Theoretical Rotational and Vibrational Investigation of Oxygen-Functionalized Interstellar PAHs

Abstract

Oxygen-functionalized polycyclic aromatic hydrocarbons (OPAHs) with OH, CHO, and CO side groups can form in the interstellar medium (ISM) mainly through the UV irradiation of PAHs in water ice. Inspired by the detection of nitrogen-containing PAHs in the ISM, this study uses highly accurate computational techniques to investigate the rotational and vibrational spectra of oxygen-functionalized pyrene derivatives (Py-OH, Py-CHO, Py-HO, and Py-O2) for comparison with experiments that could aid in their future detection. All four OPAHs exhibit strong dipole moments and rotational lines, observable in denser ISM regions near 12 GHz, 10.2 GHz, 12.6 GHz, and 9.6 GHz, respectively. The strongest IR absorption features are identified at 1179.9 cm−1 (8.47 μm) and 1385.0 cm−1 (7.22 μm) for Py-OH, 1733.4 cm−1 (5.77 μm) for Py-CHO, 1747.7 cm−1 (5.72 μm) for Py-HO, and 1613.0 cm−1 (6.20 μm) for Py-O2. The IR features of Py-OH and Py-O2 exhibit peaks in the CO stretching region around 6.0 μm, while those of Py-CHO and Py-HO are blue-shifted due to anharmonicity from the additional CH bond. This suggests PAHs with CO group and no additional peripheral CH bond better explain the observed 6.0 μm PAH emission. However, to explain the observed PAH emission bands with the OPAHs, a fully emission-cascade treatment is required in the anharmonic IR spectra. The accurate spectral data presented here are crucial for experimental classification and potential interstellar observations.