Plane of polarization rotation

Enantiomers of a chiral molecule have identical melting and boiling points, densities, and other physical and chemical properties. However, enantiomers show different behaviour towards plane-polarized light. When a beam of plane polarized light passes through an enantiomer, the plane of polarization rotates. For this reason chiral molecules are known as optical isomers and are said to be optically active. 

When a beam of plane-polarized light passes through an enantiomer, the plane of polarization rotates. 

Because a chiral object and its realized mirror image interact oppositely with the circularly polarized light (a perturbation in the bonding electrons if not an absorption), in the presence of one of the enantiomers the velocity of one of the circularly polarized components of the light will be effected more than the other so that, on recombination as the beam leaves the material, it will be seen to have had its direction of oscillation, and hence its plane of polarization, rotated relative to its original position. 

On the other hand, when plane-polarized light passes through a solution of chiral molecules, the light emerges with its plane of polarization rotated either clockwise or... 

The Martin-Puplett interferometer illustrated in Figure 19.1 has two further WGPs (Pj, and Pom) in addition to that used as the beamsplitter (B). The plane of P , located before B is set to be perpendicular to the x-axis, while the plane of B is set at an angle of 45 to both the x- and y-axes. Pm has its wires parallel to the y-axis, so that only x-polarized radiation is transmitted from P and advances toward B. The wires in B are arranged in a direction rotated clockwise by 45° from the x-axis, when viewed from the side of P, so that the component of the beam of radiation incident on B with its plane of polarization rotated clockwise by 45° from the x-axis is reflected by B, and advances toward the fixed mirror Rf along the positive direction of the y-axis. It should be noted that the plane of polarization of this beam, when viewed in the direction toward Rf, is rotated anticlockwise... 

A molecule is chiral if it cannot be superimposed on its mirror image (or if it does not possess an alternating axis of symmetry) and would exhibit optical activity, i.e. lead to the rotation of the plane of polarization of polarized light. Lactic acid, which has the structure (2 mirror images) shown exhibits molecular chirality. In this the central carbon atom is said to be chiral but strictly it is the environment which is chiral. 

The anisotropy fiinction r t) = (/ (t) -1+ 21 t)) is detemiined by two polarized fluorescence transients / (t) and/j (t) observed parallel and perpendicular, respectively, to the plane of polarization of the excitation pulse. In tlie upconversion experiment, the two measurements are most conveniently made by rotating the plane of polarization of the excitation pulse with respect to the fixed orientation of the input plane... 

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]. 

Mixtures containing equal quantities of enantiomers are called racemic mixtures Racemic mixtures are optically inactive Conversely when one enantiomer is present m excess a net rotation of the plane of polarization is observed At the limit where all the molecules are of the same handedness we say the substance is optically pure Optical purity or percent enantiomeric excess is defined as... 

Occasionally an optically inactive sample of tartaric acid was obtained Pasteur noticed that the sodium ammonium salt of optically inactive tartaric acid was a mixture of two mirror image crystal forms With microscope and tweezers Pasteur carefully sep arated the two He found that one kind of crystal (m aqueous solution) was dextrorota tory whereas the mirror image crystals rotated the plane of polarized light an equal amount but were levorotatory... 

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. 

The all trans" stmcture in Figure 4.9(a) is not chiral as it has an inversion centre i. This is called a meso stmcture. Although each CFIFCFI3 group rotates the plane of plane-polarized light in one direction, the other group rotates it an equal amount in the opposite direction. The result is that there is no rotation of the plane of polarization and, as the presence of an inversion centre tells us, the molecule is achiral. 

Iodine vapor is characterized by the familiar violet color and by its unusually high specific gravity, approximately nine times that of air. The vapor is made up of diatomic molecules at low temperatures at moderately elevated temperatures, dissociation becomes appreciable. The concentration of monoatomic molecules, for example, is 1.4% at 600°C and 101.3 kPa (1 atm) total pressure. Iodine is fluorescent at low pressures and rotates the plane of polarized light when placed in a magnetic field. It is also thermoluminescent, emitting visible light when heated at 500°C or higher. 

Polarimetry. Polarimetry, or polarization, is defined as the measure of the optical rotation of the plane of polarized light as it passes through a solution. Specific rotation [ a] is expressed as [cr] = OcjIc where (X is the direct or observed rotation, /is the length in dm of the tube containing the solution, and c is the concentration in g/mL. Specific rotation depends on temperature and wavelength of measurement, and is a characteristic of each sugar it may be used for identification (7). 

The arrangement illustrated in Figure 1 is commonly used for angular characterization of scattered light. The light source is usually a laser. The incident beam may be unpolarized, or it can be linearly polarized with provisions for rotating the plane of polarization. Typically the plane of polarization is perpendicular to the plane of... 

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... 

To be optically active, the sample must contain a chiral substance and one enantiomer must be present in excess of the other. A substance that does not rotate the plane of polarized light is said to be optically inactive. All achiral substances are optically inactive. 

What causes optical rotation The plane of polarization of a light wave undergoes a minute rotation when it encounters a chiral molecule. Enantiomeric forms of a chiral molecule cause a rotation of the plane of polarization in exactly equal fflnounts but in... 

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