Our matrix-IR spectrum of 5 is in excellent agreement with reported data 2 (see Supporting Information, SI, for matrix IR spectra and signal assignments). Upon irradiation with 365 nm, matrix-isolated 5 and 6 undergo a series of reactions, resulting in the formation of 1 and 2, and leading to the concomitant disappearance of all IR absorptions assigned to 5 and 6. In parallel, new IR bands corresponding to 1 and 2 emerged (Fig. and). The experimentally observed IR spectra of 1 and 2 match well the computed IR spectra at B3LYP/6-311++G(3df,2pd). Due to the overlapping of most bands above 1500 cm -1 , the assignments are difficult. In the range of 1500-600 cm -1 , cage vibrations dominate, with very weak intensities as compared to the stretching vibrations.