Anomalous dispersion dichroism

Fig. 1 Top Behavior of the electronic linear chiroptical response in the vicinity of an excitation frequency. Re = real part (e.g., molar rotation [< ]), Im = imaginary part (e.g., molar ellipticity [0]). Without absorption line broadening, the imaginary part is a line-spectrum (5-functions) with corresponding singularities in the real part at coex. A broadened imaginary part is accompanied by a nonsingular anomalous OR dispersion (real part). A Gaussian broadening was used for this figure [37]. Bottom Several excitations. Electronic absorptions shown as a circular dichroism spectrum with well separated bands. The molar rotation exhibits regions of anomalous dispersion in the vicinity of the excitations [34, 36, 37]. See text for further details...

To determine the absolute configuration of optically active organic compounds, there are two nonempirical methods. One is the Bijvoet method in the X-ray crystallographic structure analysis, which is based on the anomalous dispersion effect of heavy atoms. - The X-ray Bijvoet method has been extensively applied to various chiral organic compounds since Bijvoet first succeeded in determination of the absolute stereochemistry of tartaric acid in 1951. The second method is a newer one based on the circular dichroism (CD) spectroscopy. Harada and Nakanishi have developed the CD dibenzoate chirality rule, a powerful method for determination of the absolute configuration of glycols, which was later generalized as the CD exciton chirality method. 8 The absolute stereochemistry of various natural products has been determined by application of this nonempirical method. 

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].

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

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-... 

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... 

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... 

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