Knowledge Bank

The infrared absorption spectrum of the fundamental modes of $UF5$ is calculated using force constants transferred from $UF61$ to predict the wavenumbers and using the F atom polar $tensor2$ from $CH3F$ to predict the intensity. The predicted spectra are reported for several different assumed geometrical configurations of $UF5$. Comparison of these predicted spectra with the recently observed spectrum, in the UF-stretching region, of a photolysis product of $UF6$ isolated in an Ar $matrix3$ suggests strongly that the structure is square-pyramidal $(C4v)$ with the U atom above the F-atom equatorial plane. Although the model for the intensities is simple, the calculated spectrum is expected to predict the correct order of magnitude for the intensities of the fundamentals, first overtones, and binary combination bands of $UF5$, and the approximate wavenumbers for the expected absorption. Some comparisons are given between the calculations for the several structures of $UF5$ and observed spectra of other $XF5$ molecules, including $PF5$, $BrF5$, and $IF5$. We believe predictions of this kind will be of considerable help in the interpretation of spectra of transient molecules.