Chromophore chirally perturbed, intrinsically

A distorted conjugated pair of double bonds is an intrinsically chiral chromophoric system, and its overall chiroptical properties depend on the reduced symmetry of the chromophore itself as well as on the perturbing action of a dissymmetric environment. 

The diene chirality rule (hereafter referred to as DR) constitutes a simple tool for correlating the sign of the lowest energy tt —> n transition (] A —> 1B in C2 symmetry) of the distorted diene to the chirality (left or right-handed) of the chromophore. The validity of this rule is based on the assumption that the CD of the distorted chromophore is determined by its intrinsic helicity alone and that external dissymmetric perturbations have only minor effects on the optical activity. 

CD may sometimes be used to deduce the strxicture of a system. This is only really viable when the system can be considered as a collection of chromo-phores (spectroscopically well-defined subunits of a molecule) each of which is only slightly perturbed by the rest of the system. In the rest of this section we shall consider the coupling of two intrinsically achiral chromophores that are chirally oriented with respect to one another. A range of applications of this theory is given in (4). We have to treat eda/mdf transitions separately from... 

This helicity rule was criticized in 1968 by Kuriyama (36). He demonstrated that for small torsion angles in enones the chirality contribution can be neglected by an electronic interaction of the type, observed in intrinsically symmetric chromophores, which are asymmetrically perturbed. This reasoning is particularly valid when a heteroatom is homo-conjugated with the unsaturated chromophore. 

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