2018ApJ...865..114C

Aromatic hydrocarbons and their protonated ions are important constituents of the interstellar medium (ISM). The recent discovery of benzonitrile (BN; cyanobenzene, C₆H₅CN) in the ISM suggests that its protonated ion (H⁺BN) is also present. Herein, we present vibrational signatures of H⁺BN obtained via infrared photodissociation (IRPD) spectra of its clusters with up to four nonpolar ligands (L = Ar/N₂) recorded in the NH (ν_NH) and CH (ν_CH) stretch range. Protonation of BN occurs at the N atom of the nitrile group. Systematic complexation shifts (Δν_NH) observed in the IRPD spectra of H⁺BN-L_n are assigned to cluster structures by comparison to quantum chemical calculations. In the most stable H⁺BN-L_n structures, the first ligand (n = 1) forms a NH⁺... L ionic hydrogen bond (H-bond), while additional ligands (n = 2-4) are attached to the aromatic ring via π stacking. For L = Ar, a less stable π-bonded H⁺BN-Ar isomer is also detected, and its IR spectrum provides an accurate experimental estimate of ν_NH = 3555 ± 3 cm^–1 for bare H⁺BN, an intense characteristic fingerprint of this ion in the 3 µm range. Comparison of C₆H₅CNH⁺ with HCNH⁺ and CH₃CNH⁺ reveals that the acidity of the NH proton in RCNH⁺ ions increases in the order R = C₆H₅ < CH₃ < H.