NUCLEAR-MAGNETIC-RESONANCE OF HYDROGEN-BONDED CLUSTERS BETWEEN F- AND(HF)(N) - EXPERIMENT AND THEORY

Liquid state H-1 and F-19 NMR experiments in the temperature range bet ween 110 and 150 K have been performed on mixtures of tetrabutylammoni um fluoride with HF dissolved in a 1:2 mixture of CDF3 and CDF2Cl. Und er 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 mult iple hydrogen bonds to HE With increasing number of HF units the hydro gen bond protons shift towards the terminal fluorine's. The optimized gas-phase geometries of [F(HF)(n)](-), n=1 to 4, calculated using ab i nitio methods confirm the D-infinity h, C-2v, D-3h and T-d symmetries of these ions. For the first time, both one-bond couplings between a h ydrogen bond proton and the two heavy atoms of a hydrogen bridge, here (1)J(HF) and (1)J(HF'), where \(1)J(HF)\greater than or equal to\(1)J (HF')\, as well as a a two-bond coupling between the heavy atoms, here (2)J(FF), have been observed. The analysis of the differential width of various multiplet components gives evidence for the signs of these constants, i.e. (1)J(HF) and (2)J(SF)>0, and (1)J(HF')<0. Ab initio ca lculations of NMR chemical shifts and the scalar coupling constants us ing the Density Functional formalism and the Multiconfiguration Comple te Active Space method show a reasonable agreement with the experiment al parameters and confirm the covalent character of the hydrogen bonds studied.