Rotatory dispersion optical

3 Optical Rotatory Dispersion With optically active compounds, it has been common practice for many years to measure the optical rotation at 489 nm, the wavelength of the sodium D line, using a polarimeter (typical cost, 500). 

The equation [a]jj = a/lc, in which a is the observed rotation in degrees, / is the length of the sample tube in decimeters, and c is the concentration of sample in grams per 100 ml, is used to calculate the specific rotation [a]j,. The molecular rotation can be calculated (when the molecular weight of the sample is known) by multiplying the specific rotation by the molecular weight. 

I Kahlsico, Box 1166, El Cajon, Calif. 92022, sells a dipole apparatus for 1500. 

Djerassi, Optical Rotatory Dispersion, New York, McGraw-Hill Book Co., 1960. 

Chiral molecules occur in pairs related by a symmetry plane, their mirror images cannot be superimposed (enantiomers). Such molecules exhibit optical activity, i. e. they transmit left and right circularly polarized Hght in a different manner. The difference in the refraction indices for left and right circularly polarized light is called optical rotatory dispersion (ORD), the corresponding difference in absorption coef ficients is called circular dichroism (CD). ORD and CD can be related to each other by the Kramers—Kronig transformation. 

The angle of rotation a at the wavelength A is directly proportional to the concentration c  

The obtained molar rotation [m] may be influenced by the refractive index n of the solvent. The corrected molar rotation is defined by 

A plot of [a] versus A would then be a complex curve with maxima and minima. The author completely agrees with the view expressed by Djerassi78 that the usefulness of the Drude equation for structural studies is limited. 

Specific rotation [a] = 100a//c, where a is the observed rotation, / is the path length in dm and c is the concentration of the solution in g per 100 ml. Molecular dispersion, / , is defined as [a] x molecular weight/100. 

For compounds that absorb in the wavelength region studied, the dispersion curves are complicated and one or more maxima or minima may 

The oxirane ring has been found to exhibit an ORD curve at 290 nm this has the opposite sign to that of the alkyl group. 

As plane-polarized fight passes through a substance, the velocities of the two circularly polarized components are reduced. If they are reduced to the same extent, 

Physical Chemistry of Macromolecules Basic Principles and Issues, Second Edition. By S. F. Sun ISBN 0-471-28138-7 Copyright 2004 John Wiley Sons, Inc. 

FIGURE 12.2 (a) Polarized light and (b) elliptically polarized light. 

Experimentally, the velocity of light in a medium is characterized by the refractive index of the medium. The different velocities of the two circularly polarized components are thus expressed in terms of their different refractive indices. An optically active substance is one that has different refractive indices for its left and right circularly polarized lights, and mr. The optical rotation a at a given wavelength of incident light X is directly proportional to the difference between the refractive indices of the two circularly polarized components  

Customarily, the optical rotation of an optically active substance is expressed in terms of the specific rotation [ot] and the molar rotation 

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MCD Magnetic circular dichroism. See optical rotatory dispersion. 

Circular dicliroism has been a useful servant to tire biophysical chemist since it allows tire non-invasive detennination of secondary stmcture (a-helices and P-sheets) in dissolved biopolymers. Due to tire dissymmetry of tliese stmctures (containing chiral centres) tliey are biaxial and show circular birefringence. Circular dicliroism is tlie Kramers-Kronig transfonnation of tlie resulting optical rotatory dispersion. The spectral window useful for distinguishing between a-helices and so on lies in tlie region 200-250 nm and hence is masked by certain salts. The metliod as usually applied is only semi-quantitative, since tlie measured optical rotations also depend on tlie exact amino acid sequence. 

Although the usual absorption and scattering spectroscopies caimot distinguish enantiomers, certain techniques are sensitive to optical activity in chiral molecules. These include optical rotatory dispersion (ORD), the rotation by the sample of the plane of linearly polari2ed light, used in simple polarimeters and circular dichroism (CD), the differential absorption of circularly polari2ed light. 

The UV spectrum of a complex conjugated molecule is usually observed to consist of a few broad band systems, often with fine structure, which may be sharpened up in non-polar solvents. Such a spectrum can often be shown to be more complex than it superficially appears, by investigation of the magnetic circular dichroism (MCD) spectrum, or by introduction of dissymmetry and running the optical rotatory dispersion (ORD) or circular dichroism (CD) spectrum. These techniques will frequently separate and distinguish overlapping bands of different symmetry properties <71PMH(3)397). 

Electronic Spectra, Optical Rotatory Dispersion-Circular Dichroism... 

