CD Spectra of Single Chromophore Systems

The field of natural products has been particularly fertile for the application of CD spectra to structural elucidation. The method has proven especially useful for carbonyl compounds. These have been investigated very thoroughly and will therefore be discussed here in some detail. Other important naturally occurring chromophores such as alkenes, dienes, disulfides, and aromatics will be mentioned more briefly. 

For a carbonyl group in a Cjv symmetry environment, such as in formaldehyde, the dipole approximation for the n- r transition yields JWo f 0 and Mo f = 0. Although the transition is magnetically strongly allowed and polarized along the CO axis, it is electrically forbidden. The absorption therefore is of very weak intensity and the rotational strength is equal to zero. Perturbations by vibrations or by an achiral solvent can affect and to such an extent that a small nonzero electric dipole transition moment results but again this produces only a very small absorption intensity and 

The CD effect of simple alkenes occurs at shorter wavelengths and is therefore more difficult to measure. The spectra are usually interpreted on the basis of a modified octant rule. The interpretation is much less reliable. 

Dienes are inherently chiral if they are twisted around the central CC bond. This produces a rotational strength that is positive if the twisted butadiene chromophore forms a right-handed helix. But since this inherent effect is not always predominant, contributions of other substituents have to be taken into account as well in order to be able to predict the CD spectrum. Substituents in allylic positions are frequently most important. 

Typical for aromatic hydrocarbons and their derivatives are the Lb, Lj, Bb, and Bj, bands. Chirally twisted aromatic compounds, for example, hexa-helicene (2), are inherently dissymmetric n chromophores. The rotatory strength of the various transitions can be calculated by means of the common r-electron methods. 

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