Chiral, 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. 

A particularly useful probe of remote-substituent influences is provided by optical rotatory dispersion (ORD),106 the frequency-dependent optical activity of chiral molecules. The quantum-mechanical theory of optical activity, as developed by Rosenfeld,107 establishes that the rotatory strength R0k ol a o —> k spectroscopic transition is proportional to the scalar product of electric dipole (/lei) and magnetic dipole (m,rag) transition amplitudes,... 

Optical activity comes from the different refractions of right and left circularly polarized light by chiral molecules. The difference in refractive indices in a dissymmetric medium corresponds to the slowing down of one beam in relation to the other. This can cause a rotation of the plane of polarization or optical rotation. The value of specific rotation varies with wavelength of the incident polarized light. This is called optical rotatory dispersion (ORD). 

In addition to chemical correlations discussed above, several physical methods are now available for the determination of the relative and absolute configurations of chiral sulfur compounds. Among these, NMR, infrared (IR), optical rotatory dispersion (ORD), circular dichroism (CD), and X-ray analysis are the most important. Sections III-B-1 to III-B-5 outline applications of these techniques for establishing the chirality around the sulfur atom. 

The primary motivation for the development and application of vibrational optical activity lies in the enhanced stereochemical sensitivity that it provides in relation to its two parent spectroscopies, electronic optical activity and ordinary vibrational spectroscopy. Over the past 25 years, optical rotatory dispersion and more recently electronic circular dichroism have provided useful stereochemical information regarding the structure of chiral molecules and polymers in solution however, the detail provided by these spectra has been limited by the broad and diffuse nature of the spectral bands and the difficulty of accurately modeling the spectra theoretically. 

Gaffield, W., Circular dichroism, optically rotatory dispersion and absolute configuration of flavanones, 3-hydroxyflavanones and their glycosides. Determination of agyclone chirality in flavanone glycosides, Tetrahedron, 26, 4093, 1970. 

The inherent difficulty in analyzing enantiomers arises from the well-known fact that apart from their chiroptical characteristics, optical isomers have identical physical and chemical properties in an achiral environment (assuming ideal conditions). Therefore, methods of distinguishing enantiomers must rely on either their chiroptical properties (optical rotation, optical rotatory dispersion, circular dichroism), or must employ a chiral environment via diastereomer formation or interaction. Recently, it has become increasingly clear that such diastereomeric relationships may already exist in nonracemic mixtures of enantiomers via self-association in the absence of a chiral auxiliary (see Section 3.1.4.7.). 

Optical rotation, circular dichroism and optical rotatory dispersion of chiral thiophenes... 

Polarimetry, circular dichroism (CD) and optical rotatory dispersion (ORD) are the most important tools for the study of properties arising from optical activity. Although many chiral thiophenes have been prepared, there is no secure basis for a systematic discussion of the special effects of thiophene or annelated thiophene rings. For the purpose now at hand it is more expedient to discuss three different areas in which thiophene containing molecules and the related chiroptical techniques are central features. 

The term chiroptical basically refers to spectroscopic methods which afford values with opposite signs for the two enantiomers of a chiral compound [77]. Measurement of optical rotatory dispersion (ORD) and circular dichroism (CD) number among the most important chiroptical methods. 

Most measurements of the optical rotation are carried out at a single frequency, usually corresponding to the sodium D-line. However, studies of the variation of the optical rotation with the frequency of the incident light are also known, and are referred to as optical rotatory dispersion (ORD) [7], Historically, this was an important method for the determination of excitation energies in chiral molecules, but was later superseded by CD. We note that the calculation of ORD through regions of electronic absorption requires special care [27,28],... 

Organic materials with large optical rotations include cholesteric liquid crystals, molecules and polymers with chiral jt-conjugated systems, especially [n]helicenes [21, 31, 139]. The most important factor contributing to their large optical rotations is anomalous optical rotatory dispersion (ORD), which is associated with the presence of absorption (or reflection) with large rotational strength (Fig. 15.30). 

The interaction of polarized light with chiral compounds is of great interest since chiroptical techniques are extremely useful as methods of characterization. It is equally true that although most scientists are aware that enantiomerically rich solutions will rotate the plane of linearly polarized light, the origins of this effect are not as simple as might be imagined. In this first article, the phenomena of polarimetry and optical rotatory dispersion will be discussed. A subsequent note will concern the related phenomenon of circular dichroism. 

As discussed, optical rotatory dispersion (ORD) is determined by the unequal indices of refraction for left- and right-circularly polarized light in a chiral medium, within an absorption band, the ORD spectrum exhibits anomalous dispersion, which is referred to as a Cotton effect. Full understanding of ORD and anomalous dispersion requires a more detailed examination of the properties associated with refractive indices. 

Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra (71PMH(3)397) are very sensitive to the spatial disposition of the atoms in a molecule, and conformational changes may yield rather dramatic changes in the appearance of a CD or ORD spectrum of a chiral molecule. The analysis of the temperature dependence of the CD spectrum may give information on populations and free energy differences. Except for nucleosides, the use of the chiroptical method in conformational analysis is rather limited, which may be accounted for by the complexity of the theory for optical activity. 

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