Consequences of Mechanical Bonding on the Optical Activity of Cyclo[18]carbon Rings

The computed nonresonant optical activity of pairs of cyclo[18]carbon rings, linked and unlinked with the same symmetry, D_2d, are compared. For optically active, non-enantiomorphous systems, we emphasize that the observables computed here can only be measured for oriented molecules in suitably dissymmetric configurations. Mechanically bonded linked rings have one independent gyration tensor eigenvalue, ±16 bohr⁴. Breaking the mechanical bond and thereby unlinking the rings, while maintaining noncovalent contact, triples the gyration, resulting in an eigenvalue of ±54 bohr⁴. Of the excited states with B₂ and E symmetry that contribute to the optical activity, the B₂ states dominate the overall response of the link and unlink. The responses of selected low-energy states, that mimic the responses of a more comprehensive sum over many states, narrate in part the paradox that two optically inactive monomers in close proximity are more chiroptically scattering than the linked pair. The behavior described here underscores the importance of noncovalent interactions in optical activity.