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

Abstract

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, C2v, D3h and Td 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 1JHF and 1JHF where |1JHF|≥|1JHF'|, as well as a two‐bond coupling between the heavy atoms, here 2JFF, have been observed. The analysis of the differential width of various multiplet components gives evidence for the signs of these constants, i.e. 1JHF and 2JSF>0, and 1JHF|. <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.

[FHF]−—The Strongest Hydrogen Bond under the Influence of External Interactions

“…. F distances calculated here for this complex are equal to 1.008 and 1.343 Å, respectively (Table 1) which is in agreement with the previous MP2/6-31+G** calculations where 1.012 and 1.349 Å distances were found [27].…”

“…This complex was observed in earlier studies [27][28][29] and is also analyzed here; it possesses C2v symmetry and may be considered as the F3H2 − anion. The (MP2/aug-cc-pVTZ level) F-H and H…F distances calculated here for this complex are equal to 1.008 and 1.343 Å, respectively (Table 1) which is in agreement with the previous MP2/6-31+G** calculations where 1.012 and 1.349 Å distances were found [27]. Table 2 presents the binding energies and interaction energies, defined earlier, for the analyzed complexes; these energies corrected for BSSE are also included as well as the deformation energies.…”

“…(HF) 2 cluster on the other hand. Such a system has been analyzed previously both experimentally and theoretically [27][28][29].…”

“…Figure 5 presents also the continuous line passing through the points corresponding to the hydrogen bifluoride anions possessing D 8h and C 2v symmetries; theoretical results are presented (free [FHF]´anion and its complexes with Na + , Li + cations) as well as species found in the crystal structures. [27][28][29] and is also analyzed here; it possesses C 2v symmetry and may be considered as the F 3 H 2´a nion. The (MP2/aug-cc-pVTZ level) F-H and H .…”

“…There have been earlier studies where the symmetric and asymmetric hydrogen bifluoride anions were considered [26][27][28][29][30][31][32][33][34][35][36][37]. However in this study the emphasis is put on electron charge redistribution being the result of complexation.…”