Puckering Energetics and Optical Activities of [7]Circulene Conformers

The structural preference of [7]­circulene is analyzed by taking into account vibronic interactions. DFT calculations reveal that pseudo-Jahn–Teller effects cause the D 7h -symmetry structure to relax to C 2- and C s -symmetry structures, which are both ca. 9 kcal/mol lower in energy than the D 7h structure. In energy terms, the C 2-symmetry structure is 0.05 kcal/mol lower than that of the C s -symmetry. The active vibrations are attributed to low-frequency puckering modes that are coupled with π–σ excitation states. The optical activities of the C 2-symmetry structure were simulated by configuration interaction calculations, and the simulated CD/ORD spectra were reasonable and consistent with the experimental data. The optical rotatory strengths obeyed the helix rule; that is, the left-handed helix shows negative Cotton effects through the antisymmetric excited states. The calculated spectra will serve as a foundation for further investigation of optical activities of negatively curved structures.