Molecular Structure of XeF6. II. Internal Motion and Mean Geometry Deduced by Electron Diffraction

Abstract

The distribution of internuclear distances in gaseous XeF6 exhibits unusually diffuse XeF6 bonded and F–F geminal nonbonded peaks, the latter of which is severely skewed. The distribution proves the molecule cannot be a regular octahedron vibrating in independent normal modes. The instantaneous molecular configurations encountered by the incident electrons are predominantly in the broad vicinity of C3υ structures conveniently described as distorted octahedra in which the xenon lone pair avoids the bonding pairs. In these distorted structures the XeF bond lengths are distributed over a range of approximately 0.08 Å with the longer bonds tending to be those adjacent to the avoided region of the coordination sphere. Fluorines suffer angular displacements from octahedral sites which range up to 5° or 10° in the vicinity of the avoided region. Alternative interpretations of the diffraction data are developed in detail, ranging from models of statically deformed molecules to those of dynamically inverting molecules. In all cases it is necessary to assume that t1u bending amplitudes are enormous and correlated in a certain way with substantial t2g deformations. Notwith-standing the small fraction of time that XeF. spends near Oh symmetry, it is possible to construct a molecular potential-energy function more or less compatiable with the diffraction data in which the minimum energy occurs at Oh symmerty. The most notable feature of this model is the almost vanishing restoring force for small t1u bending distortions. Indeed, the mean curvature of the potential surface for this model corresponds to a υ4 force constant F44 of 10−2 mdyn/Å or less. Various rapidly inverting non-Oh structures embodying particular combinations of t2g and t1u deformations from Oh symmetry give slightly better radial distribution functions, however. In the region of molecular configuration where the gas molecules spend most of their time, the form of the potential-energy function required to represent the data does not distinguish between a Jahn–Teller first-order term or a cubic V445 term as the agent responsible for introducing the t2g deformation. The Jahn–Teller term is consistent with Goodman's interpretation of the molecule. On the other hand, the cubic term is found to be exactly analogous to that for other molecules with stereochemically active lone pairs (e.g., SF4, ClF3). Therefore, the question as to why the XeF6 molecule is distorted remains open. The reported absence of any observable gas-phase paramagnetism weighs against the Jahn–Teller interpretation. The qualitative success but quantitative failure of the valence-shell–electron-pair-repulsion theory is discussed and the relevance of the “pseudo-Jahn–Teller” formalism of Longuet-Higgins et al. is pointed out. Brief comparisons are made with isoelectronic ions.