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. 155, 054304 (2021)]. The present high-level electronic structure calculations employing, in particular, the single, double, and triple excitations and equation-of-motion coupled-cluster method accounting for single and double excitation approaches and complete basis set extrapolation technique for the 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 method. The calculated spectrum is in good agreement with the experimental data, allowing for some interpretation of the observed features to be proposed.