Chiroptical methods optical rotatory dispersion

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.). 

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. 

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. 

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. 

Chiral objects absorb left and right circularly polarized light to slightly different extents. Tltis phenomenon of circular dichroism [1] became the basis of the most widespread practical chiroptical method in the past few decades. There are some other chiroptical methods based on the interaction of chiral matter with circularly polarized light Optical rotatory dispersion is based on the analogous difference in refraction. Raman optical activity measures differences in scattered light, and circularly polarized luminescence deals with the difference in emission. 

The method and accuracy of proving the presence of a chiral stmcture in a polymer vary depending on the types of study and the stmcture of the polymer. Stmctural questions can be addressed by (1) various methods based on computer calculations or observations of molecular models, (2) achiral spectroscopic evidence (nuclear magnetic resonance (NMR) spectra, absorption spectra, fluorescence spectra, Raman spectra, X-ray diffraction (XRD), and so on), (3) viscosity or light scattering data giving information on the shape and size of an entire molecule, (4) chiroptical properties (optical activity, optical rotatory dispersion, electronic circular dichroism... 

Spectroscopic methods can work with the chiral selector associated with the ligand either in solid state or in solution. The chiroptical spectroscopies, circular dichroism, and optical rotatory dispersion, represent an important means for evaluating structural properties of selector-ligand adducts [14]. NMR can specifically investigate proton or carbon atom positions and differentiate one from the other. X-ray crystallography is a powerful technique to investigate the absolute configuration of diastereoisomeric complexes but in the solid state only. Fluorescence anisotropy is a polarization-based technique that is a measure, in solution, of the rotational motion of a fluorescent molecule or a molecule + selector complex [15]. 

Chiroptical techniques allow optical distinction between chiral molecules. In the ORD (optical rotatory dispersion) method the optical rotation is recorded as a function of the wavelength. In the CD (circular dichroism) method the difference in molar absorptivity between left and right circularly polarized light is measured as a function of the wavelength. The ORD curve of (-) humulone in methanol is displayed in Fig. 4. 

The chiroptical methods include optical rotation (OR), optical rotatory dispersion (ORD), electronic circular dichroism (BCD), vibrational circular dichroism (VCD), and Raman optical activity (ROA). These are nondestructive methods that can be measured directly in solution and without the need for crystallization. The power of the above-mentioned methods for the stereochemical investigation of chiral organic compounds resides in the fact that the two mirror-image CPL beams interacting with an asymmetric molecule is a manifestation of diastereo-meric discrimination. ... 

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