Enantiotropic atoms

Liquid crystals based on aliphatic isocyanides and aromatic alkynyls (compounds 16) show enantiotropic nematic phases between 110 and 160 °C. Important reductions in the transition temperatures, mainly in clearing points (<100 °C), areobtained when a branched octyl isocyanide is used. The nematic phase stability is also reduced and the complexes are thermally more stable than derivatives of aliphatic alkynes. Other structural variations such as the introduction of a lateral chlorine atom on one ring of the phenyl benzoate moiety or the use of a branched terminal alkyl chain produce a decrease of the transition temperatures enhancing the formation of enantiotropic nematic phases without decomposition. 

A single stereogenic center on the bridge between the target C-H bond and the diazocarbonyl can be sufficient to induce high diastereoselectivity. This is illustrated [lOJ by the cyclization of 21, prepared from farnesol (20), which has two enantiotropic H atoms on the alcohol methy-... 

Enantiotropic Used to describe two atoms or groups that upon replacement with a third group give enantiomers. 

The classification of polymorphic substances into monotropic and enantiotropic classes from the standpoint of observed phenomena is not appropriate as a basis for the lattice theory of polymorphism. In the lattice theory, one begins by considering whether the lattices of the polymorphic forms are related in structure or not. Only in the case of lattices that are structurally related in structure or not. Only in the case of lattices that are structurally related will it be possible for mutual transformation to take place at a transition point fixed by the temperature and pressure of the system. If the lattices differ in such a way that atoms or molecules must be completely regrouped during the transformation (changing their state of bonding), no point of contact for mutual reversible transformation will exist. 

Dithiolate compounds are similar to the carboxylates, with sulfur atoms taking the place of the oxygen atoms in the polar head. Each of the tetrakis(alkyldithiolato)dinickel(II) complexes ((101) R = C H2 +i, n = 4- ) shows a monotropic lamellar mesophase between 80and 90°C, which has been described as discotic lamellar (Dl) phase (see Figure 22). In the case where R is a branched chain, an enantiotropic Colh and a monotropic Dl mesophases have been found for tetrakis( 1 -ethylpentyldithiolato)dinickel(II) complex. ... 

If the remaining hydrogen atom in the iV-methylamide group is also replaced by a second methyl group, the solvent dimethylformamide results, which does not possess any hydrogen bond donor atoms. In the contact region of this solvent with C50, only an enantiotropic N phase and a monotropic lamellar L phase appear (Fig. 5.13). 

Mononuclear cyclopalladated liquid crystalline materials whose molecular structure consists of two different thermotropic ligands, connected by a palladium atom, 55, have been also prepared. The mesomorphic properties depend on the length of the chain. Monotropic or enantiotropic nematic and/or smectic phases can be found [131]. 

New compounds containing two or even four palladium atoms and eight or twelve flexible side chains have been synthesized by Praefcke et al. [138]. The compound, 58, shows an enantiotropic discotic hexagonal columnar mesophase over a wide range of temperature and a lyotropic nematic phase in various organic solvents. X-ray diffraction studies reveal an oblique arrangement of disordered columns. 

We investigated the effect of alkyl chain length of cyanobiphenyl compounds that had a primary alcohol end group (compounds I-n, where n is the number of carbon atoms of the alcohol) on the growth inhibition of A549 cells [198]. In addition, a compound bearing a secondary alcohol was synthesized (compound n ) for comparison. Figure 11.31 shows the structure ofthe compounds, and their phase transition temperatures are shown in Table 11.2. AU compounds except I-l and II-4 show an enantiotropic nematic phase [199-201] I-l and II-4 are monotropic nematic. 

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