An experimental value for the B_1u C-H stretch mode in benzene.

We here present experimental infrared spectra on two (C₆H₆)(C₆D₆) benzene dimer isomers in the gas phase. The spectra show that the two benzene molecules in the dimer are symmetrically inequivalent and have distinct IR signatures. One of the two molecules is in a site of low symmetry, which leads to the IR activation of fundamental modes that are IR forbidden by symmetry in the monomer. In the spectra, all four fundamental C–H stretch modes of benzene are observed. Modes in the dimer are shifted up to 3 cm^–1 to the red, compared to the modes that are known for the monomer. For the ν₁₃ B_1u C–H stretch fundamental mode of benzene, a first experimental value of 3015 ⁺²_–5 cm^–1 is determined, in excellent agreement with anharmonic frequency calculations presented here