Molecular chirality, types

It is clear from the above examples that the presence of chiral centers in molecules leads to stereoisomers. There is another type of molecule which itself is chiral but has no chiral center. The molecular chirality arises from the presence of a screw axis in the molecule. Allenes and biphenyls are common examples of such compounds, and because they are chiral, they exist as enantiomers. 

The book provides systematic and detailed descriptions of the numerous approaches to chiral resolution. The first chapter is an introduction to basic concepts of molecular chirality and liquid chromatography. Chapters 2 through 9 discuss the chiral resolution of various classes of chiral stationary phases. Chapter 10 deals with chiral resolution using chiral mobile phase additives. These discussions elaborate the types, structures, and properties of the chiral phases,... 

The following sections address the various types (A-E in Fig. 4.61) of chiral dendrimers. Individual publications which have shed light on the relation between the molecular chirality of the dendritic building blocks and the macroscopic chirality of the molecules as well as applications-relevant properties of chiral dendrimers are presented. 

LI.2 Synthetic polymeric type CSPs. With the aim of mimicking nature and naturally occurring biopolymeric SOs like polysaccharides or proteins, researchers have developed several approaches for the preparation of new types of synthetic macromolec-ular SOs. These new polymeric SOs may be divided into (a) SOs synthesized from achiral monomers including helical polyacrylates and molecular imprint type CSPs and (b) SOs synthesized from chiral monomers including polyacrylamides and network polymers based on tartaric acid diamides. 

Despite the independent nature of these two attributes, stereotopicity and chirotopicity of intramolecular sites are intimately intertwined. When molecular half-spaces are considered in terms of these two attributes, one finds five types of molecular faces two that are homotopic -achirohomotopic (H,H) and chirohomotopic (H, H ), enantiotopic faces (E a), and two types of diastereotopic faces - achirodiastereotopic (D,F) and chirodiastereotopic (D, F ) (Figure 12.1)." Of these five t) es of molecular faces, types (H,H), (E, 3) and (D,F) are foimd only in achiral molecules, while (H, H ) and (D, F ) faces are found only in chiral molecules. 

SmC a It is the most symmetric, antiferroelectric-type phase (P = 0) that borders SmA phase. It is helical but the helicity originates not from the molecular chirality but is due to specific NNN interactions. The pitch is short and incommensurate to the layer periodicity. In Fig. 13.21 the top view on the first five layers is shown and one may conclude that the helical pitch may be as short as 51, but it vary with temperature. Due to short helical pitch the phase does not show ORP. 

Fig. 13.22 Chiral antiferroelectric SmC A phase. Alternating tilt plane (a) and layer polarization (b) and the long-pitch helical structure (c). Note that the unit cell consisting of two layers rotates as a whole forming two geared helices of the same handedness. This type of rotation is controlled by molecular chirality inherent in all phases shown in Fig. 13.21...

Literature data evidence different types of liquid crystalline mesophase, depending on the ordering type, mesogene, molecular chirality, etc. [75,76]. 

Hu et al. [31] reported a new type of macro molecular chiral catalysts for asymmetric catalysis using Suzuki coupling polymerization and obtained optically active ephedrine-bearing dendronized polymers. Their finding showed that the optically active dendronized polymers have characteristics joined features like huge numbers of catalytic sites, more solubility and nanoscopic dimensions towards more acceptable in comparison to its existing chiral catalysts of linear polymeric and dendritic nature. 

We contend that molecular chirality appears as a four-dimensional symmetry which is incompletely interpreted in three dimensions. The type of anticipated error is demonstrated by the way in which three-dimensional chirality is projected into two dimensions, as in Fig. 6. The two-dimensional chiral system is defined here in the plane that supports the triangular base of a three-dimensional chiral tetrahedron. The symmetry element, shown as a solid vertical line, represents an inversion (I) in three dimensions and a twofold rotation (R) in two. The horizontal broken line represents a twofold rotation in three dimensions, but a reflection (M) in two dimensions. To complete the argument, the three-dimensional reflection that operates diagonally also appears as a two-dimensional reflection. Two-dimensional inversion is equivalent to rotation. In summary,... 

Many types of molecular chirality are observed. They can be shortlisted into four categories ... 

In this chapter, we will discuss the present status of CHIRBASE and describe the various ways in which two (2D) or three-dimensional (3D) chemical structure queries can be built and submitted to the searching system. In particular, the ability of this information system to locate and display neighboring compounds in which specified molecular fragments or partial structures are attached is one of the most important features because this is precisely the type of query that chemists are inclined to express and interpret the answers. Another aspect of the project has been concerned with the interdisciplinary use of CHIRBASE. We have attempted to produce a series of interactive tools that are designed to help the specialists or novices from different fields who have no particular expertise in chiral chromatography or in searching a chemical database. 

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]. 

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