Knowledge Bank

Infrared spectra are reported for several $(C6H6)m-(HCN)n$ and $(C6D6)m-(HCN)n$ clusters in liquid helium nanodroplets. A large electric field can be used to collapse the rotational structure in these spectra, which aids in the initial searching. In the 1:1 clusters the $HCN\nu CH$ stretching vibration at $3253.2cm-1(C6H6-HCN)$ and $3252.9cm-1(C6D6-HCN)$ are red-shifted by approximately $60cm-1$ compared to the monomer, indicative of strong intermolecular hydrogen bonds between the CH-group and the aromatic $\pi$-clouds of the benzenes. Zero field spectra show considerable rotational structure consistent with a symmetric rotor structure as determined in the gas-phase by microwave $spectroscopy.a$ Moreover several trimers and tetramers of HCN and Benzene have been observed, all of which are further red-shifted than the 1:1 complexes. Pressure-dependency studies allow the size assignments of these clusters. The frequency shifts and rotational contours of the bands allows one to infer the structures of these clusters in liquid helium.