R. Peschar

Structure of mono-acid even-numbered β-triacylglycerols

The crystal structure of the β polymorph of tripalmitin (1,2,3-trihexadecanoylglycerol, β-PPP) has been determined by single-crystal X-ray diffraction. The molecules crystallize in space group P1¯ in an asymmetric tuning-fork conformation. This structure and the already-known crystal structures of β-tricaprin (β-CCC) and β-xadtrilaurin (β-LLL) could be matched in an overlap model. Apart from a difference in chain length, the three structures are almost identical. The overlap model can be used to predict the crystal structure of the other members of the CnCnCn-type (n = even) TAG series reasonably accurately. This is demonstrated by predicting the crystal structure for β-trimyristin (β-xadMMM) and successively comparing the experimental and calculated X-ray powder diagrams.

Structure of CnCn+2Cn-type (n = even) β′-triacylglycerols

The crystal structures of the β′ phase of CLC (1,3-didecanoyl-2-dodecanoylglycerol) and MPM (1,3-ditetradecanoyl-2-hexadecanoylglycerol) have been determined from single-crystal X-ray diffraction and high-resolution X-ray powder diffraction data, respectively. Both these crystals are orthorhombic with space group Iba2 and Z = 8. The unit-cell parameters of β′-CLC are a = 57.368u2005(6), b = 22.783u2005(2) and c = 5.6945u2005(6)u2005A and the final R value is 0.175. The unit-cell parameters of β′-xadMPM are a = 76.21u2005(4), b = 22.63u2005(1) and c = 5.673u2005(2)u2005A and the final Rp value is 0.057. Both the β′-CLC and β′-MPM molecules are crystallized in a chair conformation, having a bend at the glycerol moiety. The zigzag planes of the acyl chains are orthogonally packed, as is typical for a β′ phase. Furthermore, unit-cell parameters of some other members of the CnCn+2Cn-type triacylglycerol series have been refined on their high-resolution X-ray powder diffraction pattern. Finally, the crystal structures are compared with the currently known structures and models of triacylglycerols.

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. Grid search and Rietveld refinement have been used to determine and refine the structure, respectively. Both substances crystallize in space group Pbar 1 with Z = 2. The unit-cell parameters for β-MMM are a = 12.0626u2005(6), b = 41.714u2005(1), c = 5.4588u2005(3)u2005A, αxa0=xa073.388u2005(4), β = 100.408u2005(5) and γ = 118.274u2005(4)°. For β-SSS the unit-cell parameters are a = 12.0053u2005(7), b = 51.902u2005(2), c = 5.4450u2005(3)u2005A, α = 73.752u2005(5), β = 100.256u2005(6) and γ = 117.691u2005(5)°. Soft-distance restraints have been applied to the molecules during refinement. For β-MMM the final Rp value obtained is 0.053 and for β-SSS the final Rp value is 0.041.

Geometrical aspects of real‐time powder diffraction using a normal generator and a linear diode‐array detector

A real-time powder diffractometer has been constructed, with which it is possible to follow structural changes taking place in a specimen in minutes or even seconds. Arrangements are available to control and change the temperature of the sample in the diffractometer. A formula describing the line width resulting from the non-conventional geometry has been derived. This formula can be used for tuning the equipment to meet practical requirements. Experiments to assess this formula are presented

Structures of mono‐unsaturated triacylglycerols. II. The β2 polymorph. Corrigendum

A replacement Fig. 3 to the paper by van Mechelen et al. (2006), Acta Cryst. B62, 1131–1138 is given.

Structure determination of two metal-organic complexes from high-resolution synchrotron powder diffraction data.

The crystal structures of [1,2-bis(2,6-diisopropylphenylimino)acenaphthene-N,N]carbonylchlororhodium(I) (1) and [N,N-ethylene-bis(3-methylsalicylideneiminato)-O,N,N,O](tetrahydrofurfuryl)-cobalt(II) (2) have been determined from high-resolution synchrotron X-ray powder diffraction data. Compound 1 is the first neutral Rh complex, in contrast with findings in the literature, containing a bidentate nitrogen ligand, and compound 2 is the first three-dimensional structure of a (five-coordinated) tetrahydrofurfurylcobalt(III) complex. Grid-search and Rietveld refinement have been used to determine and refine the structures, respectively. Crystals of 1 are orthorhombic, space group Pbca, Z = 8, with cell parameters a = 21.729 (2), b = 27.376 (3), c = 11.580 (1) A. Crystals of 2 are monoclinic, space group P2(1)/n, Z = 4, a = 16.6701 (6), b = 9.4170 (4), c = 13.7088 (7) A and beta = 96.520 (3) degrees. Chemical diagrams for the two compounds are given. Soft restraints were applied during Rietveld refinement; for 1 converging to R(p) = 8.4%, R(w) = 11.0%, GoF = 2.3, and for 2 converging to R(p) = 8.5%, R(w) = 11.4%, GoF = 7.6.

Intermolecular —CH3⋯O2N— contacts in two polymorphic modifications of (1E)-N'-[(E)-2-cyano-1-(dimethylamino)-2-nitrovinyl]-N,N-dimethyl­ethanimidamide

The title compound was synthesized and isolated in two crystal modifications. The structure of the orthorhombic modification was determined by the X-ray powder diffraction method and the structure of the monoclinic modification was determined using the X-ray single-crystal diffraction technique. The molecules in both polymorphs are E,E isomers. Intermolecular H(3)C....NO(2) contacts and their role in the formation of the polymorphic modifications are analyzed.

The fat bloom mechanism of chocolate clarified from X-ray powder diffraction data

Lamellar structure and its formation from melt has been studied in ultra-long monodisperse alkanes using realtime versions of small-angle neutron and X-ray (SANS and SAXS) and low-frequency-shift Raman scattering. Pure and binary linear unlabeled and deuterium endlabeled alkanes were examined, as well as an asymmetrically methyl-branched alkane, C192H385CH(CH3)C99H199. It was possible to study the structure and the transformation of the initial transient Non-Integer Folded (NIF) form into a variety of secondary phases, whose formation depends on temperature [1,2]. These secondary phases include the extended-chain form and two mixed folded-extended forms: the double-layer and the triple-layer superlattice. The semicrystalline form, which features in pure alkanes as the transient NIF, becomes a stable high-temperature form in either binary mixtures of linear alkanes [3] or in pure branched alkanes. The rich layer polymorphism in the asymmetrically branched alkane was found to be due to the anchoring of the methyl branch at the crystal surface. Two semicrystalline and two superlattice forms were found [4] see Figure. The exceptionally high chain tilt angle (46°) was caused by the inability of surface overcrowding to be alleviated by chain folding. The study provides new insights into the complex problem of chain folding in crystalline polymers, exactly 50 years after its discovery.

Progress in structure determination of larger organic and organo-metallic compounds from powder diffraction data

The method of the joint probability distribution function has been used to derive accurate phase estimates for proteins via: a) the method of the isomorphous derivatives (SIR, MIR); b) the anomalous dispersion techniques (SAD, MAD); c) the method of the isomorphous derivatives when anomalous dispersion effects are present (SIRAS, MIRAS). For SIR, MIR, SIRAS and MIRAS cases the program automatically provides phase values (and a structural model) starting from the diffraction data. In the SAD and MAD cases the program first locates the anomalous scatterers, automatically assigned structure factor phases and refines them, and lastly provides a structural model. Some experimental results will be described.