Mid‐infrared Spectroscopy of Protonated Benzonitrile, 2‐Cyanonaphthalene, and 9‐Cyanoanthracene for Astrochemical Consideration

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The mid‐infrared (700–2300 cm −1 ) absorption spectra of three protonated cyanosubstituted polycyclic aromatic hydrocarbons (CN‐PAHs), benzonitrile, 2‐cyanonaphthalene, and 9‐cyanoanthracene, measured by infrared multiple photon dissociation using the free‐electron laser Free‐Electron Laser for Infrared eXperiments are reported. The frequency of the CN stretch is found to be 2191 ± 10 cm −1 for protonated benzonitrile and 2175 ± 10, and 2140 ± 10 cm −1 for 2‐cyanonaphthalene, and 9‐cyanoanthracene respectively, showing a clear redshifting of the CN stretch frequency as the size of the aromatic system increases, contrary to neutral cyano‐PAHs that show nearly no shift as function of size. Quantum chemical calculations are performed to complement the experimental results at B3LYP/aug‐cc‐pVTZ level of theory. Density functional theory calculations reproduce the fingerprint region for all three CN‐PAHs, although they overestimate the CN stretching vibration frequency. These results likely rule out small protonated cyano‐PAHs as major contributors to the unidentified infrared bands observed in space. However, the largest species investigated in this study shows a promising match with the 4.75 μm band.