Diffraction asymmetric

Crazing Incidence Asymmetric Bragg (CIAB) Diffraction... 

Simulated SWCNT ED patterns will be presented below. Tbe most striking difference with tbe MWCNT ED patterns is tbe absence of tbe row of sharp oo.l reflexions. In tbe diffraction pattern of ropes there is still a row of sharp reflexions perpendicular to the rope axis but which now corresponds to the much larger interplanar distance caused by the lattice of the tubes in the rope. The ho.o type reflexions are moreover not only asymmetrically streaked but also considerably broadened as a consequence of the presence of tubes with different Hamada indices (Fig. 3). 

Depending on the size and packing (space group) of the asymmetric unit in the crystal and the resolution available, many tens of thousands of diffraction spots must be recorded to determine a structure. 

The reaction with PPh2CCH leads to the formation of [Au(QF5)(PPh2CCH)[ [53] whose P H NMR spectrum shows a singlet at 17.2ppm, in the H NMR spectrum the resonance of the C = CH proton is observed at 3.46 ppm. The IR spectrum shows, besides the pentafluorophenyl absorptions, a band at 3271 cm due to the V(Csch) and another absorption at 2056 cm for the asymmetric C = C stretch. The structure of this complex was studiedby X-ray diffraction, the Au(I) atom is an almost linearly coordinated and the Au—C and Au—P distances are in the range of the values found for similar complexes. The excitation and the emission data in the solid state at 77 K are 331 and 445 nm. 

The crystal structure of the complex [PhCH2PPh3][Au(C6F5)Cl] has been determined by X-ray diffraction it consists of an anion-cation pair where the anion shows a two coordinate Au(I) atom bonded to a Cl atom and a C Fs ring and the coordination is almost linear (molecules with two different angles were found but the differences between the two molecules in the asymmetric unit are not significant 177.8(5) and 179.8(4) A) [78]. 

In 2005, Carretero et al. reported a second example of chiral catalysts based on S/P-coordination employed in the catalysis of the enantioselective Diels-Alder reaction, namely palladium complexes of chiral planar l-phosphino-2-sulfenylferrocenes (Fesulphos). This new family of chiral ligands afforded, in the presence of PdCl2, high enantioselectivities of up to 95% ee, in the asymmetric Diels-Alder reaction of cyclopentadiene with A-acryloyl-l,3-oxazolidin-2-one (Scheme 5.17). The S/P-bidentate character of the Fesulphos ligands has been proved by X-ray diffraction analysis of several metal complexes. When the reaction was performed in the presence of the corresponding copper-chelates, a lower and opposite enantioselectivity was obtained. This difference of results was explained by the geometry of the palladium (square-planar) and copper (tetrahedral) complexes. 

Secondly, the molecular structure of [TpPr 2]Zn 2(/u,-7)1,7)2-C03), as determined by x-ray diffraction (151,153), reveals that the carbonate ligand is coordinated in an asymmetric manner, with unidentate coordination to one zinc center and bidentate coordination to the other zinc center, i.e., fi-ri1,ri2-C03 (Scheme 24). Such coordination is noticeably distinct from that of [TpBut Me]Zn 2(/x.-r)1,7 1-C03), in which the carbonate ligand bridges in a symmetric unidentate mode. 

The absence of asymmetric carbon atoms in IER could explain why the strain-induced crystallization is more pronounced in this case. Numerous strong reflections of the type shown in Figure 11 were obtained by X-ray diffraction of stretched IER samples at 30 °C. It is therefore evident that a well-formed crystal lattice exists. 

The structural changes that accompanied the [2 + 2] photodimerization of the metastable a -polymorph of ort/zo-ethoxy-tranx-cinnamic acid have been studied [93]. In this study, the photochemical reaction was carried out at 293 K, and observed in situ by single-crystal X-ray diffraction. In the structure of the title compound, the three molecules in the asymmetric unit are arranged to form two potential reaction sites, but only one of these was found to be photoreactive. Since only two out of three molecules in the asymmetric unit take place in the photodimerization reaction, the crystal of the final product contains an ordered arrangement of the photodimer and the unreacted monomer. 

Dicyanofuroxan (3,4-dicyano-l,2,5-oxadiazole 2-oxide), the precursor to the NCCNO (see structure 11) species, has been studied in the solid and gas phases to obtain both structural and electronic information. The solid-state structure determined by X-ray diffraction gives an orthorhombic space group PnaZ, with a= 10.2578(14), b = 10.8818(12), and c= 10.2259(15) A. There are two independent molecules with similar geometries in the asymmetric unit. The gas-phase molecule was characterized by Hel photoelectron, Hel and HLtti(3i7 photoionization, and IR spectroscopy. The... 

Although X-ray and neutron diffraction and scattering methods give only approximate estimates of hydration numbers they can provide precise measures of ion-water distances in solution. In calcium chloride and bromide solutions of various concentrations, Ca-0 distances of between 2.40 and 2.44 A have been reported (167,168,171,172) Ca-0 — 2.26A was claimed in an early X-ray investigation of molar calcium nitrate solution (167,186). EXAFS and LAXS studies showed a broad and asymmetric distribution of Ca-0 distances centered on a mean value of 2.46 A (174). 

Myelin in situ has a water content of about 40%. The dry mass of both CNS and PNS myelin is characterized by a high proportion of lipid (70-85%) and, consequently, a low proportion of protein (15-30%). By comparison, most biological membranes have a higher ratio of proteins to lipids. The currently accepted view of membrane structure is that of a lipid bilayer with integral membrane proteins embedded in the bilayer and other extrinsic proteins attached to one surface or the other by weaker linkages. Proteins and lipids are asymmetrically distributed in this bilayer, with only partial asymmetry of the lipids. The proposed molecular architecture of the layered membranes of compact myelin fits such a concept (Fig. 4-11). Models of compact myelin are based on data from electron microscopy, immunostaining, X-ray diffraction, surface probes studies, structural abnormalities in mutant mice, correlations between structure and composition in various species, and predictions of protein structure from sequencing information [4]. 

Seven crystal systems as described in Table 3.2 occur in the 32 point groups that can be assigned to protein crystals. For crystals with symmetry higher than triclinic, particles within the cell are repeated as a consequence of symmetry operations. The number of asymmetric units within the unit cell is related but not necessarily equal to the number of molecules in a unit cell, depending on how the molecules are related by symmetry operations. From the symmetry in the X-ray diffraction pattern and the systematic absence of specific reflections in the pattern, it is possible to deduce the space group to which the crystal belongs. 

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