Low-temperature NMR spectra of fluoride-acetic acid hydrogen-bonded complexes in aprotic polar environment

Using low-temperature NMR ( 1 H, 19 F) technique in the slow exchange regime, the solutions containing tetrabutylammonium (TBA) acetate and HF have been studied in an aprotic freon mixture, CDF 3 /CDF 2 Cl, exhibiting a dielectric permittivity, which increases strongly by lowering the temperature. Two different hydrogen bonded anionic clusters, a 1:1 cluster of the type AcO δ − ⋯H⋯F −1+ δ + ([AcOHF] − ) and a 2:1 cluster of the type AcOH⋯F − ⋯HOAc ([(AcOH) 2 F] − ) have been detected in equilibrium with each other, both forming ion pairs with the TBA countercation. [AcOHF] − exhibits an extremely strong hydrogen bond, with a proton shared between partially negatively charged oxygen and fluorine atoms. The NMR chemical shifts and scalar spin–spin coupling constants, 1 J(FH), have been measured in the temperature range between 110 and 160 K, where separate NMR signals are observed for both species. In addition, H/D isotope effects on the 19 F NMR chemical shielding have been measured for both clusters. In contrast to the related complexes [(FH) n F] − ( n =1–4) studied previously, the NMR parameters of [AcOHF] − and of [(AcOH) 2 F] − depend strongly on temperature. This effect is associated with the increasing polarity of the solvent with decreasing temperatures, established earlier, which displaces the proton from fluorine to oxygen. As a motive power of this conversion, preferential solvation of the compact fluoride ion as compared to acetate is proposed.