Polarized hght

The rotation of plane polarized hght (either -I- or -) is not a man-made convention. It is a physical effect that is measured in the lab. It is impossible to predict whether a compound will be -l- or - without actually going into the lab and trying. If a stereocenter is R, this does not mean that the compound will be +. It could just as easily be. In fact, whether a compound is -l- or - will depend on temperature. So a compound can be + at one temperature and - at another temperature. But clearly, temperature has nothing to do with R and S. So, don t confuse R/S with +/-. They are totally independent and unrelated concepts. 

The presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarized hght is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory (+) or to the left, levorotatory (—). The direction of rotation is independent of the stereochemistry of the sugar, so it may be designated d(—), d(+), l(—), or l(+). For example, the naturally occurring form of fructose is the d(—) isomer. 

If two structures are nonsuperimposable mirror images, what can you say about the direction that each wih rotate the plane of polarized hght Would a mixture of the two be optically active Explain. 

A spectroscopic technique that measures the differential absorption of left and right circularly polarized hght as... 

Dextrorotatory stereoisomer that reflects plane-polarized hght to the right. 

Compounds in which rotation is restricted may exhibit cis-trans isomerism. These compounds do not rotate the plane of polarized hght (unless they also happen to be chiral), and the properties of the isomers are not identical. The two most important types are isomerism resulting from double bonds and that resulting from rings. 

As indicated, P is the difference between the two observed intensities divided by their sum. Fluorescence polarization is measured by placing a mechanically or electrically driven polarizer between the sample cuvet and the detector. A diagram of a fluorescence polarization measurement system is shown in Figure 3-16. In the normal instrumentation mode, the sample is excited with polarized hght to obtain maximum sensitivity. The polarization analyzer is positioned first to measure the intensity of the emitted fluorescence hght in the vertical plane ly), and then the polarization analyzer is rotated 90° to measure the emitted fluorescence hght intensity in the horizontal plane (Ij,). P is then calculated manually or automatically by use of equation (13). 

K. Ichimura, Y. Hayashi, and N. Ishizuki, Photocontrol of in-plane alignment of a nematic liquid crystal by a photochromic spiropyran monolayer absorbing linearly polarized hght, Chem. Lett. 1992,1063-1066. 

A.29.9 A polarization microscope can be used to evaluate the hydrolysis that should be occurring in the first stage of this microfluidic device. The intensity of transmitted hght through these microfluidic stages can be measured with a CCD, digitizing camera or appropriate photomulitplier system. The polarizer at the condenser level could be set to transmit polarized hght at... 

Fig. 10.28. Sequence of snapshots of martensitic microstructnre corresponding to different load levels (courtesy of R. James and C. Chu) as obtained using polarized hght. Different shades correspond to different variants of the Cu-Al-Ni martensite. Biaxial stress state applied using machine shown in fig. 10.27, with stresses applied along (Oil) and (Oil) directions (a) =...

Polarized light (Section 7.4) Light in which the electric field vectors vibrate in a single plane. Polarized hght is used in measuring optical activity. 

Optical isomerism is concerned with chirality, and some important terms relating to chiral complexes are defined in Box 19.2. The simplest case of optical isomerism among fi -block complexes involves a metal ion surrounded by three didentate ligands, for example [Cr(acac)3] or [Co(en)3] (Figures 3.16b and 19.12). These are examples of tris-chelate complexes. Pairs of enantiomers such as A-and A-[Cr(acac)3] or A- and A-[Co(en)3]Cl3 differ only in their action on polarized hght. However, for ionic complexes such as [Co(en)3], there is the opportunity to form salts with a chiral counter-ion A. These salts now contain two different types of chirality the A- or A-chirality at the metal centre and the (-I-) or (—) chirality of the anion. Four combinations are possible of which the pair (A-(- -) and A-(—) is enantiomeric as is the pair A-(—) and A-(- -). However, with a given anion chirality, the pair of salts A-(—) and A-(—) are diastereomers (see Box 19.2) and may differ in the packing of the ions in the solid state, and separation by fractional crystallization is often possible. 

Optical isomers. Optical isomers exist for octahedral complexes that do not possess a center of inversion or a mirror plane of symmetry. The complex and its mirror image are not superimposable. One isomer will rotate the plane of polarized light to the left, the other will rotate polarized hght to the right. The complexes are said to be chiral and optically active. Some examples are [Co(ox)3] cis [Rh(en)2Cl2] and cis, cis, cis [PtCl2Br2(NH3)2]. 

Although Pasteur s work opened the door to understanding the relationship between structure and optical activity, it was not until 1874 that the Dutch chemist van t Hoff and the French chemist LeBel independently came up with a basis for the observed optical activity tetrahedral carbon atoms bonded to four different atoms or groups of atoms. Thus, two enantiomers, which are identical to one another in all other chemical and physical properties, will rotate plane-polarized hght to the same degree, but in opposite directions. 

Figure 10 shows extinction spectra for x and y-polarized hght that is scattered and absorbed by a double chain of gold conjugates with the following... 

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