Chirality molecular twist

A review of the older literature on compounds with a stereogenic axis is available22, as are reviews on planar chiral molecular structures 23, on the stereochemistry of twisted double bond systems 24, on helical molecules in organic chemistry 25, and on the synthesis and stereochemistry of chiral organic molecules with high symmetry 26. 

In this way, the above two methods are very effective to control ee and choose one of the chiral domains. It is noted that both methods use no chiral molecular species. Instead, CPL and twist cell geometry are used as chiral stimuli, which act as a symmetry-degeneracy-breaking field. This field triggers and accelerates a preferential formation of one of the two possible chiral conglomerates, which is fixed in a B4-like phase. The monochiral films obtained by achiral molecules open... 

Figure 15. Schematic illustration of a chiral bilayer (A), with the molecules tilted with respect to the local layer normal. (The arrows indicate the direction of the molecular tilt, projected into the layer plane.) The favored twist between the chiral molecules leads the whole membrane to curve into (B), a wound ribbon, then fuse into (C), a cylindrical tube. The observed CD spectra come from the chiral molecular packing common to all three figures, not from the micrometer-scale helical structure in (B) and (C). Reproduced from ref. 171 (Schnur et al.. Science 1994,264,945) with permission of the American Association for the Advancement of Science.

Figure 3. Examples of molecular twists present in chiral substances.

First it was argued to be due to a chiral molecular configuration characteristic of the particular type of bent-shape molecules, such as twisted or propeller shape (conformational chirality). The concept of conformational chirality was supported by simulations by Earl et al. [61], and was demonstrated by the observation that doping calamitic cholesteric liquid crystal by achiral bent-core molecules can lead to a decrease of the helical pitch, indicating an enhanced rotatory power of the mixture [62]. Unfortunately there is no proof that the decrease of the pitch is not due to a decrease of the twist elastic constant caused by the addition of bent-core units. Although the conformational chirality is usually not questioned in the solid B4 phase [20], its role has been questioned by Walba et al. [20] by arguing that these chiral conformations have very short lifetime, therefore they average out in fluid smectic, such as SmCP or SmCo phases. 

Alcohol (3R,4R)-(- -)-31 (entry 19) is a starting material for the synthesis of a light-powered chiral molecular motor [CD( )237.2]-( )-44a, which rotates in one rotational direction by the use of hght energy as shown in Figure 55.10, where cis-olefin [CD( )238.0]-44c is one of the motor rotation isomers.As these compounds take twisted structures, their ACs or hehcities are defined as (P,P) or MM)-... 

The story of a hght-powered chiral molecular motor originates from the smdy how to determine the AC of chiral olefin [CD(- -)239.0]-45, which also takes a similar twisted structure and, hence, shows a strong positive CD band at 239.0 nm. To determine the AC of [CD(- -)239.0]-45, we first tried to prepare derivatives containing a heavy atom, but aU attempts were unsuccessful. So we adopted the next strategy, where the methyl group at the chiral position was used as an internal reference of AC in X-ray crystallography. 

In order to obtain information on the poly(PDBF) molecular structure, molecular dynamics (MD) simulations were performed for a PDBF 20-mer model created on the basis of the DBF oligomer conformation in crystal (see X-ray Crystal Analyses section) under a constant NVT condition with COMPASS force field at 300 K for 8.6 ns. The simulation afforded the structure shown in Fig. 40 in which the neighboring fluorene moieties stack on top of each other in a chiral, slightly twisted arrangement. The entire chain formed a relatively long-pitched helical structure in the simulation. Hence, the local twist and the helix may contribute to the CD spectra. 

Other more exotic types of calamitic liquid crystal molecules include those having chiral components. This molecular modification leads to the formation of chiral nematic phases in which the director adopts a natural helical twist which may range from sub-micron to macroscopic length scales. Chirality coupled with smectic ordering may also lead to the formation of ferroelectric phases [20]. 

A very different model of tubules with tilt variations was developed by Selinger et al.132,186 Instead of thermal fluctuations, these authors consider the possibility of systematic modulations in the molecular tilt direction. The concept of systematic modulations in tubules is motivated by modulated structures in chiral liquid crystals. Bulk chiral liquid crystals form cholesteric phases, with a helical twist in the molecular director, and thin films of chiral smectic-C liquid crystals form striped phases, with periodic arrays of defect lines.176 To determine whether tubules can form analogous structures, these authors generalize the free-energy of Eq. (5) to consider the expression... 

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