Chiralic molecules, order parameter

The overall observed retention of the enantiomers, and thus the elution order, is based on several kinetically and thermodynamically controlled parameters concerned with stereorecognition nonstereoselective interactions of all partners SO(R), SA(R S), and particularly of the [SO(RI-SA(KI] and [SO(K)-SA(Si] complexes with the achiral stationary phase, also play a role (Figure 21). Therefore the retention order may be reversed for a specific pair of enantiomers depending on whether a covalently bound CSP or a CMPA is applied, but using the same chiral molecule (part) as chiral selector. These general principles, shown schematically for a CLEC system, are further complicated by the complexity of the entire system, hence they are difficult to anticipate and each case must be studied individually. 

These experiments clearly confirm the theoretical concept of the helical twist of cholesteric phases developed by Goossens and Vertogen 79,86), which is based on the hindered rotation of chiral molecules around their long molecular axis and the introduction of the order parameter D. Furthermore they support the concept of the flexible spacer, described in Chap. 2.1. 

A theoretical study of numerous chiral molecules including bridged biaryls 5 and 6 has been undertaken using a molecular Monte Carlo simulation approach coupled with calculations of molecular chirality based on a chirality order parameter. The method successfully predicts the helical twisting powers <2003JCP10280>. 

Early studies on monolayers of chiral molecules like 2-hydroxyalkanes, amphiphilic amino acids, 2-methylhexacosanoic acid esters, and hydroxy-hexadecanoic acid and its esters have been reviewed. The interesting question about monolayers of chiral molecules is whether the parameters which can be determined and the phase transitions are different for pure enantiomers and racemates. For components of biomembranes like phosphatidylcholines 10 this appears not to be the case," but for synthetic compounds like iV-(a-methylbenzyl-stearamide) 11 specific interactions between the molecules of the enantiomers are observed (Chart 2). ° In recent years, advanced techniques have been developed to probe the order in monolayers at the air-water interface, including surface X-ray diffraction, and microscopic techniques, viz. fluorescence microscopy, and Brewster angle microscopy (BAM). The X-ray diffraction technique has been used to identify homochiral and heterochiral two-dimensional domains in mono-layers of racemic amphiphilic amino acids on subphases containing glycine. Fluorescence microscopy requires the introduction in the monolayer of a small... 

Figure 10.9 shows the snapshot after 100 ps of mixture of 5CB/p-azo-9 (98/ 2 mol%) obtained by MD simulation. It seems likely that ti—ti stacking interaction between 5CB molecules is enhanced through the intermolecular interaction between the chiral azobenzene molecule and 5CB molecule. As a result, the order parameter, S, was increased by mixing trans-p-azo-9 in 5CB. However, S of 5CB mixed with cis-p-a.zo-9 was almost constant. The molecular orientation of the LC molecules doped with the trans-azohenzene is stable relatively to that doped with... 

Azobenzenes are a well-known family of photochromic compounds that can isomerize from its tmns form to cis form upon UV irradiation (Fig. 5.4a). The cis isomer can be switched back to trans form either by visible light or thermal relaxation. The rod shape of the trans form can stabilize calamitic LCs, while the cis form is bent and normally decreases the order parameters of LC phases. Owing to the dramatic molecular shape and property change between the trans and cis isomers, azobenzenes are the most investigated photochromic molecules to function as either mesogens or dopants in chiral LCs. 

The decrease in order parameter in LCs caused by the photoisomerization of azobenzene molecules are also known to induce phase transition from chiral smectic phase to cholesteric phase. This type of transition is mainly observed in chiral mesogenic compounds. In 1999, My et al. conducted a systematic research on the phase transition behavior of a series of chiral azobenzene LCs [44]. A photo... 

The flexoelectric effect is a phenomenon where a space variation of the order parameter induces polarization. Chiral polar smectics are liquid crystals formed of chiral molecules and organized in layers. All phases in tilted chiral polar smectic liquid crystals have modulated structures and they are therefore good candidates for exhibiting the flexoelectric effect. The flexoelectric effect is less pronounced in the ferroelectric SmC phase and in the antiferroelectric SmC. The flexoelectric effect is more pronounced in more complex phases the three-layer SmCpu phase, the four-layer SmCFi2 phase and the six-layer SmCe a phase. 

S and D are the Saupe order parameters that describe the order of the molecules in the absorbing sample. s% (i = 1, 2, 3) are the diagonal elements of the transition moment tensor. They are proportional to the squares of the components of the transition moments (<(/< > i = 1,2,3)) given with respect to a molecule fixed-coordinate system (xj) joc(Circular dichroism of chiral anisotropic phases without suprastructural chirality). 

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