Crystals derivative

The reason for this can be seen as follows. In a perfect crystal with the ions held fixed, a positive hole would move about like a free particle with a mass m depending on the nature of the crystal. In an applied electric field, the hole would be uniformly accelerated, and a mobility could not be defined. The existence of a mobility in a real crystal derives from the fact that the uniform acceleration is continually disturbed by deviations from a perfect lattice structure. Among such deviations, the thermal motions of the ions, and in particular, the longitudinal polarisation vibrations, are most important in obstructing the uniform acceleration of the hole. Since the amplitude of the lattice vibrations increases with temperature, we see how the mobility of a... 

Williams and coworkers have reported a DKR of ot-bromo [56a] and a-chloro esters [56b]. In the latter case, the KR is catalyzed by commerdally available cross-linked enzyme crystals derived from Candida cylindracea lipase. The racemization takes place through halide 5 2 displacement. The DKR is possible because the racemization of the substrate is faster than that of the produd (carboxylate). For the ester, the empty ii (C=0) orbital is able to stabilize the Sn2 transition state by accepting... 

Synthesis of the first mesoionic nematic and smectic A liquid crystals derived from sydnones has been described and their self-organization into liquid crystal phases has been studied by optical, calorimetric, and powder X-ray diffraction methods <2005CC1552>. 

It was quickly recognized that chirality would play an important role in discotic liquid crystals, not only for the possibility of creating cholesteric and ferroelectric liquid crystals but also as a tool for studying the self-assembly of these molecules as a whole, both in solution and in the solid state. However, initial studies revealed that expression of chirality in discotic liquid crystals was not as straightforward as for liquid crystals derived from calamitic molecules. More recently, with the increase in interest in self-assembly and molecular recognition, considerably more attention has been directed to the study of chiral discotics and their assemblies in solution. The objective of this chapter is... 

Discotic liquid crystals based on carbohydrates are intrinsically chiral due to the chiral nature of their core.49,50 Numerous discotic liquid crystals derived from these mesogens have been reported however, the expression of chirality in the subsequent mesophases has only rarely been seen. An example of the latter is the a-anomer of penta- O -decanoylglucopyranosc (47), which forms a discotic mesophase in which the molecules pack helically in the columns... 

Investigators of cellulosic liquid crystals have two main motivations to study mesophase formation primarily from a scientific viewpoint or a technolomcsd vie oint. The main focus of the latter has been on the potential of preparing high strength/high modulus regenerated cellulose fibers. Another potentim use of cellulosic liquid crystal derivatives is as chiroptical filters (S,lfi). 

The adduct formed by two lithium atoms with polycondensed aromatic hydrocarbons crystallizes with two solvating molecules of TMEDA. The structure of the crystals derived from naphthalene (73) and anthracene (74) was elucidated by XRD. This arrangement of the unsolvated lithium atoms, in 7 -coordination fashion on the opposite sides of two contiguous rings, was found by MNDO calculations to be the most favorable one for naphthalene, anthracene and phenanthrene (75) . 

Nettleship, J. E., et al. (2005). Sample preparation and mass spectrometric characterisation of crystal-derived protein samples. Acta Crystallogr. D 61, 643-645. 

TABLE 1. Physical Properties of Selected Liquid-Crystal Derivatives Derived Using Optical Anisotropic Materials"... 

Kaida et al. (1,2) prepared optically anisotropic polymerizable liquid-crystal derivatives, (I) and (II), respectively, containing internal esters that were used in optical disc applications. 

Final proof for the inferred structure (1) for cyclosporin A and a first insight in the shape of the molecule resulted from X-ray analysis and high-resolution NMR spectra. The preparation of a crystallized derivative containing a heavy atom was achieved by an addition reaction using iodine and thallium(I) acetate. Instead of the expected iodoacetoxy derivative, the cyclic product (11) was obtained. Obviously, the reaction proceeded by a selective addition of iodine to the double bond of the MeBmt unit followed by an internal cyclization with the participation of the OH group. Iodocyclosporin A (11) reverted easily with Zn powder in acetic acid into genuine cyclosporin A by rranr-elimination. X-ray analysis [6] revealed that iodocyclosporin A assumes a rather rigid backbone conformation. The amino acids 1-6 adopt an antiparallel, markedly twisted /i-pleated sheet conformation, whereas the residues 7-11 form a loop. 

Cook, A. G., Baumeister, U., Tschierske, C., Supramolecular dendrimers Unusual mesophases of ionic liquid crystals derived from protonation of DAB dendrimers with facial amphiphilic carboxylic acids. J. Mater. Chem. 2005, 15, 1708-1721. 

