Biomedical subjects

René Peschar

Publications and source records attributed to René Peschar.

6 recordsLinked to original sources

PubMed2006

High-spin- and low-spin-state structures of [Fe(chloroethyltetrazole)6](ClO4)2 from synchrotron powder diffraction data.

The spin-crossover complex [Fe(teec)(6)](ClO(4))(2) (teec = chloroethyltetrazole) exhibits a 50 % incomplete spin crossover in the temperature range 300-30 K. Time-resolved synchrotron powder diffraction experiments have been carried out to elucidate its structural behavior. We report crystal structure models of this material at 300 K (high spin) and 90 K (low spin), as solved from synchrotron powder diffraction data by using Genetic Algorithm and Parallel Tempering techniques and refined with Rietveld refinement. During short synchrotron powder diffraction experiments (five minutes duration) two distinguishable lattices were observed the quantities of which vary with temperature. The implication of this phenomenon, that is interpreted as a structural phase transition associated with the high-to-low spin crossover, and the structural characteristics of the high-spin and low-spin models are discussed in relation to other compounds showing a similar type of spin-crossover behavior.

Journal Article↗

PubMed2005

Low-spin state structure of [Fe(chloroethyltetrazole)6](BF4)2 obtained from synchrotron powder diffraction data.

The complex [Fe(teec)6](BF4)2 (teec = chloroethyltetrazole) shows a two-step complete spin-crossover transition in the temperature range 300-90 K. Time-resolved synchrotron powder diffraction experiments have been carried out in this temperature range, and crystal structure models have been obtained from the powder patterns by using the parallel tempering technique. Of these models, the low-spin state structure at 90 K has been refined completely with Rietveld refinement. Its structural characteristics are discussed in relation to the high-spin state model and other spin-crossover compounds. The complex shows a remarkable anisotropic unit-cell parameter contraction that is dependent on the applied cooling rate. In addition, the possible important implications for the interpretation of spin-crossover behavior in terms of structural changes are discussed.

Journal Article↗

PubMed2005

Structural analysis of a melaminium polyphosphate from X-ray powder diffraction and solid-state NMR data.

The crystal structure of the environmentally friendly flame retardant melaminium polyphosphate (MPoly) (2,4,6-triamino-1,3,5-triazinium x PO(3))(n)was determined by a direct-space global optimization technique from X-ray powder diffraction data. Solid-state NMR was used to corroborate the proposed hydrogen-bonding model and to determine the average degree of polymerization (n > 100). An analysis of the crystal structure of MPoly reveals aspects of molecular geometry and packing that are characteristic for melamine-containing compounds and polyphosphate salts. A comparison of MPoly with the crystal structures of its precursors melaminium orthophosphate (MP) and melaminium dihydrogenpyrophosphate (MPy) provides insight in the mechanism of the endothermic dehydration processes that takes place in the reaction path MP --> MPy --> MPoly. Solid-state NMR characterization of various samples of the same batch showed inhomogeneities in the MPoly composition. Various quantities of orthophosphates were found, which cannot be assigned to be MP.

Journal Article↗

PubMed2004

Structures of Fe(II) spin-crossover complexes from synchrotron powder-diffraction data.

Crystal structure determination and analysis have been carried out for the two spin-crossover compounds [Fe(teeX)(6)](BF(4))(2) (teeX is haloethyltetrazole; X = I: teei; X = Br: teeb), in both their high-spin (near 300 K) and their low-spin states (T = 90 K), using high-resolution powder-diffraction data collected at the ESRF (Grenoble, France) and SPring8 (Japan) synchrotron radiation facilities. The structures of teei have been solved using various direct-space structure determination techniques (grid search, genetic algorithm and parallel tempering) and refined with the Rietveld method using geometrical restraints. In the case of teeb, a structural model was found but a full refinement was not successful because of the presence of a significant amount of an amorphous component. Analysis of the structures (space group P2(1)/c, Z = 2) and diffraction data, and the absence of phase transitions, show the overall structural similarity of these compounds and lead to the conclusion that the gradual spin-crossovers are likely to be accompanied by small structural changes only.

Journal Article↗

PubMed2002

5,5-Dimethyl-2-[6-methyl-2-(methylsulfanyl)pyrimidin-4-yloxy]-1,3,2-dioxaphosphorinane-2-thione.

The title compound, C(11)H(17)N(2)O(3)PS(2), is a cyclic thiophosphoryl pyrimidine derivative exhibiting insecticidal properties. The crystal structure determination gives evidence for the presence of the thione isomer of the compound. The pyrimidine nucleus is planar and its substituents have small deviations from the least-squares plane. The dioxaphosphorinane ring adopts a chair conformation. The lack of classical hydrogen bonds and the weak intermolecular interactions lead to a 'loose' packing characterized by channels in the structure.

Crystallography, X-Ray↗

PubMed2001

Structure of C15-, C17- and C19-mono-acid beta-triacylglycerols.

The crystal structures of beta-1,2,3-tris(pentadecanoyl)glycerol (beta-C15C15C15), beta-1,2,3-tris(heptadecanoyl)glycerol (beta-C17C17C17) and beta-1,2,3-tris(nonadecanoyl)glycerol (beta-C19C19C19) have been determined from high-resolution X-ray powder diffraction data. Grid search and Rietveld refinement have been used to determine and refine the structures, respectively. As in beta-1,2,3-tris(tridecanoyl)glycerol (beta-C13C13C13) and the even-numbered mono-acid triacylglycerols, all three odd-numbered monoacid triacylglycerols crystallize in space group P1 with Z = 2 in an asymmetric tuning-fork conformation and have a lateral acyl chain packing resulting in a layered structure.

Journal Article↗