Infrared absorption intensities of hexafluorobenzene. Assignment of the 317 cm−1 band

The infrared intensifies of the two low-frequency fundamentals of hexafluorobenzene have been measured in the vapor phase, and the value of the intensity of the Fermi doublet at 1000 cm⁻¹ has been checked. The intensity of the 317 cm⁻¹ band is very low, contrary to the estimate made by STEELE and WHIFFEN. Although both the low-frequency bands show the PQR structure of a parallel vibration, estimation of the PQR spacing expected for this molecule confirms the assignment of the 317 cm⁻¹ band to the e_1u fundamental, ν₂₀, made by STEELE and WHIFFEN. The CF bond moments are presented, together with a discussion of the nature of the bond moment hypothesis and an estimate of the vibronic effects to be expected in this molecule.