Pseudo Jahn-Teller effects and optical activities of negatively curved hydrocarbons

As a fundamental study on energetics of negatively curved structures, puckering energetics of [n]circulene is studied in view of vibronic interactions. In [7]circulene, the D7h geometry is subject to pseudo Jahn-Teller effect (PJT) and unstable with respect to the E1'', E2'', and E3'' vibrational modes. It is distorted into Cs or C2 geometries to form valleys of the potential surface ca. 9 kcal/mol below the D7h geometry. The C2 structure is slightly stable than Cs one, which is consistent with experiments. The resultant C2 geometry is optically active, and the CD (circular dichroism) and ORD (optical rotatory dispersion) spectra are well simulated. This type of optical activities is interesting in that they should be proportional to the PJT-distortion. PJT-induced optical activity in [2m+1]circulenes should be mainly originated from vibronic coupling constants in the initial and final states, which leads to helical rule characteristic of C2 molecules.