We report the observation of the C-H stretch vibration of 1,2,4,5-tetracyanobenzene (TCNB) and (TCNB)(2) embedded in superfluid helium droplets. The asymmetric C-H stretch of TCNB was observed at 3053.488(3) cm(-1). The corresponding band of the (TCNB)(2) is slightly blueshifted and was observed at 3054.651(2) cm(-1). The intensity of the IR band is increased compared to the monomer absorption. Based on a comparison of our experimental results with predicted IR bands, we conclude that (TCNB)(2) is formed in a C(2h) structure in the droplet.

The IR spectrum of benzene and benzene dimer embedded in superfluid helium droplets in the region of the C–H stretch vibration (3000–3110 cm-1) was observed. By analyzing the pressure dependence of the spectra it was possible to assign these spectra to benzene monomer and dimer. Based on a comparison of the observed position of the dimer bands to experimental bands recorded in the gas phase and to theoretical frequency predictions the authors assign these to the tilted T-shaped benzene dimer structure. None of the other two possible structures (PD or PTS) were found. The above assignment is firm. For the monomer, we find in helium nanodroplets a splitting of the ν 8+ν 19 combination band observed in the gas phase. We assign this to an additional Fermi resonance which is caused by the matrix shift in helium nanodroplets.