The degenerate coupled oscillator DCO model

Coupled oscillator models are extensions to the simple models developed for electronic circular dichroism. They are well known under the name exciton theory (see e.g. Harada and Nakanishi, 1972). These models, extended to vibrational transitions, describe the coupling of pairs of electric dipole transition moments. They predict equal amounts of positive and negative VCD intensity  

When two identical achiral chromophors a and h are present in the same molecule and form a chiral arrangement, they can interact with each other and alleviate the degeneracy of the transition. In this way the VCD shows a so-called couplet a positive effect closely followed (going to longer wavelength) by a negative one (a positive couplet) or vice versa (a negative couplet). In the equation above i/ is the transition frequency. Tab the separation of the dipoles, and /z the dipole moment. 

The applicability of the model have been studied using steroids with two C=0 -functionalities located in six-membered rings (Narayanan and Keiderling, 1983). The DCO model proves to be satisfactory for all six but one cases. 

---