Nomenclature polarized-light-rotating

RGURE 1 Pasteur separated crystals of two stereoisomers of tartaric acid and showed that solutions of the separated forms rotated polarized light to the same extent but in opposite directions. These dextrorotatory and levorotatory forms were later shown to be the (R,R) and (S,S) isomers represented here. The RS system of nomenclature is explained in the text. 

Another point connected to chirality is the nomenclature of enantiomers. In the beginning, the optical isomers were distinguished with (+) and (—) signs or d (dextro) and / (levo), indicating the direction in which the enantiomers rotate a plane of polarized light. In this nomenclature, (+) or d stands for a rotation to the right (clockwise), whereas (—) or l indicates a rotation to the left (counterclockwise). The main drawback of such an assignment is that one cannot derive the number of chirality centers from it. Rather, it is necessary to apply the R/S... 

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

In 1891 Emil Fischer devised a nomenclature system that would allow scientists to distinguish between enantiomers. Fischer knew that there are two enantiomers of glyceraldehyde that rotated plane-polarized light in opposite directions. He did not have the sophisticated tools needed to make an absolute connection between the structure and the direction of rotation of plane-polarized light. He simply decided that the (-L) enantiomer would be the one with the hydroxyl group of the chiral carbon on the right ... 

The direction of rotation of plane-polarized light is not known because there is no correlation to nomenclature systems such as L or d, or J or S. 

In general, compounds which contain asymmetric carbon atoms rotate the plane of polarization of plane-polarized light. For this reason they are said to be optically active. When the molecular symmetry is such that the optical activity of one portion of the molecule is cancelled by that of the second portion of the molecule, the compounds are said to be internally compensated and are called meso compounds. The tartaric acid with the formula (X) is such a compound and has been known as the meso-tartaric acid. The tartaric acids identified as (VIII) and (IX) have been known as d-tartaric acid and Z-tartaric acid because of the sign of their optical rotations (dextro and levo, respectively). (The nomenclature of these acids is discussed later in this chapter.) The compounds (VIII) and (IX) are non-superimposable mirror images, called enantiomorphs. The existence of such pairs of asymmetric isomers is the fundamental basis of optical activity. The asymmetry may be in either the molecular structure or the crystal structure. Asymmetric carbon atoms are not always present in optically active molecules. 

Depolarized Scattering and Rotational Diffusion Most flexible and semiflexible polymers are isotropic and hence do not depolarize incident polarized light. For particles with shape anisotropy there will be a tensor polarizability and the scattered fight will have depolarized components. Depolarized scattering principles and nomenclature are covered in Sections 8.1,8.2.4, and 8.4. 

In reading about the history of science, we often find that famous scientists in previous eras made important contributions in many different fields. Such is the case for the French scientist Jean Baptiste Biot (Biographic Photo 2.1), whose name is associated with major discoveries in mathematics, astronomy, physics, and chemistry. Biot discovered in 1815 that the phenomenon of rotation of the plane of polarized light (a topic introduced and described very briefly in Chapter 1), which was known for quartz crystals, could also be observed in naturally occurring liquids such as oils of lemon and turpentine [1]. We now know that these two substances are not pure compounds, but complex mixtures. The main constituent in lemon oil is (-h )-limonene, sometimes referred to as d-limonene (a nomenclature that we will discuss shortly), and the main component in "French" turpentine is (-I- )-a-pinene. It is interesting to note that the molecule present in turpentine in North America is (—)-a-pinene. The structures of these compounds are given in Figure 2.1. Note that the molecule limonene has one asymmetric... 

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