Vibronic coupling and Jahn–Teller effect in negatively charged benzene and [18]annulene

Abstract

The vibronic (vibrational–electronic) interactions and Jahn–Teller distortions in the mono- and trianions of benzene and [18]annulene, which show delocalized D6h structures in their neutral states, are discussed. E2g modes of vibration remove the orbital degeneracies in these highly symmetric molecules to lead to D2h distortions. We calculate the electronic structures, vibrational modes, and linear vibronic coupling constants of benzene and [18]annulene as well as their deutero-forms using the B3LYP method, a hybrid (Hartree–Fock/density functional theory) method. The C–C stretching E2g mode of 1656 cm−1 and the C–H stretching E2g mode of 3184 cm−1 give large vibronic coupling constants in the mono- and trianions of benzene. On the other hand, in [18]annulene the lowest E2g mode of 116 cm−1 affords extremely large coupling constants in its mono- and trianions to contribute to the Jahn–Teller distortions. The lowest mode of [18]annulene is related to a deformation of the carbon ring of [18]annulene and analogous to acoustic mode of phonon in solid. Thus, the different types of E2g mode play an important role in the Jahn–Teller distortions in negatively charged benzene and [18]annulene.