Vibrational spectrum and force field of uranium hexafluoride

Abstract

A total of 44 bands in the infrared and Raman spectra of 238UF6 vapor has been measured using long-path absorption cells and multiple-reflection Raman cells. The types of contours exhibited by these bands, and analogous contours in the spectrum of SF6, are discussed. Overtone and combination bands with resolved Q branches are used to estimate the anharmonicity corrections for the stretching fundamentals. The infrared contours of ν3 (at 228 K) and ν4 (at 256 K) have yielded estimates for the Coriolis constants ζ3 and ζ4 that satisfy the sum rule. The 235UF6–238UF6 frequency shifts in these fundamentals have also been measured. All of these data, plus vibrational amplitudes from the electron diffraction study of Seip, have been combined to determine the general quadratic harmonic force field of UF6. Only three valence force constants are appreciably different from zero: fr=3.85±0.05, fα≈fα−fa′a= 0.12±0.01, and frr (cis interaction)=0.30±0.02, all in mdyn/Å. The large differences between this force field and that previously determined for WF6 are attributed to the effects of f-orbital contributions in UF6.