Azetidine, 7V-bromo-, 7, 240 Azetidine, AT-r-butyl- N NMR, 7, 11 Azetidine, AT-t-butyl-3-chloro-transannular nucleophilic attack, 7, 25 Azetidine, 3-chloro-isomerization, 7, 42 Azetidine, AT-chloro-, 7, 240 dehydrohalogenation, 7, 275 Azetidine, 7V-chloro-2-methyl-inversion, 7, 7 Azetidine, 3-halo-synthesis, 7, 246 Azetidine, AT-halo-synthesis, 7, 246 Azetidine, AT-hydroxy-synthesis, 7, 271 Azetidine, 2-imino-stability, 7, 256 Azetidine, 2-methoxy-synthesis, 7, 246 Azetidine, 2-methyl-circular dichroism, 7, 239 optical rotatory dispersion, 7, 239 Azetidine, AT-nitroso-deoxygenation, 7, 241 oxidation, 7, 240 synthesis, 7, 246 Azetidine, thioacyl-ring expansion, 7, 241 Azetidine-4-carboxylic acid, 2-oxo-oxidative decarboxylation, 7, 251 Azetidine-2-carboxylic acids absolute configuration, 7, 239 azetidin-2-ones from, 7, 263 synthesis, 7, 246... 

Freeing a solution from extremely small particles [e.g. for optical rotatory dispersion (ORD) or circular dichroism (CD) measurements] requires filters with very small pore size. Commercially available (Millipore, Gelman, Nucleopore) filters other than cellulose or glass include nylon, Teflon, and polyvinyl chloride, and the pore diameter may be as small as 0.01 micron (see Table 6). Special containers are used to hold the filters, through which the solution is pressed by applying pressure, e.g. from a syringe. Some of these filters can be used to clear strong sulfuric acid solutions. 

P. Crabbe, Top..Stereochem. 1 93 (1967) C. Djerassi, Optical Rotatory Dispersion, McGraw-Hill, New Vbrk, 1960 P. Crabbe, Optical Rotatory Dispersion and Circular Dichroism in Organic Chemistry, Holden D, San Francisco, 1965 E. Chamey, The Molecular Basis of Optical Activity. Optical Rotatory Dispersion and Circular Dichroism, John Wiley Sons, New Vbrk, 1979. 

The optical rotatory dispersion curves of steroidal ketones permit a distinction to be made between the conformations, and assignment of configuration is possible without resorting to chemical methods (see, e.g. ref. 36) which are often tedious. The axial halo ketone rule and, in the more general form, the octant rule summarize this principle and have revealed examples inconsistent with the theory of invariable axial attack in ketone bromination. 2-Methyl-3-ketones have been subjected to a particularly detailed analysis. There are a considerable number of examples where the products isolated from kinetically controlled brominations have the equatorial orientation. These results have been interpreted in terms of direct equatorial attack rather than initial formation of the axial boat form. 

The effect of an a-substituted oxirane group on the optical rotatory dispersion of steroidal ketones should be interpreted with caution an inverted octant rule for a-epoxy ketones has been proposed/although recent data indicate that the normal octant rule may still be valid. 

P. Crabbe, Optical Rotatory Dispersion and Circular Dichroism in Organic Chemistry, Holden-Day, Inc., San Francsico, 1965, p. 166. 

In addition to the expected 2,2-dimethyl- and 2a-methyl- compounds (7) and (8) the 2 -methyl-3-ketone (9) is obtained. Chemical evidence and optical rotatory dispersion measurements indicate that ring A in (7) and (9) is in the boat conformation. 

The P configuration at C-6 is based on mechanistic considerations and analysis of optical rotatory dispersion curves. 

The mass spectra of enamines (215,216), and optical rotatory dispersions (217) of optically active enamines have also been used for structural assignments. 

Chirooptical properties give more subtle information on the conformational behavior of biopolymers and peptides in solution. In early experiments, optical rotation and optical rotatory dispersion (ORD) have been recognized as valuable techniques, followed more recently by significant progress and refinements in the equipment which have resulted in the routine measurements of applied circular dichroism (CD). 

Other methods have also been used, including optical rotatory dispersion, circular dichroism (CD), and asymmetric synthesis (see p. 147). 

If two different three-dimensional arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about bonds, they are called conformationsIf they are not interconvertible, they are called configurations Configurations represent isomers that can be separated, as previously discussed in this chapter. Conformations represent conformers, which are rapidly interconvertible and are thus nonseparable. The terms conformational isomer and rotamer are sometimes used instead of conformer . A number of methods have been used to determine conformations. These include X-ray and electron diffraction, IR, Raman, UV, NMR, and microwave spectra, photoelectron spectroscopy, supersonic molecular jet spectroscopy, and optical rotatory dispersion (ORD) and CD measurements. Some of these methods are useful only for solids. It must be kept in mind that the conformation of a molecule in the solid state is not necessarily the same as in solution. Conformations can be calculated by a method called molecular mechanics (p. 178). 

Moffitt W. Optical rotatory dispersion of helical polymers. / Chem Phys 1956 25 467-78. 

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