Anomalous optical rotatory dispersion

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). 

Fig. 15.30 Anomalous optical rotatory dispersion (ORD) and circular dichroism (CD) spectra for the positive Cotton effect of a single, isolated electronic transition.

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. 

Although the existence of circular dichroism and anomalous optical rotatory dispersion for the visible d—d transitions of transition metal complexes was discovered by Cotton (7), the first resolution of an octahedral complex was achieved by Werner (2), for the chloroamminebis-(ethylenediamine)cobalt(III) ion (I, X = C1, Y = NHs). In the course of a few years he resolved (3) the trisethylenediaminecobalt(III) ion (II), a number of bis- and tris-chelated octahedral complexes of cobalt, chro-... 

For many years no attempt was made to determine the absolute stereochemistry of transition metal complexes, although Kuhn (8) calculated the absolute configuration of (—)n-[Co(C204)3]3- from its anomalous optical rotatory dispersion, using classical coupled oscillator theory. He later (9) extended his theory to other tris-chelated cobalt complexes such as (+)n-[Co(en)3]3+. However, in 1955 Saito (10) showed by anomalous X-ray dispersion that Kuhn s suggested configuration for (+)n-[Co(en)3]3+ was incorrect. 

Optical rotatory dispersion (ORD) involves the measurement of the rotation of plane-polarized light by a chiral compound as a function of the wavenumber. Circular dichroism (CD) is the unequal absorption of right and left circularly-polarized light as a function of its wavenumber. CD and anomalous ORD curves observed for chiral solute... 

Copolymers of L-proline and sarcosine have been prepared by Fasman and Blout (1961). These were found to exist in two forms, analogous to poly-L-proline. Form I, which is obtained directly from the copolymerization mixture showed anomalous optical rotatory dispersion and relatively low viscosity. On dissolving form I in 2-chloroethanol, form II is obtained which exhibits normal optical rotatory dispersion and relatively high viscosity. Fasman and Blout have suggested that the transition, form I —> form II, involves a conversion of the structure from ds- amide bonds to fmns-amide bonds. 

Figure 15. Circular dichroism curve (-—) plotted on the left-hand ordinate and optical rotatory dispersion (ORD) curve (-—) plotted on the right-hand ordinate of d-camphor-10-sulfonate. It is seen that the band width at [df]/e and the distance between the ORD extrema coincide better than does the band width at [0 ]/2, i.e., r. The amplitude of the ORD anomalous dispersion curve (Cotton effect) is indicated as [Amp]. Reproduced, with permission, from [3].

As a consequence of a CD the normal plain curve of the wavelength dependence of a, the "optical rotatory dispersion" (ORD) becomes anomalous and the plain curve is superposed by an S-shaped curve. Both, CD and anomalous ORD, are called "Cotton effect". Quantitatively CD and ORD are linked through the Kronig-Kramers relationship. No stringent coupling of the extinction coefficient (Emax) with the CD effect exists, because contains... 

O BRIEN and TOOLE (1) studied the.optical rotatory dispersion (ORD) of complexes formed by several colorless metal ions with optically active amines. They concluded that the effects were independent of the nature of the central metal ion and of the nature of the active amines, but dependent on the number of ligand molecules coordinated. SHIMURA (2) showed that for [Co(NHj) -leuc] (C10 )j [leuc a NHj,CH(CO )CjHj(CHj)j] the presence of the optically active ligand caused an anomalous dispersion curve in the region of the first absorption band of Co(III), The present paper reports the study of the contributions to the optical activity from the dissymmetric spiral configuration... 

In the region of the selective reflection, the optical rotatory dispersion is anomalous and changes sign approximately at the peak wavelength of the reflection band. 

Polavarapu PL, Zhao C. Ab initio predictions of anomalous optical rotatory dispersion. J. Am. Chem. Soc. 1999 121 (l) 246-247. 

The active bromides exhibit strongly anomalous rotatory dispersion. Kudo and Shimura reported that the circular dichroism (CD) intensity of the optically active ammine-hexol is affected by the anion coexisting in the solution, that the antipodes racemize readily in neutral and basic aqueous solutions, and that the racemization rate in acidic aqueous... 

Figure 10 shows the relationships between the spectral behavior of the two polarized beams with respect to rotation, absorption, and refraction. Only optically active substances interact differently with two polarized beams, so that this difference leads to observable effects. Whereas in the region of anomalous rotatory dispersion the ORD curve shows a point of inflection, the CD curve has a maximum or minimum [38]. The extreme of the CD curve coincides approximately with the point of inflection of the ORD curve. In analogy... 

The rotatory dispersion of amino acids in the visible range of the spectrum has been studied by several workers (Karrer and Kaase, 1919 Waser, 1923 Pertzoff, 1927). Recently, Patterson and Erode (1943) have published detailed results with thirteen a-amino acids. With most compounds studied, the dispersion is normal, i.e., the specific rotation [a] at a particular wave length can be expressed by a one-term Drude equation which has the form a = a/(X — Xo ), where a and Xo are constants. It is likely that, if measurements were extended to the far ultraviolet, amino acids, like many other optically active compounds, would exhibit anomalous dispersion, especially in those parts of the spectrum in which absorption of amino and carboxyl groups becomes significant. It appears, especially from the work of Patterson and Erode (1943) that the l forms of amino acids fall into three classes Group I consists of those amino acids which have a normal and positive dispersion for this class, to which most purely aliphatic amino acids belong, Xo has... 

Figure 4 Optical rotatory curves, as would be obtained for simple dispersion (both positive and negative curves are shown) and anomalous dispersion (illustrating a positive Cotton effect).

The pure (—) alcohol (I) yields, via the chloride (II), a diextrorotatory alcohol (III). Both I and III yield methyl-n-propylcarbinol on catalytic reduction but, whereas IV is the optically pure (—)-carbinol, V is the dl form. Hills, Kenyon, and Phillips drew the very remarkable but not unreasonable conclusion that, in I, the olefinio system is in a state of induced asymmetry (a conclusion supported by the fact that the rotatory dispersion of I is anomalous, presumably owing to the optical contribution of the induced system), and that, in III, the optical activity is due solely to the residual induced asymmetry, the contribution from the fixed center having been destroyed by racemization or inversion during the two replacement reactions. 

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