SE838: Reexamining the aromatic infrared bands: PAH anharmonicity matters

The aromatic infrared bands (AIBs) observed in astronomical IR emission spectra are primarily attributed to specific vibrational transitions of polycyclic aromatic hydrocarbons (PAHs), but these attributions need to be carefully reexamined. Most of these attributions are based on previous harmonic frequency quantum chemical calculations of potential PAH AIB carriers. Due to their pronounced anharmonicity, however, PAHs can have vibrational transitions involving significant resonances between various vibrational modes (e.g., Fermi and Darling-Dennison resonances), complicating the precise attribution of the AIBs. Thus, quantum chemical approaches to AIB attribution should account for PAH anharmonicity. To tackle this challenge, we introduce two complementary methods for interpreting the emission spectra of potential PAH AIB carriers based on anharmonic frequency quantum chemical calculations: vibrational carbon character analysis (VCCA) and local vibrational mode character analysis (LVMCA). The application of VCCA and LVMCA to various potential PAH AIB carriers underscores the importance of accounting for the anharmonicity that greatly influences PAH vibrational transitions. Equipped with these two methods, the astrochemical community can reexamine the AIBs through the lens of PAH anharmonicity, enabling more precise attributions of these spectral features for studying astronomical PAHs.