Pentacene ethers 37-39 (Fig. 3.8) were prepared in an attempt to reduce the HOMO energy level relative to functionalized pentacene 29 [52]. These cyclic ether compounds were surprisingly stable, soluble and easily crystallized. Saturated diox-ane compound 37 has an oxidation potential of 0.580 V vs SCE (similar to that of unfunctionalized pentacene), but the axial hydrogen atoms on the dioxane ring prevent -stacking interactions in the crystal. Derivatives 38 and 39 were synthesized to overcome this structural issue. Although the change of oxidation potential... 

It is well known that the configurational entropy can be evaluated from the heat capacitance of the crystal, derived from DSC measurements, by the integral... 

Naphthyl p-quinone liquid crystals derivatives have been prepared and are described (3). 

In addition to simplifying the workup procedures, use of catalytic amounts of the titanium-tartrate complex in the epoxidation of low molecular weight alcohols makes in situ derivatization of the crude epoxy alcohol feasible (Table 5)42, Catalytic epoxidation and in situ derivatization allowed the preparation of glycidol, an epoxy alcohol accessible only in very low yield by the stoichiometric method. The in situ derivatization method also makes possible the enhancement of enantiomeric excess through crystallization. Derivatives of low molecular weight alcohols, such as the 4-nitrobenzoates, undergo significant enantiomeric enrichment upon crystallization (Table 5). 

Liquid crystals derive both their beauty and their usefulness from their ordered, yet liquid, state. At rest, they possess at least some orientational order at rest, but lack the full three-dimensional positional order of solid crystals. Orientational order implies that the molecules tend to point in the same direction, while positional order implies that the molecules centers of mass tend to lie on lattice points. Spherically symmetric molecules, for example, can have positional order, but no orientational order. Thus, when one heats a simple positionally ordered crystalline solid to its melting point, all of its positional order is destroyed in a single step, producing an isotropic liquid. Orientational order is, however, possible for rod-like or disk-like molecules. Crystalline solids made of such molecules can melt into isotropic liquids in multiple steps at intermediate stages of melting, there can be orientational order with no, or only partial, positional order. That is, the molecules might tend to be aligned in a common direction, but the centers of mass of the molecules take on random positions. Such intermediate phases are liquid crystals.------------------------... 

The arrangement of chiral molecules in thermotropic liquid crystals is more complex, since entire volumes of space - rather than the bounded twisted ribbons discussed above - must be ed subject the constraint of a preferred twist between neighbouring molecules. The simplest examples of such mesophases are the cholesteric liquid crystals, discovered last century, (c/. section 5.1.8). This class of thermotropic liquid crystals derives its generic name from chiral cholesterol derivatives (shown below), which were found a century ago to exhibit peculiar optical changes as they were heated. 

Some enolate crystals derived from achiral substrates undergo spontaneous resolution upon crystallization. These include the aggregates (139) and (150) and perhaps others. Presumably this occurs to yield the conglomerate. 

Kranz, H., Vill, V. and Meyer, B. (1996). Prediction of Material Properties from Chemical Structures. The Clearing Temperature of Nematic Liquid Crystal Derived from their Chemical Structures by Artificial Neural Networks. J.Chem.Inf.Comput.Sci, 36,1173-1177. 

Secondary nucleation occurs when crystals of the solute are present. Less supersaturation is required to generate new particles under this condition. Referring to the types of secondary nucleation shown in Fig. 4-2, all of those are preformed crystals derived by various means from the parent, except for the case where molecular aggregates (clusters) are stripped from the surface. Except for pure breakage effects, as in milling, supersaturation is required in all types of secondary nucleation to satisfy the energetics. 

Table 4.10 Examples for the most commonly used dielectrically negative liquid crystals deriving their dipole moment from lateral difluorination. The virtual clearing points TNi.vin are extrapolated from the Merck mixture ZLI-4792, Af from ZLI-2857 [41].

Tsiourvas D, Felekis T, Sideratou Z et al (2004) Ionic liquid crystals derived from the protonation of poly(propylene imine) dendrimers with a cholesterol-based carboxylic acid. Liq Cryst 31 739-744... 

Qiarge transfer excitation of 1 1 CT crystals derived from bis [1,2,5] thia-diazolotetracyanoquinodimethane and arylolefins gave [2+2] cycloadducts... 

Cross-linked enzymes are commercial biocatalysts and can be reused in organic solvent and aqueous solution. They are purchased as crystals derived from a single cross-linked enzyme. 

Early results (39,40,113) on the hydrogenation of carbon monoxide on Ni, Fe, and mixed Ni-Fe amorphous alloys with P and/or B as metalloids are summarized in Ref. 45. Experiments with 15 different amorphous catalysts and their crystallized derivatives in a flow reactor, at atmospheric pressure, and in the temperature range of 493-643 K led to the following main findings ... 

---