Plane polarised light

Figure 2.58 Schematic representation of the generation of elliptically polarised tight riu the reflect ion of plane-polarised light from a reflective surface.

The optical isomers have identical physical properties except for their effect on plane-polarised light. 

William Nicol (1768-1851) developed the Jirst prism that produced plane polarised light... 

Enantiomers possess identieal physieal properties namely, melting point, boiling point, refraetive index, ete. They only differ with respeet to the rotation of plane polarised light. If one of the enantiomer is dextro rotatory, the other will be laevo rotatory. 

Optical isomerism of drug molecules is widespread. Many drug molecules only contain one or two chiral centres. A simple example is the naturally occurring neurotransmitter adrenaline. When a compound has no symmetry about a particular carbon atom the carbon atom is said to be a chiral centre. When a compound contains one or more chiral centres it is able to rotate plane-polarised light to the right (+) or the left (-). A chiral centre arises when a carbon atom has four structurally different groups attached to it. 

To relate the (+) (dextrorotatory) and (-) (laevorotatory) forms of a molecule to an absolute (R or S) configuration is complex and requires preparation of a crystal of the compound suitable for analysis by X-ray crystallography. In contrast the direction in which a molecule rotates plane-polarised light is easily determined using a polarimeter. 

It should be noted that in older literature the terms d and I are used to denote (+) and (-) respectively and D and L are used to denote R and 5 respectively. A mixture containing equal amounts of (+) and (-) adrenaline or indeed enantiomers of any drug is known as a racemic mixture and of course will not rotate plane-polarised light. The physical separation of enantiomers in a racemic mixture into their pure (-t) and (-) forms is often technically difficult. 

Chirality is an important aspect of aroma chemicals since enantiomers of the same compound may possess different organoleptic characters. Chirality means the occurrence of one or more asymmetric carbon atoms in an organic molecule. Such molecules exhibit optical activity and therefore have the ability to rotate plane-polarised light by equal amounts but in opposite directions. In other words, two stereoisomers which are mirror images of each other are said to be enantiomers. If two enantiomers exist in equal proportions, then the compound is called racemic. Enantiomers can be laevorotatory (, I, -, S), meaning rotating the plane of the polarised light to the left or dextrorotatory (°, d, -f, R), that is. 

The asymmetry of the two cm-for ms manifests itself in the form of crystals which are enantiomorphous, and in the action of the substances in solution on plane polarised light. 

In these experiments, the molecule of interest is in solution and excited with plane polarised light. The fluorescence emitted perpendicularly to... 

Whereas enantiomers (e.g. 4a and 4b) have indentical chemical and physical properties (except their effect on plane polarised light), diastereoisomers (e.g. 4a, or 4b and 4c) frequently differ in their chemical properties, and have different physical properties. 

Enantiomers have identical physical and chemical properties to one another except the direction in which they rotate plane polarised light (clockwise or anticlockwise). They may be separated by interaction with a second chiral species. This gives two diastereoisomers (if the two chiral centres are the same we can describe the diastereoisomers as optically pure meso AA and the racemic or rac form which itself occurs as two pairs of enantiomers, AA and AA) which do differ in their physical properties e.g. have different NMR spectra, can be separated by achiral chromatography etc). For example, Scheme 3.1 shows the experimental resolution of [Co(en)3]3+ using tartrate. 

Fig. 4.5. Photomicrograph of a leucite tephrite from Vulsini showing color zoning (from deep green salite to colourless diopside) for clinopyroxene phenocrysts. Plane polarised light.

Optical isomerism is also a type of configurational isomerism and is so named because of the ability of optical isomers to rotate plane-polarised light clockwise or counterclockwise. The existence of optical isomers has very important consequences for life, because optical isomers generally have significant differences in their biological activity. Apart from their biological activity and their effects on plane-polarised light, optical isomers have identical chemical and physical properties. 

A solution of each enantiomer or optical isomer can rotate plane-polarised light. One antiomer will rotate plane-polarised light clockwise while the other (the mirror image) will rotate it counterclockwise by the same amount. A mixture of the two isomers (a racemate) will not rotate plane-polarised light at all. In all other respects, the two isomers are identical in physical and chemical properties and are therefore indistinguishable. The asymmetric centres in the molecules shown (Following fig. B) have been identified with as asterisk. The structure lacking the asymmetric centre is symmetric or achiral and does not have optical isomers. A structure can also have more than one asymmetric centre. 

The assignment of an asymmetric centre as (R) or (S) has nothing to do with whichever direction the molecule rotates plane-polarised light. Optical rotation can only be determined experimentally. By convention, molecules which rotate plane-polarised light clockwise are written as (+) or d. Molecules which rotate plane-polarised light counterclockwise are written as (-) or 1. The (R) enantiomer of lactic acid is found to rotate plane-polarised light counterclockwise and so this molecule is defined as (R)-(-)-lactic acid. 

The only flat-panel technology with the potential to pose a realistic challenge to LCDs in the medium term is OLED technology. The first factories for OLEDs using either small molecules or polymers have started production, if in relatively low volumes, see Table 1.1. Higher production volumes can be confidently expected as the market acceptability and awareness of the capability of OLEDs increases. A combination of the modulation of plane polarised light provided by an OLED back-light by an LCD to create a hybrid OLED-LCD may become a major commercial product in the near future. Oriented main-chain polymers or anisotropic polymer networks in the nematic liquid crystal-... 

Interference between the e-ray and the o-ray, which have travelled with different velocity through the nematic medium, gives rise to the coloured appearance of LCDs operating with plane polarised light. For a wave at normal incidence, the phase difference in radians between the o-ray and the e-ray caused by traversing a birefringent film of thickness d and birefringence An, is referred to as the optical retardation, <5 ... 

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