Helical twist

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]. 

At the end of 2003, new research results led to sensational headlines Minerals Cooked Up in the Laboratory Call Ancient Microfossils Into Question was the title chosen by Richard A. Kerr for his article in Science dealing with synthetically prepared silicate carbonates. Their microstructures show morphologies which look exactly like those of filaments which had been assigned as cyanobacterial microfossils of the Precambrian Warrawoona chert formation in western Australia. The synthetic structures consist of silicate-encapsulated carbonate crystals, and in part have a helically twisted morphology reminiscent of biological objects. Simple... 

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... 

A quantitative study of the cholesteric induction and of the chiral transfer from dopant to phase requires the definition of the helical twisting power 3. This quantity expresses the ability of a chiral dopant to twist a nematic phase and can be numerically expressed in Eq. (2) where p is the cholesteric pitch, c the dopant molar fraction, and r its enantiomeric excess its sign is taken to be positive or negative for right-handed (P), or left-handed (M) cholesterics, respectively. This relation holds for molar fractions <0.01-0.05 ... 

From a cholesteric induction experiment, one can obtain chiral information on the induced cholesteric (namely, pitch and handedness) and therefore the helical twisting power of the dopant in that solvent (at a certain temperature). If a model or molecular theory relating molecular chirality to mesophase chirality is available, one can infer stereochemical information about the dopant (absolute configuration, preferred conformation). 

Helical Twisting Powers of Compounds 15-24 Determined at Room Temperature in... 

A last example of a dopant whose chirality has been investigated by the LC technique is represented by helicenes and related molecules. Once again, compounds very different spectroscopically, such as 46-49, and hence hardly comparable with chiroptical techniques, are very similar in shape and give helical twisting powers of the same sign and of comparable intensity80 the twisting powers of helicenes have been successfully calculated by the shape model of Ferrarini et al.73... 

In the held of thermotropic cholesterics, the most promising approach seems to be that reported by Nordio and Ferrarini22 23 for calculating helical twisting powers. It allows one to tackle real molecules with rather complex structures and to describe them in detail. The model is currently being extended to include a better description of nematic solvents and specific solute-solvent interactions. Once tested also for conformationally mobile molecules, this model could allow the prediction of the handedness of single-component cholesterics, and, in the held of induced cholesterics, very interesting information on solute molecules could be obtained. 

The structural constraints imposed by the fused cyclic structure of the catenates and the ability to cleanly separate product mixtures greatly facilitates crystallization, and single crystal X-ray structures have been obtained for a number of ladder polysilanes.3 8 For the all-anti molecules, the non-planarity of the SI rings induces a unidirectional helical twist to the ladder structure, as is evident in Figure 63. 

Tullius, T.D. and Dombroski, B.A. (1985) Iron(II) EDTA used to measure the helical twist along any DNA molecule. Science 230, 679-681. 

Widom, J. (1992) A relationship between the helical twist of DNA and the ordered positioning of nucleosomes in all eukaryotic cells. Proc. Natl. Acad. Sci. USA 89, 1095-1099. 

The optical character of chalcedony is distinct from that expected for the normally uniaxial mineral, quartz, and signals the fibrous nature of a particular sample. The direction of fiber elongation is often parallel to the [1120] crystallographic direction of the quartz structure (Fig. 2.19A), but other fiber directions have also been determined within a single sample (Frondel, 1985). The presence of helically twisted fibers are suspected from the variations in extinction and birefringence noted along the fiber length (Fig. 2.19C). More detailed information on the optical or other physical and chemical properties of quartz and its many varieties can be found in volume 3 of Palache et al. (1962) and in Frondel (1985). 

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