Rotating optical compensator

Molecules of inheient structural asymmetry aie anisotropic they are optically active and exhibit optical rotation in solution. The typical optically active center is a carbon atom with four different substituents. In addition, any structural dissymmetry that results in a spatial left- and right-handedness will cause optical activity. Compounds of these types of come in a right-hand l R) and left-hand (L) form. When equal amounts of these two forms are mixed (racemic mixtures) there is no optical rotation because the activity of the two forms exactly cancel. Internal compensation of optically active centers m complex molecules is also found. Left- and right-handed optical isomers were first studied by Pasteur well over 100 years ago. and extensive surveys are found in most organic chemical texts. 

There is an interesting theoretical report concluding that the polymerization giving an atactic polymer can be asymmetric synthesis polymerization without using chiral catalysts [95]. According to the report, when DP of an atactic polymer exceeds 70, the polymer sample cannot be a racemic mixture because the number of possible diastereomers is far larger than that of polymer chains and only one antipode of enantiomer can exist for some of the diastereomeric chains. However, the polymer sample does not show optical activity due to compensation of optical rotations contributed from different diastereomers (cryptochirality). 

The Bertrand lens, an auxiliary lens that is focused on the objective back focal plane, is inserted with the sample between fully crossed polarizers, and the sample is oriented to show the lowest retardation colors. This will yield interference figures, which immediately reveal whether the sample is uniaxial (hexagonal or tetragonal) or biaxial (orthorhombic, monoclinic, or triclinic). Addition of the compensator and proper orientation of the rotating stage will further reveal whether the sample is optically positive or negative. 

This particular example represents one class of stereoisomers known as enantiomers, which may be defined as two molecules that are mirror images but are nonetheless nonsuperimposable. Such molecules are said to possess opposite configuration. If these isomers are separated (resolved), the separate enantiomers have been found to rotate the plane of plane-polarized light. This phenomenon of optical activity has been known for well over a century. A 50-50 mixture of two enantiomers is optically inactive or racemic, since the rotation of light by one enantiomer is precisely compensated by the rotation of tight in the opposite direction by the other enantiomer. 

In saccharimeiers with double compensation (Fig. 42), the place of the parallel-faced quartz plate H is taken by a second pair of wedges, one (K) stationary and the other (H) movable, having rotations of opposite sign to M and N (and carrying also another vernier and the corresponding scale) the other optical parts correspond with those of saccharimeters with simple compensation. When the zeros of the two scales coincide with those of... 

Step II. Insert a first order red compensator into the optical path. Take note of the color of fibers as the stage is rotated. Several possibilities exist. 

The sum-frequency response at aqueous interfaces is very weak because of the small number of adsorbed molecules present and the poor polarizability of most liquids. To compensate for the low sum-frequency efficiency, pulsed lasers are used. Since the sum-frequency intensity increases with the peak intensity of the incident beams, picosecond and femtosecond pulses are optimal, although these shorter pulses result in larger IR bandwidths. Nanosecond systems are generally much simpler to operate and have narrower IR bandwidths, but can contribute to significant heating of the interface unless an optical coupling scheme such as total internal reflection (TIR) [6] or other mechanisms such as sample rotation [32] are employed. 

Figure 4.6-8 Optical rotation exhibited by a 0.2 mm thick sample of a mixture of cholesteryl chloride and cholesteryl myristate (molar ratio 1.67) at 1900 cm Scanning the temperature changes the pitch. At 59.5 °C the pitch corresponds to 1900 cm , at about 48 °C the twisting influences of the mixture components are mutually compensated so that the sample is nematic, at lower temperatures the structure is countercurrent. Above and below T em the rotatory dispersion follows a curve as derived by de Vries (1951).

M] or [rh] Molecular rotation, defined as [a] x MW/100. Specific rotation corrected for differences in MW. The symbol [M] and the term molecular rotation are now deemed incorrect, and the term molar rotation denoted by [d ] is preferred. meso- Denotes an internally compensated diastereoisomer of a chiral compound having an even number of chiral centres, e.g., me o-tartaric acid. Formally defined as an achiral member of a set of diastereomers that also contains chiral members, mutarotation Phenomenon shown by some substances, especially sugars, in which the optical activity changes with time. A correct presentation is, e.g., [a]n ° + 20.3 -101.2 (2h)(c, 1.2 in HjO). 

The theory of quadratic variations in optical activity with respect to the electric field strength was first formulated for macromolecules by Tinoco and Hammerle," and then developed by others." The earliest e qperi ments are due to Tinoco in solutions of poly-y-L-sJutamate in ethjdene dichloride of late, this experiment has been extended to transient optical rotation changes by Jennings and Baily." Also, electric field effects on the optical rotatory power of a compensated cholesteric liquid crystal have been stuped." ... 

The main optical directions of the anisotropic layer (and, correspondingly, the orientation angle in FB or EB) are found when the compensator is switched on (6 = 0 or rjn = 0 in Eq. (106)) and the half-shaded position is established by rotating the whole optical system (limb Bi). The orientation angle a is determined as the half-angle between two half-shaded positions one of which corresponds to the clockwise rotation of the rotor of the dynamo-optimeter and the other to the counter-clockwise rotatioa... 

The other example of note is the optically active tartaric acids (Fig. 11). Tartaric add contains two asymmetric carbon atoms. The dextro- and levo-tartaric adds are enantiomers. However, a third isomer is possible in which the two rotations due to the two asymmetric carbon atoms compensate and the molecule is optically inactive as a whole. That is, the molecule contains a plane of symmetry. This form, meso-tartaric acid, was also discovered by Pasteur, differs from the two optically active tartaric adds in being internally compensated, and is not resolvable. Thus, the tetrahedral model for carbon and the asymmetric carbon atom proposed by van t Hoff were able to completely explain the observations of Pasteur relating to the three isomers of tartaric add. 

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