Vibrational modes and electronic structural changes generating infrared intensities in charged conjugated π-electron systems: a case study on the chrysene radical cation

Abstract

The vibrational modes responsible for infrared (IR) intensities (called intensity-carrying modes) and the electronic structural changes that are important for generating the IR intensities are analyzed theoretically for the chrysene radical cation. The intensity-carrying modes are derived with a recently developed theoretical method. It is shown that the strongest intensity-carrying mode consists of a linear combination of many CC stretches, and induces a dipole derivative approximately along the longest molecular axis. The direction of the dipole derivative and the vibrational pattern of this intensity-carrying mode are closely related to the phase relationship in the singly occupied molecular orbital (SOMO), as in the cases of the radical cations of other aromatic molecules, and are rationalized by considering that charge fluxes are generated by this vibrational mode. The changes in the electronic structure that are important for generating the IR intensities are examined by using a model Hamiltonian developed recently. It is shown that the dipole derivative of the strongest intensity-carrying mode is mainly generated by the mixing of one particular one-electron excited electronic configuration into the wave function of the ground electronic state.