Nuclear magnetic resonance of hydrogen bonded clusters between F− and (HF)n: Experiment and theory

Abstract Liquid state 1 H and 19 F NMR experiments in the temperature range between 110 and 150 K have been performed on mixtures of tetrabutylammonium fluoride with HF dissolved in a 1:2 mixture of CDF 3 and CDF 2 Cl. Under these conditions hydrogen bonded complexes between F − and a varying number of HF molecules were observed in the slow proton and hydrogen bond exchange regime. At low HF concentrations the well known hydrogen bifluoride ion FHF − is observed, exhibiting a strong symmetric H‐bond. At higher HF concentrations the species F(HF) 2 − , F(HF) 3 − are formed and a species to which we assign the structure F(HF) 4 − . The spectra indicate a central fluoride anion which forms multiple hydrogen bonds to HF. With increasing number of HF units the hydrogen bond protons shift towards the terminal fluorine's. The optimized gas‐phase geometries of F(HF) n − , n = 1 to 4, calculated using ab initio methods confirm the D ∞h , C 2v , D 3h and T d symmetries of these ions. For the first time, both one‐bond couplings between a hydrogen bond proton and the two heavy atoms of a hydrogen bridge, here 1 J HF and 1 J HF where | 1 J HF |≥| 1 J HF '|, as well as a two‐bond coupling between the heavy atoms, here 2 J FF , have been observed. The analysis of the differential width of various multiplet components gives evidence for the signs of these constants, i.e. 1 J HF and 2 J SF >0, and 1 J HF |. <0. Ab initio calculations of NMR chemical shifts and the scalar coupling constants using the Density Functional formalism and the Multi‐configuration Complete Active Space method show a reasonable agreement with the experimental parameters and confirm the covalent character of the hydrogen bonds studied.