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

Using low-temperature NMR (1H, 19F) technique in the slow exchange regime, the solutions containing tetrabutylammonium (TBA) acetate and HF have been studied in an aprotic freon mixture, CDF3/CDF2Cl, exhibiting a dielectric permittivity, which increases strongly by lowering the temperature. Two different hydrogen bonded anionic clusters, a 1:1 cluster of the type AcOd2· · ·H· · ·F21 þdþ ([AcOHF]2) and a 2:1 cluster of the type AcOH· · ·F2· · ·HOAc ([(AcOH)2F]2) have been detected in equilibrium with each other, both forming ion pairs with the TBA countercation. [AcOHF]2 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, 1JðFHÞ; 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 19F NMR chemical shielding have been measured for both clusters. In contrast to the related complexes [(FH)nF]2 ðn ¼ 1–4Þ studied previously, the NMR parameters of [AcOHF]2 and of [(AcOH)2F]2 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. q 2004 Elsevier B.V. All rights reserved.