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

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.