Inherently dissymmetric chromophore

The presence of an inherently dissymmetric chromophore in a molecule clearly provides a key to obtaining stereochemical information from both electronic CD and, as demonstrated by the CH2CH2C H fragment, from VCD spectra. As with any spectroscopic structural probe, the characteristic spectral features of the chromophore must not significantly mix with nor be overlapped by other vibrations in the molecule. 

Many chromophores are suitable for use in the exciton chirality method. One of the features required for such a chromophore is its planarity or near-planarity. Nonplanar (inherently dissymmetric) chromophores would contribute to the CD spectra by other mechanisms. The other limiting factor is the position of the transition in the spectral region studied. For example, the 1B transition in the alkyl-substituted benzene chromophore appears near the short-wavelength recording limit around 200 nm, making its use in the exciton chirality method less attractive. Furthermore, the direction of polarization of the lB transition in alkyl-substituted benzene derivatives is not readily determined. In such cases calculation of the rotatory strength is more reliable than qualitative analysis. 

Typical for aromatic hydrocarbons and their derivatives are the Li, L 8, and B3 bands. Chirally twisted aromatic compounds, for example, hexa-heiicene (2), are inherently dissymmetric chromophores. The rotatory strength of the various transitions can be calculated by means of the common r-electron methods. 

The fourth, the so-called inherent dissymmetric chromophore, does not possess local symmetry. Therefore, the transitions belonging to these chromophores are magnetically and electrically allowed. Inherent dissymmetric chromophores are often found with so-called form chiral molecules for which atropisomers like binaphthols are typical representatives. Further examples are chromophores that come into being by exciton coupHng. In both cases the dissymmetry factor g is in the order of 10 to 10 and the CD is easily measurable in spite of the fact that the absorption coefficients of these compounds are often very high (e between 10" and 10 ). 

The assignment by semiempirical and empirical rules known as sector and helicity rules. Sector rules can be applied to inherent symmetric chrom-ophores. Helicity rules have been deduced for inherent dissymmetric chromophores. 

Electronic circular dichroism for compounds without a chromophore To determine the absolute configuration of compounds without a chromophore in the accessible spectral region one or more proper groups can be derivatized to obtain an appropriate chromophoric system. The substitution of di-hydroxysteroids with two p-methoxybenzoates can be mentioned as an example. The substitution leads to an inherent dissymmetric chromophore for which the exciton chirality method can be applied. For chiral diols and amino alcohols another interesting possibility does exist. Complexing of these diols and amino alcohols with a transition metal... 

More important for CD spectroscopy is the classification of chromophores with respect to symmetry operations of the second kind (i, a, and with n>2). An inherently dissymmetric chromophore possesses no symmetry element of the second kind and thus is chiral by itself. In an inherently symmetric chromophore a symmetry element of the second kind exists and thus it is achiral by itself. In Figure 4 the enone chromophore is shown as inherently symmetric and inherently dissymmetric. The local symmetry of a... 

Helicity rules have been developed which can be applied to inherently dissymmetric chromophores, such as, e.g. the enone chromophore shown in Figures 12 and 13, where the chosen atoms are only approximately on a helical line. Many papers have been published concerning the enone chromophore as summarized in Patai and Rappoport by Gawronski. 

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