Heavy atom tunnelling on XeF6 pseudorotation.

The uncertainty on XeF6 geometry persisted for over five decades, with elusive multiple C3v 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.