Anomalous ORD

However, if the dependence of the angle of rotation a upon the wavelength is measured in the region of the absorption band of the optically active substance under investigation, then superposition of an S-shaped component on the normal ORD curve is observed in this region. Circular dichroism effects are responsible for this anomalous ORD curve. 

Unlike ORD, circular dichroism (CD) depends upon the differential absorption of the two circularly polarised part-beams of incident light by an optically active compound. The combined action of anomalous ORD and CD is summarised as the Cotton effect. Therefore Cotton effects can be determined both... 

However signs do change with the "handedness" of the natural isomer so a higher level of distinction is achieved using CD or polarimetric detection. An enormous advantage is derived from the fact that achiral excipients, such as lidocaine, procaine, and benzocaine, often deliberately added to illicit drug preparations to complicate the chromatographic identifications of amphetamine and methamphetamine, present no interference problem whatsoever to CD detection. ORD detection was developed for the analysis of mixtures of pseudoephedrine and ephedrine [47], which because of the connection between anomalous ORD and CD, should be applicable to CD detection in the UV. 

Derivatization is the preferred option for aminoacids (making use of the anomalous ORD or CD effects introduced with the chromophore) to move the Cotton bands to longer wavelengths and to lower the limits of detection. Derivatizations might include N-acetylation as in the case of aminoglycosides [67], dansylation [72], or binding to metal complexes [73,74], see later. On the other hand aminoacids have an auxiliary role to play in the analysis of other substances, where advantage is taken of their chiroptical properties. 

The lower intensity, long wavelength absorption bands of a, P-unsaturated ketones vary considerably with the polarity of the solvent. The same is also true for anomalous ORD spectra and CD spectra both of which have absorbance by a chromophore as a pre-requisite. The chiroptical spectra however are much more sensitive than absorbance spectra to the interaction between the solvent and the solute. 

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... 

Fig. 2 Composite diagram of the three chiroptieal dispersion spectra (a) A positive plain ORD curve (b) a positive anomalous ORD curve (Cotton effect) (c) a positive CD curve for a single Cotton effect.

Fig. 3 The anomalous ORD spectrum consisting of several overlapping Cotton bands of the unsaturated rigid ketone, bicyclo(2,2,l)hept-5-enone, which shows how complex the spectrum can be, even for a single molecule.

Fig. 2b is an idealized illustration of a single, uncomplicated Cotton effect. In reality, the occurrence of a complete curve in the electronic spectrum is rare. Complete dispersions are more likely to be observed in the vibrational spectral range because of the increased spectral resolution. However, even there, dispersions are too often complicated by extensive band overlap. The same is true for electronic spectra where hidden absorption bands coupled vibronic excitations and interferences from bands associated with other chiral chromophores contribute to producing anomalous ORD curves that are so complex they have little utility in quantitative analytical applications (Fig. 3). 

At the time that Cotton was correctly interpreting the physical origins of anomalous ORD behavior, he proposed that there is also a difference between the absolute absorbances of the two circular polarized beams by a chiral medium (dichroism) and that the magnitude of the dichroism is proportional to the absorbance difference. Convention has dictated that the difference is always written as the absorbance of the left rotating beam minus the absorbance of the right, A A = Al — Ar O. Using the Beer-Lambert law to convert A to molar units, the dichroism expression can be rewritten as As = — br, where b has the... 

Anomalous ORD and CD both originate from light absorption by a chiral species and as such contain the same information. A mathematical equation, the Kronig-Kramers transform, relates one to the other over the wavelength range of the absorption, namely, [ (/I)] = -2/71 - X )dX. When the... 

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... 

The Cu(II) complex of L-alanine was the first chromophoric derivative of an amino acid whose anomalous ORD spectrum was observed (Lifschitz, 1925). Subsequently, Pfeiffer and Christeleit (1937a,b) studied a series of Cu(II)-amino acid complexes (6) and found a close relationship between... 

A dichroic band is commonly referred to as a Cotton effect, named after the discoverer of the phenomenon, the French physicist Aime Cotton (1859-1961), even though, historically, this term was first used to describe the anomalous ORD effect. It is thanks to the work of Cotton, who investigated the use of ellipticity measurements for CD studies, that until today the output of CD instmments is usually measured in ellipticity 6 (in mdeg), which is related to CD through ... 

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