Vibrational Spectra of Anionic Polycyclic Aromatic Hydrocarbons in the CH Stretching Region: Anthracene, Pyrene, Perylene, and Fluoranthene

The CH stretching region of the infrared spectra of polycyclic aromatic hydrocarbon radical anions (2700-3100 cm-1) is characterized by complicated band patterns that defy straightforward assignments from harmonic calculations. We compare experimental infrared photodissociation spectra of argon-tagged anthracene, pyrene, perylene, and fluoranthene anions with spectra computed using different methods in the framework of vibrational perturbation theory, starting from electronic structure calculations based on density functional theory. We trace the complex behavior in the CH stretching region to Fermi interactions between levels with one vibrational quantum of CH stretching excitation and vibrational states with two quanta in lower frequency modes characterized by CH wagging, CC stretching, and CCC bending motions. The resulting vibrational states are strongly mixed in the basis of the harmonic normal modes. Taking all 2:1 Fermi resonances in the relevant frequency range into account typically results in the best agreement between theory and experiment.