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

Abstract

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.