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

Using low-temperature NMR (H-1, F-19) technique in the slow exchange regime, the solutions containing tetrabutylammonium (TBA) acetate and HF have been studied in an aprotic freon mixture, CDF3/CDF2CI, exhibiting a dielectric permittivity, which increases strongly by lowering the temperature. Two different hydrogen bonded anionic clusters, a 1:1 cluster of the type (AcOdelta-...H...) F-1 (+ delta+) ([AcOHF](-)) and a 2:1 cluster of the type (AcOHF-...HOAc)-F-... ([(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 F-19 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)2F](-) 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. (C) 2004 Elsevier B.V. All rights reserved.