Vibrational Spectra of PF5 and AsF5: Height of the Barrier to Internal Exchange of Fluorine Nuclei

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Abstract
The infrared spectra of gaseous PF5 and AsF5 have been obtained over the spectral range 30–2000 cm−1, and Raman spectra have been observed for the liquid state. The observed frequencies for both molecules are satisfactorily assigned on the basis of a D3h structure. A normal‐coordinate calculation is made with the help of a valence‐force system and simplifying assumptions about off‐diagonal elements in the potential‐energy matrix. An approximate potential diagram for internal exchange of axial and equatorial fluorine nuclei is evaluated from the potential constants of species E′. From the calculated exchange rates it is concluded that it is unlikely that the axial—equatorial chemical shifts in the NMR spectra of PF5 and AsF5 will be resolved at low temperatures.