Vibrational Excitons in CH-Stretching Fundamental and Overtone Vibrational Circular Dichroism Spectra

A set of vibrational circular dichroism (VCD) spectra in the CH-stretching fundamental
region for about twenty compounds belonging to the class of essential oils was empirically analyzed by
the use of a sort of vibrational exciton mechanism, involving three centers. Through a general formula
applicable to many coupled dipole oscillators, the rotational strengths of the previously identified
vibrational excitons are evaluated. The results are then critically reviewed by the use of recent ab
initio methodology, as applied to selected molecules of the original set. Further insight is gained by
model calculations adding up the contribution of the coupling between electric dipole moments associated
with normal mode behavior and that of the polarizability from polarizable groups. The former
part is responsible for the excitonic behavior of the VCD spectra. For the same selected molecules we
have also investigated whether some excitonic behavior is taking place in the second overtone region,
and have concluded that this is not the case.