Polarized light properties

The Cahn-Ingold-Prelog (CIP) rules stand as the official way to specify chirahty of molecular structures [35, 36] (see also Section 2.8), but can we measure the chirality of a chiral molecule. Can one say that one structure is more chiral than another. These questions are associated in a chemist s mind with some of the experimentally observed properties of chiral compounds. For example, the racemic mixture of one pail of specific enantiomers may be more clearly separated in a given chiral chromatographic system than the racemic mixture of another compound. Or, the difference in pharmacological properties for a particular pair of enantiomers may be greater than for another pair. Or, one chiral compound may rotate the plane of polarized light more than another. Several theoretical quantitative measures of chirality have been developed and have been reviewed elsewhere [37-40]. 

The experimental facts that led van t Hoff and Le Bel to propose that molecules having the same constitution could differ m the arrangement of their atoms m space concerned the physical property of optical activity Optical activity is the ability of a chiral sub stance to rotate the plane of plane polarized light and is measured using an instrument called a polarimeter (Figure 7 5)... 

Section 7 4 Optical activity, or the degree to which a substance rotates the plane of polarized light is a physical property used to characterize chiral sub stances Enantiomers have equal and opposite optical rotations To be optically active a substance must be chiral and one enantiomer must be present m excess of the other A racemic mixture is optically inactive and contains equal quantities of enantiomers... 

Enantiomers. Two nonsuperimposable structures that are mirror images of each other are known as enantiomers. Enantiomers are related to each other in the same way that a right hand is related to a left hand. Except for the direction in which they rotate the plane of polarized light, enantiomers are identical in all physical properties. Enantiomers have identical chemical properties except in their reactivity toward optically active reagents. 

Polypropylene molecules repeatedly fold upon themselves to form lamellae, the sizes of which ate a function of the crystallisa tion conditions. Higher degrees of order are obtained upon formation of crystalline aggregates, or spheruHtes. The presence of a central crystallisation nucleus from which the lamellae radiate is clearly evident in these stmctures. Observations using cross-polarized light illustrates the characteristic Maltese cross model (Fig. 2b). The optical and mechanical properties ate a function of the size and number of spheruHtes and can be modified by nucleating agents. Crystallinity can also be inferred from thermal analysis (28) and density measurements (29). 

The detection of Hquid crystal is based primarily on anisotropic optical properties. This means that a sample of this phase looks radiant when viewed against a light source placed between crossed polarizers. An isotropic solution is black under such conditions (Fig. 12). Optical microscopy may also detect the Hquid crystal in an emulsion. The Hquid crystal is conspicuous from its radiance in polarized light (Fig. 13). The stmcture of the Hquid crystalline phase is also most easily identified by optical microscopy. Lamellar Hquid crystals have a pattern of oil streaks and Maltese crosses (Fig. 14a), whereas ones with hexagonal arrays of cylinders give a different optical pattern (Fig. 14b). 

It is possible to obtain pure enantiomers of chiral compounds. One property of separated enantiomers is to cause the rotation of the plane of polarized light by opposite... 

Today, we would describe Pasteur s work by saying that he had discovered enantiomers. Enantiomers, also called optical isomers, have identical physical properties, such as melting point and boiling point, but differ in the direction in which their solutions rotate plane-polarized light. 

Some physical properties of the three stereoisomers are listed in Table 9.3. The (+)- and (-j-tartaric acids have identical melting points, solubilities, and densities but differ in the sign of their rotation of plane-polarized light. The meso isomer, by contrast, is diastereomeric with the (+) and (-) forms. As such, it has no mirror-image relationship to (+)- and (-)-tartaric acids, is a different compound altogether, and has different physical properties. 

Except for their effect on plane-polarized light, two enantiomers of a chiral compound have identical physical properties. For example, the two isomers of lactic acid shown below have the same melting point, 52°C, and density, 1.25 g/mL. 

There are plastics that are transparent and translucent in the unpiginented state. They have a range of optical properties that make them interesting for a wide spectrum of optical applications that extends from windows to lens systems to sophisticated applications involving action via polarized light. Used for over a half century are aircraft canopies (thermoformed) and windows in many different structures. 

Birefringence is the property of an anisotropic optical media that causes polarized light with one orientation to travel with... 

There is the microtoming optical analysis test. In this procedure thin slices (under 30 tixri) of the plastics are cut from the product at any level and microscopically examined under polarized light transmitted through the sample. Rapid quality and failure analysis examination occurs by this technique. This technique has been used for many years in biological studies and by metallurgists to determine flaws, physical and mechanical properties. Examination can be related to stress patterns, mechanical properties, etc. 

This is an extracellular deposition of an insoluble protein, which has adopted a (3-sheet structure due to an unknown event that induced misfolding of unstable proteins. The name amyloid has been given according to the amyloid staining properties, which are similar to carbohydrate deposits, e.g., amyloid can be identified with Congo red and seen under polarized light (birefringence test). 

Both columnar and laminar structures are optically active to polarized light and usually have similar physical properties. 

They rotate the plane of polarized light in opposite directions, though in equal amounts. The isomer that rotates the plane to the left (counterclockwise) is called the levo isomer and is designated ( —), while the one that rotates the plane to the right (clockwise) is called the dextro isomer and is designated (-t-). Because they differ in this property they are often called optical antipodes. 

In general, it may be said that enantiomers have identical properties in a symmetrical environment, but their properties may differ in an unsymmetrical environment. Besides the important differences previously noted, enantiomers may react at different rates with achiral molecules if an optically active catalyst is present they may have different solubilities in an optically active solvent., they may have different indexes of refraction or absorption spectra when examined with circularly polarized light, and so on. In most cases these differences are too small to be useful and are often too small to be measured. 

Brewster-angle microscopy dispensed with the need for a probe molecule [41,42], Brewster-angle microscopy is based solely on the reflectivity properties of p-polarized light. The reflectance of p-polarized light at the water-air interface vanishes at 53.1° (using refractive indices at 20°C) if the interface is perfectly sharp in reality there is a deep minimum near zero. The presence of a monolayer gives an intervening layer of different re-... 

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