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

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Abstract
The vibronic (vibrational–electronic) interactions and Jahn–Teller distortions in the mono- and trianions of benzene and [18]annulene, which show delocalized D 6h structures in their neutral states, are discussed. E 2g modes of vibration remove the orbital degeneracies in these highly symmetric molecules to lead to D 2h 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 E 2g mode of 1656 cm −1 and the C–H stretching E 2g 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 E 2g 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 E 2g mode play an important role in the Jahn–Teller distortions in negatively charged benzene and [18]annulene.