Jahn–Teller Effect in Circulenes

To compare with the Jahn–Teller effect in [5]circulene, corannulene monoanion, ESR spectra of [6]circulene, coronene monoanion was observed in solution. Although the ESR spectra of doped corannulene monoanion have shown complicated hyperfine splitting due to the Jahn–Teller effect, those of coronene monoanion exhibited equally spaced 13 peaks due to the 12 equivalent hydrogens and no hyperfine structure due to the Jahn–Teller effect down to 183 K, which signifies that the energy barrier between the minima of coronene monoanion is lower than that of corannulene monoanion. The quantum chemical calculations were performed to obtain the Jahn–Teller potential of corannulene and coronene monoanion. For corannulene, the calculated barrier between the equivalent minima was 7.9 meV for the E2(16) mode which is the strongest coupling mode, while that for coronene monoanion was 0.2 meV. In order to elucidate the warping terms on the potentials, it is important to include nonlinear terms, at least fifth-order and sixth-order anharmonic vibration for corannulene and coronene, respectively, and fourth-order nonlinear vibronic coupling for both molecules. The symmetries of the Jahn–Teller potentials and the optimized structures of a minimum and a saddle point are discussed from the group-theoretical point of view.