Vibronic coupling and Jahn–Teller effects in negatively charged [30]annulene Original Research Article

The vibronic interactions and Jahn–Teller distortions in the mono- and trianions of [30]annulene are discussed comparatively with those of benzene and [18]annulene. We calculate the electronic structures, vibrational modes, and linear vibronic coupling constants of [30]annulene as well as its deutero-form using the B3LYP method, the hybrid density functional theory method of Becke and Lee, Yang, and Parr. The lowest-frequency E2g mode of 38 cm−1 which causes deformation of the carbon ring is important compared to any other mode in causing the Jahn–Teller distortions. Such a low-frequency vibrational mode is characteristic of nanosized molecular systems, being analogous to acoustic mode of phonon in solid. We find that such a vibrational mode plays an important role in the Jahn–Teller distortions as the ring size of annulene becomes large from benzene to [30]annulene. We also study the H/D isotope effect and find that the vibronic coupling constant of the E2g mode of 1513 cm−1 becomes larger by substituting deuteriums for hydrogens in negatively charged [30]annulene.