Temperature-induced displacement of the proton site in strong F–H–F hydrogen bond and mechanism of phase transition in 1,4 diazabicyclo[2.2.2]octane dihydrogen difluoride

The crystal of [C6H13N2]+[HF2]− is built of ionic pairs tied by strong N–H···F hydrogen bonds. A very strong F–H···F hydrogen bond linking the anion is symmetric at higher temperatures, while upon cooling the H-atom shifts continuously to a clearly non-centred site. This first observation of the thermally induced proton migration in the strongest known hydrogen bond is explained by the potential-energy fluctuations due to the large-amplitude motions of the fluorine atoms. The crystal undergoes a continuous phase transition at 150.6 K, accompanied by the symmetry change from Cmc21 to P1. The transition mechanism involves an order–disorder contribution related to the behaviour of the hydrogen difluoride anion.