The forbidden far-infrared ν6 band of SF6

The far infrared spectra of SF₆ in the 33-μm region in the gas phase at different pressures and in the liquid phase have been studied. A small band situated at 351 cm^?1 on the high-frequency side of two difference bands situated at 304.5 cm^?1 has been observed in the gas phase. Since the integrated intensity of the 351-cm^?1 band varies linearly with the density, it cannot be collision induced. It appears that it is the forbidden ν₆ band that becomes active by Coriolis interaction. This band is seen in the liquid at about the same frequency when it is deconvoluted from the neighboring broadened and split difference bands.