The Jahn-Teller and pseudo-Jahn-Teller effects in hexafluorobenzene radical cation: nonradiative decay and radiative emission.

Due to the highly symmetric structure of the hexafluorobenzene (HFBz, D6h point group symmetry) molecule, its radical cation possesses degenerate electronic states and vibrational modes. Therefore, this provides a unique platform to investigate multi-mode Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) effects in its ionic states. In this work, the first four energetically low-lying electronic states of HFBz+, viz., X̃2E1g, Ã2A2u, B̃2B2uand C̃2E2g, are considered and the high-level ab initio electronic structure calculations are performed. Equation of motion-ionization potential-coupled cluster singles and doubles (EOM-IP-CCSD) method is used for this purpose. Among these electronic states, X̃2E1g and C̃2E2g states are doubly degenerate, and the X̃2E1g state is energetically well separated from the other excited states. However, the rest of the states are closer in energy and they form low-energy conical intersections (CIs) leading to JT and PJT interactions. A vibronic coupling model is developed in a diabatic electronic basis employing symmetry selection rules, dimensionless normal displacement coordinates of the vibrational modes and Taylor series expansion of the elements of electronic Hamiltonian. Both time-independent and time-dependent quantum mechanical methods are employed in the nuclear dynamics calculations. The multi-configuration time-dependent Hartree (MCTDH) method is used for this purpose. Theoretical results are compared and found to be in good agreement with the available experimental data. The impact of the fluorination on the structure and dynamics of excited states is discussed in relation with the parent benzene radical cation.