Enantiomers alignment

An important aspect of chiral liquid crystals is that they have the potential to be used to determine the enantiomeric purity of any substrate because they do not depend on any direct interaction between the liquid crystal and the substrate, but only depend on the two enantiomers aligning differently relative to the applied magnetic field. Chiral liquid crystals have been applied to a broad variety of compound classes including aliphatic hydrocarbons. " ... 

Stretched gelatin gels as chiral alignment media for the discrimination of enantiomers by NM R spectroscopy. Angew. Chem. Int. Ed. 2005, 44, 3145-3147. 

Because of the tetrahedral stereocenter of the amino acid, three-point binding can occur with proper alignment for only one of the two enantiomers. 

Fig. 16 STM images (b 11.7x 11.7 nm2, c 11.5 x 11.5 nm2) and structure models (d,e) for the enantiomorphous lamella structures induced by adsorption of ISA (a) on hopg from a 1-heptanol solution [47]. For opposite enantiomers, opposite lamella tilt angles (0) are observed. The large lamella ALi is built up from pure ISA enantiomers, while the smaller AL2 lamella consists of coadsorbed achiral 1-heptanol molecules. The ISA chirality is transferred to the coadsorbed solvent molecules via opposite alignment angles (p (f,g). Reprinted with permission from Wiley...

A purely organic chiral nitroxide which shows liquid crystalline behaviour as well as intriguing magnetic properties and a dependence on the enantiomeric nature has been reported [180]. The reason for studying the compounds was to increase the sensitivity of mesophases to magnetic and electric fields. The racemic modification of the radical, which displays a nematic phase, proved to be more sensitive to alignment than the cholesteric phase with the enantiomers present. It was proposed that the compounds may also be used to study the dynamic nature of mesophases by electron paramagnetic resonance spectroscopy. 

Many of the alignment media presented so far possess stereogenic centres, but only media with a large chiral super structure are referred to as chiral alignment media, since only those are capable of distinguishing enantiomers (see also Section 6.3). 

If enantiomers can be distinguished by RDCs, the determination of absolute chirality should also be possible in principle. A general approach for this task has not yet been derived, but two examples using the conformational difference of enantiomers bound to the chiral alignment medium250 and the comparison of experimental multiplet patterns with similar compounds of known chirality269 were successful in determining the correct absolute chirality... 

In addition to applications in achiral environments, chiral alignment media have been shown to allow the effective distinction of enantiomers and the measurement of enantiomeric excess (ee). This fast and non-destructive way of measuring ee can be of special interest for quality control of many small molecule syntheses. 

Chiral discriinination is most clearly manifested in the moving frame, and the. rcc-butyl chlorides are no exception in this context. The (3,1) element of (v(/)J (0)> for the S enantiomer is oscillatory with a positive peak (Fig. 10). The same element for the R enantiomer mirrors this behavior, and in the racemic mixture the element vanishes for all t. In the aligned sample (Fig. 10b) the normalized amplitude of the first peak of the cross-correlation function is reduced by a factor of about S, but thereafter there are signs that the function is considerably more oscillatory in nature. Again the behavior of the R enantiomer mirrors that of the S enantiomer, and the same function in the racemic mixture vanishes for all t. 

Then, draw a mirror plane and arrange the substituents in the mirror image so that they are a reflection of the groups in the first molecule, forming B. No matter how A and B are rotated, it is impossible to align all of their atoms. Because A and B are mirror images and not superimposable, A and B are a pair of enantiomers. Two other pairs of enantiomers are drawn in Figure 5.5. 

In this case, the atoms of A and B do not align, making A and B nonsuperimposable mirror images—enantiomers. A and B are two of the four possible stereoisomers for 2,3-dibromopentane. 

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