Until now, we discussed tetrahedral, axial, and helical chirality. Additionally, octahedral systems such as chelate complexes occur regularly in the stereochemical literature and can be chiral as well. An example includes the octahedral
A-form A-form chelate phen
A-form A-form chelate phen
Fig. 2.8 A-form (left) and A-form (right) of an octahedral complex with three phen chelate ligands. For assigning the A- or A-form to a three-dimensional octahedral complex, the upper vertex is to be taken as a viewpoint. Its projection on the equatorial plane is represented as a straight line. After rotation that the bent line is on the top, the complex is the A-enantiomer if the straight line is on the right and the A-enantiomer if the straight line is on the left
[Cr(phen)3]2+ chelate complex in which the central metal ion M is Cr2+ surrounded by three 1,10-phenanthroline (phen) chelate ligands. Other central metal ions and/or chelating ligands such as ethylenediamine (en) are possible as well. The chirality of octahedral complexes composed of two or three chelate ligands is assigned by the A- (delta) and A- (lambda) symbol as illustrated in Fig. 2.8. In order to assign these stereochemical descriptors to an octahedral chelate system, an analogous three-dimensional projection is required. For the systematic stereochemical nomenclature of similar (but not analogous) octahedral systems with e.g. two chelate ligands the interested reader is referred to Herrero and Uson (1995).
An optical method capable of differentiating between two enantiomers is referred to as a chiroptical technique. As indicated in Sect. 2.2, the optical rotation of a chiral molecule in solution depends on the wavelength of the incoming linearly-polarized light. Rotation angle a and specific rotation [a] change as a function of the wavelength, the measurement of which is called the optical rotation dispersion (ORD).
Often, the optical rotation changes only slightly by changing the wavelength, and ORD curves show no extrema (minimum or maximum) in the observed wavelength range in the visible spectrum. In this case, curves are called plain curves or normal dispersion curves. One distinguishes between positive and negative ORD plain curves. Positive ORD curves are obtained for positive values of the optical rotation. They are characterized by an increasing optical rotation towards short wavelengths in the ultraviolet range of the spectrum and show a negative slope. Negative ORD plain curves are for negative values of the optical rotation and show a positive inclination with more negative values towards shorter wavelengths. Achiral molecules and racemic mixtures are not optically active resulting in ORD curves with [a] = 0 over the whole wavelength range. If one enantiomer of a given chi-ral molecule presents a positive ORD curve, the other enantiomer does necessarily present a negative ORD curve. Normal dispersion curves are obtained in a region of the spectrum without any absorption maxima of an analyte's chromophore, i.e., the functional group absorbing the visible or near ultraviolet light.
An abnormal dispersion curve is obtained if the wavelength of the incoming linearly-polarized light approaches the absorption maximum of a chromophore in the vicinity of a stereogenic center. Here - and this is a peculiar optical property of chiral molecules - the abnormal ORD curve describes an extremum, say a maximum, passes zero with a high slope, and enters in a minimum! The specific rotation changes its sign by a minute variation of the wavelength! This phenomenon in the abnormal ORD curve is called the Cotton effect. The Cotton effect of an optically active analyte is usually observed in solution and depends here on parameters like the dipole-character of the solvent, pH-value, temperature, etc.
The Cotton effect with its important change of sign for the optical rotation can be predicted even for an invisible or inaccessible region of the spectrum. A specific region of the ORD spectrum might be inaccessible due to solvent absorption and oxygen absorption below 200 nm and also ozone O3 produced by the lamp of the polarimeter absorbs below 250 nm. Moreover, other wavelength-dependent optical effects may hide a region of the ORD spectrum. For the calculation and prediction of the Cotton effect, one uses the Drude equation 2.4 which denotes the specific rotation [a] given in experimental units of degrees per decimeter versus wavelength (A) in nanometer. Previously to Drude, it had already been shown by Biot that the angle of rotation a is inversely proportional to the square of the wavelength A of the light shining through a chiral medium. A positive Cotton effect calculated by the Drude equation is illustrated in Fig. 2.9.
In the Drude equation, A is a compound specific constant and AC denotes the critical wavelength at which the dispersion curve passes zero. If A becomes AC the denominator becomes zero giving an infinite value for [a]. The critical wavelength AC coincides with the maximum in the absorption spectrum of the chiral structure. Moreover it should be noted that modern molecular theories of optical rotation all provide equations of this form for transparent regions (Barron 2004).
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