Preferred orientation complex

Huisgen has also studied the effects of substitution in the keten in the reactions of a series of alkylphenyl ketens to ethyl cis- and trans-propenyl ethers. With the cis-enol ethers the thermodynamically less stable cyclobutanone is always produced. This is the same result as that found in the addition of ketens to cyclopentadiene and other cis-olefins, and the mechanistic implications are the same. With the trans-enol ether, the thermodynamically more stable product is formed, and this observation can be rationalized in terms of a [tc2 + k2 J cycloaddition if the preferred orientation complex has the substituent on the keten between the alkoxy-group and a hydrogen rather than between a methyl group and a hydrogen on the enol ether. In all the cases studied, the cis-enol ether reacted more rapidly than its trans-isomer. This cis trans reactivity ratio is not found in [2 + 2] additions proceeding via zwitterionic intermediates. For example, the rate ratio for the reaction of TCNE with cis- and trans-1-alkenyl ethers is very close to unity. 

The stereoselectivity of the catalyzed reaction appears to be associated with the complexation step, which is product determining. The preferred orientation of approach of the complex is anti to the oxygen substituent, which acts as an electron acceptor and more electronegative groups enhance reactivity. The preferred conformation of the alkene has the hydrogen oriented toward the double bond and this leads to a syn relationship between the alkyl and oxygen substituents.170... 

Fig. 5.3. (a) View of (S)-BINAP-ruthenium complex showing the chiral environment, (b) Relationship of reactant to chiral environment showing preferred orientation. The binaphthyl rings are omitted for clarity. Adapted from J. 

The mathematical expression of N(6, q>, i//) is complex but, fortunately, it can be simplified for systems displaying some symmetry. Two levels of symmetry have to be considered. The first is relative to the statistical distribution of structural units orientation. For example, if the distribution is centrosymmetric, all the D(mn coefficients are equal to 0 for odd ( values. Since this is almost always the case, only u(mn coefficients with even t will be considered herein. In addition, if the (X, Y), (Y, Z), and (X, Z) planes are all statistical symmetry elements, m should also be even otherwise = 0 [1]. In this chapter, biaxial and uniaxial statistical symmetries are more specifically considered. The second type of symmetry is inherent to the structural unit itself. For example, the structural units may have an orthorhombic symmetry (point group symmetry D2) which requires that n is even otherwise <>tmn = 0 [1], In this theoretical section, we will detail the equations of orientation for structural units that exhibit a cylindrical symmetry (cigar-like or rod-like), i.e., with no preferred orientation around the Oz-axis. In this case, the ODF is independent of t/z, leading to n — 0. More complex cases have been treated elsewhere [1,4]... 

The lead precursor in PbSe film deposition was lead acetate complexed with triethanolamine. The growth rate for PbSe was 0.18-0.16nm/cycle. The films were polycrystalline (i.e., cubic) without preferred orientation. The stoichiometry of SILAR-grown PbSe was found to be 1 1 within the limits of the RBS technique. Impurities detected were 5 at.% of oxygen and 8 at.% of hydrogen.103... 

A geometric isomer of the vinylallene mentioned above also undergoes [4 + 21-cycloaddition with butadiene to furnish a cydohexene derivative in 90% yield with excellent diastereomeric purity (>99 1). The preferred formation of a jt-allylpalla-dium complex from the axially oriented complex accounts for the trans selectivity (Scheme 16.79) [89]. 

The first ingredient in any theory for the rheology of a complex fluid is the expression for the stress in terms of the microscopic structure variables. We derive an expression for the stress-tensor here from the principle of virtual work. In the case of flexible polymers the total stress arises to a good approximation from the entropy of the chain paths. At equilibrium the polymer paths are random walks - of maximal entropy. A deformation induces preferred orientation of the steps of the walks, which are therefore no longer random - the entropy has decreased and the free energy density/increased. So... 

Each sphere has a lamellar structure and a single preferred orientation but shows systematic variations in the optical extinction patterns, which indicate some variation from a strictly lamellar arrangement this is most noticeable at the poles of spheres. As they grow and meet obstructions to their enlargement in particular directions, their extinction behavior becomes increasingly complex, especially when the pitch nears solidification and the complete mosaic structure is formed. 

For the case of adsorbed complex molecules, which generally have a preferred orientation with respect to the substrate atomic lattice in their energy minimum configuration, the possibility of 2-D molecular rotations needs to be considered. These rotations require thermal activation, analogous to lateral transport. In the simplest case they imply the overcoming of a unique rotation energy barrier Er, which may be higher, equal or lower than the... 

Toraya s WPPD approach is quite similar to the Rietveld method it requires knowledge of the chemical composition of the individual phases (mass absorption coefficients of phases of the sample), and their unit cell parameters from indexing. The benefit of this method is that it does not require the structural model required by the Rietveld method. Furthermore, if the quality of the crystallographic structure is poor and contains disordered pharmaceutical or poorly refined solvent molecules, quantification by the WPPD approach will be unbiased by an inadequate structural model, in contrast to the Rietveld method. If an appropriate internal standard of known quantity is introduced to the sample, the method can be applied to determine the amorphous phase composition as well as the crystalline components.9 The Rietveld method uses structural-based parameters such as atomic coordinates and atomic site occupancies are required for the calculation of the structure factor, in addition to the parameters refined by the WPPD method of Toraya. The additional complexity of the Rietveld method affords a greater amount of information to be extracted from the data set, due to the increased number of refinable parameters. Furthermore, the method is commonly referred to as a standardless method, since the structural model serves the role of a standard crystalline phase. It is generally best to minimize the effect of preferred orientation through sample preparation. In certain instances models of its influence on the powder pattern can be used to improve the refinement.12... 

K for 13 h in a Teflon-lined autoclave. There was, however, deviation in the peak intensities compared to the standard diffraction pattern, indicating the preferred orientation of the tubular ZnO (wurtzite). TEM images showed that the walls of the ZnO tubes were not very smooth due to the build-up by polycrystalline nanoparticles. The Zn(NH3)42 complex is hydrolyzed in the presence of ammonia and nanoparticles of ZnO are produced along with the nanotubes as seen in the TEM images. As the temperature and duration of the reaction are increased, there is increased evolution of ammonia and the ZnO nanoparticles assemble along certain orientations and aggregate to the hollow tubular structures. 

The second mode of crystal formation that occurs in dentin is via matrix vesicles. These are phospholipid delimited packages of specialized enzymes, macromolecular complexes and ions, that induce the precipitation of amorphous calcium phosphate. At some point the latter crystallizes into carbonated apatite crystals, that have no preferred orientation [62], These appear smaller and denser than the crystals that form in the collagen framework. 

Similar considerations hold also for syndio-specific polymerization catalysts, for which the C -symmetric zirconocene complex shown in Figure 19 is a prototype. Here the two coordination sites have opposite chirality. The preferred orientation of the C(oc)-C((3) segment of the polymer chain and hence the preferred enantiofacial orientation of the inserting olefin will thus alternate with each consecutive insertion, by which the Zr-CH2(polymer) bond moves from one coordination site to the other. 

All in all, liquid-crystalline media are not generally useful solvents for controlling the rates and stereochemistries of chemical reactions. In each case, careful consideration of the fine details regarding the structure of educts and activated complex, their preferred orientations in a liquid-crystalline solvent matrix, and the disruptive effects that each solute has on the solvent order has to be made. A mesophase effect can only be expected when substantial changes in the overall shape of the reactant molecule(s) occur during the activation process [734],... 

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