PAHsing to Compute Vibrational Spectra

The spectral assignment of several CN-functionalized polycyclic aromatic hydrocarbons (CN-PAHs) is provided from quantum chemical computations in direct support of James Webb Space Telescope observations. As much as 20% of the carbon in the universe is believed to be tied up in polycyclic aromatic hydrocarbons (PAHs). While hints to their presence in space have been known for more than half a century, only recent detections of strongly dipolar, CN-functionalized PAHs have clearly shown that PAHs and PAH-like molecules are present in astronomical regions like the Taurus Molecular Cloud. Part of this lack of clear attribution has been due to a lack reference, experimental spectral data as well as the difficulty in characterizing the spectra of PAHs even when they're produced due to the largely blended spectral features that characterize PAHs as they aggregate even in the gas phase. As such, computation can treat one PAH or CN-PAH at a time and blend the spectra together from the bottom up. This work showcases that the region around the CN stretch in known CN-PAHs is relatively narrow and that other IR regions become much more complicated upon CN functionalization than in standard PAHs.