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 D_7h 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 C_s or C₂ geometries to form valleys of the potential surface ca. 9 kcal/mol below the D_7h geometry. The C₂ structure is slightly stable than C_s 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 [2 m +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. (paper)