Heavy atom tunnelling on XeF6 pseudorotation

Abstract

The uncertainty on XeF₆ geometry persisted for over five decades, with elusive multiple C_3v equivalent minima as an outcome of the Jahn–Teller effect. Herein we theoretically prove that XeF₆ shows a genuine fluorine quantum mechanical tunnelling rearrangement, rapidly “jumping” between isomers even close to 0 K. The isoelectronic anions IF₆⁻ and TeF₆²⁻ behave in the same way as XeF₆, but with a lower and shorter energetical barrier they have a faster tunnelling rate. A complete isotopic analysis revealed a large k(¹⁸F)/k(¹⁹F) kinetic isotope effect. We speculate that it would be possible to experimentally observe the tunnel effect in XeF₆ by means of cryogenic NMR or IR.