Vibrational Spectroscopy of Benzonitrile–(Water)1–2 Clusters in Helium Droplets

Polycyclic aromatic hydrocarbons are considered as primary carriers of the unidentified interstellar bands. The recent discovery of the first interstellar aromatic molecule, benzonitrile (C6H5CN), suggests a repository of aromatic hydrocarbons in the outer earth environment. Herein, we report an infrared (IR) study of benzonitrile–(D2O) n clusters using mass-selective detection in helium nanodroplets. In this work, we use isotopically substituted water, D2O, instead of H2O because of our restricted IR frequency range (2565–3100 cm–1). A comparison of the experimental and predicted spectra computed at the MP2/6-311++G­(d,p) level of benzonitrile–(water)1–2 clusters reveals the formation of a unique local minimum structure, which was not detected in previous gas-phase molecular beam experiments. Here, the solvent water forms a nearly linear hydrogen bond (H-bond) with the nitrile nitrogen of benzonitrile, while the previously reported most stable cyclic H-bonded isomer is not observed. This can be rationalized by the stepwise aggregation process of precooled monomers. The addition of a second water molecule results in the formation of two different isomers. In one of the observed isomers, a H-bonded water chain binds linearly to the nitrile nitrogen similar to the monohydrated benzonitrile–water complex. In the other observed isomer, the water dimer forms a ring-type structure, where a H-bonded water dimer simultaneously interacts with the nitrile nitrogen and the adjacent ortho CH group. Finally, we compare the water-binding motif in the neutral benzonitrile–water complex with the corresponding positively and negatively charged benzonitrile–water monohydrates to comprehend the charge-induced alteration of the solvent binding motif.