Vibrational Structuring in Optical Activity. I. Vibronic Coupling in the Circular Dichroism of Dimers

The characteristics of circular dichroism arising from near-degenerate electronic states under the influence of vibration are considered and compared to the ordinary absorption. A ``pseudo-Jahn—Teller effect'' operates between limits of strong and weak coupling, the influence of which on ordinary absorption has been considered earlier. An exciton or coupled-oscillator model is used which seems especially appropriate to optically active biaryls. For intermediate coupling, overlapping band systems of equal net rotatory strength but opposite sign will give a resultant dichroism altogether different in appearance from the ordinary absorption.