Gas-phase spectroscopic characterization of neutral and ionic polycyclic aromatic phosphorus heterocycles (PAPHs)

Polycyclic aromatic hydrocarbons (PAHs) constitute an essential family of compounds in interstellar (ISM) and circumstellar (CSM) media. Recently, formation routes for the corresponding polycyclic aromatic phosphorus heterocycles (PAPHs) in astrophysical environments have been proposed. In order to contribute to a better understanding of the phosphorus chemistry in the ISM, infrared (IR) spectra and selected properties of PAPHs were computed at the density functional theory level for neutral, cationic, and anionic species. Our results reveal that several protonated PAPHs do not have planar backbones, and all species have permanent dipole moments between 2D and 4D. Closed-shell PAPHs have similar ionization potentials compared to the parent PAHs, below the Lyman threshold limit. In addition, all PAPHs show positive electron affinities higher than those of PAHs. Protonation preferably occurs on the heteroatom but with lower proton affinities than those of the corresponding nitrogen analogues (polycyclic aromatic nitrogen heterocycles). In general, neutral species have similar IR spectra profile with the most intense bands around 800 cm−1 (12.5 μm) related to C−H wagging. Charge and protonation affect the IR spectra mainly by decreasing the intensities of these modes and increasing the ones between 1000 (10.0 μm) and 1800 cm−1 (5.6 μm). The P−H stretching appears in a different spectral region, between 2300 (4.3 μm) and 2700 cm−1 (3.7 μm). Our results are discussed in the context of distinct sources where PAHs and phosphorus are detected. PAPHs, in particular the coronene derivatives, can contribute to the unidentified infrared emission band at 6.2 μm.