Assigning the CH stretch overtone spectrum of benzene and naphthalene with extension to anthracene and tetracene using 2- and 3-quanta anharmonic quantum chemical computations.

Abstract

The CH stretch overtone region (5750-6300 cm-1) of benzene and naphthalene is assigned herein using anharmonic quantum chemical computations, and the trend of how this extends to larger polycyclic aromatic hydrocarbons (PAHs) is established. The assignment of all experimental bands to specific vibrational states is performed for the first time. Resonance polyads and the inclusion of 3-quanta vibrational states are both needed to compute accurate vibrational frequencies with the proper density-of-states to match the experimental band shape. Hundreds of 3-quanta states produce the observed band structure in naphthalene, anthracene, and tetracene, and this number is expected to increase drastically for larger PAHs. The width and shape of the main peak are consistent from naphthalene to anthracene, necessitating further exploration of this trend to confirm whether it is representative of all PAHs in the CH stretch overtone region. Understanding observations of PAH sources in the 1-3 μm region from the NIRSpec instrument aboard JWST requires new computational data, and this study provides a benchmark and foundation for their computation.