Coupled oscilator mechanism

The coupled oscillator mechanism involves coupling between the transition moments of two adjacent chromophores these must have a spatial relationship in which the interacting moments are non-parallel. (If the moments are parallel, only the absorption spectrum is affected hyperchromism is shown if the chromophores are arranged in a head-to-tail mode, and hypochromism if they are stacked one over the other in the head-to-head mode 20). By knowing the optical activity, it is possible to deduce the relative configuration between the given chromophores, and vice versa. 

Optical activity arises from the coupling of given electric-allowed transitions with a chiral orientation (coupled oscillator mechanism or two-electron mechanism) or from the electric or magnetic moments of a transition being pertubed by a chiral static field (asymmetrically perturbed field mechanism or one-electron mechanism) in the given one molecule. A similar mechanism of the optical activity can be expected for molecular assemblies which are composed of chiral and achiral ones. This type of optical activity is called induced optical activity and depends on types of inter-molecular interaction modes. 

The electronic transitions of the bound chromophore are coupled to the transitions of the protein, e.g., aromatic amino acid side chains and/or peptide bonds by the coupled oscillator mechanism [68-70]. 

In contrast to the trans-disubstituted phenylcyclopropanes cis-disubstituted phenylcyclopropanes, such as 23 and 30, do not show any detectable CD signals a. This is reasonable, if the optical activity of the two lowest energy electronic bands of phenylcyclopropanes are generated essentially by the coupled-oscillator mechanism (exciton mechanism for the degenerate case, such as 19). In cis-phenylcyclopropanes the interacting transition moments will lie (approximately) in the same plane. Hence, in the coupled-oscillator mechanism there will be only a very small Cotton effect (which is zero, if... 

---