Infrared Signatures of Protonated Benzonitrile

Abstract

Abstract 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.