Molecular Structure of XeF6. I. Analysis of Electron-Diffraction Intensities

Abstract

A gas‐phase electron‐diffraction investigation of xenon hexafluoride has been carried out in an effort to obtain structural information which might shed light on the curious properties of the compound. Elaborate precautions were taken to prevent decomposition or contamination of the sample (99.8 mole % pure). Several innovations were introduced into the structure analysis to minimize difficulties encountered in conventional analyses of molecules containing both heavy and light atoms. Two different sets of analyses (I and II) employing two different levels of approximation in electron scattering theory were conducted to test the adequacy of the expressions usually adopted. Analysis I was based on Hartree–Fock x‐ray elastic scattering factors, Heisenberg–Bewilogua inelastic scattering factors, and (modified) Born phase‐shift corrections for Thomas–Fermi atomic fields. Improvements in Analysis II included the new electron elastic scattering factors and Born phase‐shift corrections calculated by the partial wave method by Cox and Bonham, and Hartree–Fock inelastic scattering factors. The Hartree–Fock phase‐shift correction was not in complete agreement with experiment but was markedly better than the Thomas–Fermi correction. The effect of ionic character on phase shift was investigated theoretically and shown to be significant. A mean Xe–F bond length of 1.890 ± 0.005 Å was found, but the radial distribution function for Xe–F bonds corresponded to that of a composite for nonequivalent bonds. Amplitudes of bending oscillations are notably large. The diffraction data are not compatible with a regular octahedral XeF₆ molecule vibrating in independent normal modes. A more detailed exposition of alternative structures and internal motion is presented in Paper II.