molecules chosen as targets for this blind test. The first four targets, two rigid molecules, one semi-flexible molecule and a 1:1 salt, matched the criteria for the targets from CSP2007, while the last two targets belonged to two new challenging categories - a larger, much more flexible molecule and a hydrate with more than one polymorph. Each group submitted three predictions for each target it attempted. There was at least one successful prediction for each target, and two groups were able to successfully predict the structure of the large flexible molecule as their first place submission. The results show that while not as many groups successfully predicted the structures of the three smallest molecules as in CSP2007, there is now evidence that methodologies such as dispersion-corrected density functional theory (DFT-D) are able to reliably do so. The results also highlight the many challenges posed by more complex systems and show that there are still issues to be overcome.

The non-perovskite compound LaTi(2)Al(9)O(19) was synthesized and structurally characterized by conventional X-ray powder diffraction and shown to be isostructural with SrTi(3)Al(8)O(19), as confirmed by bond-valence sum calculations. The dielectric properties of LaTi(2)Al(9)O(19) at 1 MHz were measured. The crystal structure of La(3)Ti(5)Al(15)O(37), which is referred to as the most complex structure solved ab initio from X-ray powder diffraction (XRPD) to date, is shown to be incorrect.

In the range 308-90 K single crystals of [Ni(H(2)O)(6)](NO(3))(2)·(15-crown-5)·2H(2)O continue to diffract well as they undergo three phase transitions. Structures have been determined at 28 different temperatures; data were collected at six more but could not be refined satisfactorily. The transitions identified structurally are in good agreement with those found by thermal analysis. Phase I (above ca 292 K; I2/m, Z = 2, Z' = ¼) is disordered because the 15-crown-5 molecule is located on a 2/m site. Phase II (ca 292-248 K; P2(1)/m, Z = 2, Z' = ½) is less disordered. Phase III (ca 248-208 K; B2(1), Z = 28, Z' = 7 in a commensurate approximation) is modulated with some regions resembling phase II and the rest resembling the fully ordered phase IV (P2(1)/c, Z = 4, Z' = 1) that is stable below ca 208 K. The modulated structure could be determined well because crystals flash-cooled to 90 K usually retain the phase III structure. The unusual phase III may be a consequence of the differing thermal contraction of two types of alternating layers, one composed of neutral 15-crown-5 molecules and the other composed of the cations, anions and lattice water molecules, which form a hydrogen-bonded network. The two kinds of layers are linked by O(ether)···H(2)O(axial)-Ni hydrogen bonds.