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,, 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.