Vibronic coupling in the ground and excited states of the imidazole radical cation

Vibronic interactions in the ground and two excited states of the imidazole radical cation, X2A" (π-1), A2A' (nσ-1) and B2A" (π-1), and the associated nuclear dynamics were studied theoretically. The results were used to interpret the recent photoelectron measurements [M. Patanen, et al., J. Chem. Phys. 154, 054304 (2021)]. The present high-level electronic structure calculations employing in particular the CC3 and EOM-IP-CCSD approaches and complete basis set extrapolation technique for evaluation of the vertical ionization energies of imidazole indicate that the A 2A' and B 2A" states are very close in energy and subject to non-adiabatic effects. Our modeling confirms the existence of pronounced vibronic coupling of the A 2A' and B 2A" states. Moreover, despite the large energy gap of nearly 1.3 eV the ground state X 2A" is efficiently coupled to the A 2A' state. The modeling was performed within the framework of the three-state linear vibronic coupling problem employing Hamiltonians expressed in a basis of diabatic electronic states and parameters derived from ab initio calculations. The ionization spectrum was computed using the multi-configuration time-dependent Hartree (MCTDH) method. The calculated spectrum is in good agreement with the experimental data allowing some interpretation of the observed features to be proposed.