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

Liquid state ¹H and ¹⁹F 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 CDF₃ and CDF₂Cl. 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 ¹J_HF and ¹J_HF where |¹J_HF|≥|¹J_HF'|, as well as a two-bond coupling between the heavy atoms, here ²J_FF, have been observed. The analysis of the differential width of various multiplet components gives evidence for the signs of these constants, i.e. ¹J_HF and ²J_SF>0, and ¹J_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.