TY - JOUR

T1 - Low-temperature NMR spectra of fluoride-acetic acid hydrogen-bonded complexes in aprotic polar environment

AU - Golubev, Nikolai S.

AU - Tolstoy, Peter M.

AU - Smirnov, Sergei N.

AU - Denisov, Gleb S.

AU - Limbach, Hans Heinrich

N1 - Funding Information: This work is financially supported by the Russian Foundation for Basic Research, Grants 03-03-32272 and 03-03-04009, the Deutsche Forschungsgemeinschaft, Bad Godesberg, and the Fonds der Chemischen Industrie, Frankfurt. P.M.T. thanks the Fonds for a Kékulé scholarship. Authors are grateful to the referee for valuable comments and suggestions.

PY - 2004/8/20

Y1 - 2004/8/20

N2 - 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/CDF 2Cl, exhibiting a dielectric permittivity, which increases strongly by lowering the temperature. Two different hydrogen bonded anionic clusters, a 1:1 cluster of the type AcOδ-⋯H⋯F -1+δ+ ([AcOHF]-) and a 2:1 cluster of the type AcOH⋯F-⋯HOAc ([(AcOH)2F]-) 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, 1J(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 19F NMR chemical shielding have been measured for both clusters. In contrast to the related complexes [(FH) nF]- (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.

AB - 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/CDF 2Cl, exhibiting a dielectric permittivity, which increases strongly by lowering the temperature. Two different hydrogen bonded anionic clusters, a 1:1 cluster of the type AcOδ-⋯H⋯F -1+δ+ ([AcOHF]-) and a 2:1 cluster of the type AcOH⋯F-⋯HOAc ([(AcOH)2F]-) 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, 1J(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 19F NMR chemical shielding have been measured for both clusters. In contrast to the related complexes [(FH) nF]- (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.

KW - Acetic acid

KW - Fluoride anion

KW - Hydrogen bond

KW - Isotope effect

KW - Low-temperature NMR

UR - http://www.scopus.com/inward/record.url?scp=3142657308&partnerID=8YFLogxK

U2 - 10.1016/j.molstruc.2004.01.024

DO - 10.1016/j.molstruc.2004.01.024

M3 - Article

AN - SCOPUS:3142657308

VL - 700

SP - 3

EP - 12

JO - Journal of Molecular Structure

JF - Journal of Molecular Structure

SN - 0022-2860

IS - 1-3

ER -