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Bicamphor: a prototypic molecular system to investigate vibrational excitons.

Vibrational circular dichroism (VCD) and IR spectra have been recorded in the fingerprint and carbonyl stretching regions for endo,endo-bicamphor (1), exo,exo-bicamphor (2), endo,exo-bicamphor (3), 3,3'-bicamphorylidene (4), and exo,exo-bis-thiocamphor (5). The C2 symmetry possessed by these molecular systems (except in one case), as well as their limited conformational mobility associated with well-defined degrees of freedom, allow for optimal test of the vibrational circular dichroism exciton chirality (VCDEC) rule introduced by Taniguchi and Monde. Density functional theory calculations are employed not only to predict the entire aspect of the VCD and IR spectra but also to study how the VCDEC rule may be impacted by the coupling between C═O stretchings and from C═O stretchings with other vibrational modes and by the rotation about the C-C bond connecting the two camphors. Comments are provided about the limitations and potentialities of the VCDEC method and about the manifestation of different vibrational excitons in other regions of the VCD spectra, either in the mid-IR or in the CH-stretching regions.