Low-Temperature NMR Studies of the Structure and Dynamics of a Novel Series of Acid−Base Complexes of HF with Collidine Exhibiting Scalar Couplings Across Hydrogen Bonds^†

Abstract

The low-temperature 1H, 19F, and 15N NMR spectra of mixtures of collidine-15N (2,4,6-trimethylpyridine-15N, Col) with HF have been measured using CDF3/CDF2Cl as a solvent in the temperature range 94−170 K. Below 140 K, the slow proton and hydrogen bond exchange regime is reached where four hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3 could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants across the 19F1H19F and 19F1H15N hydrogen bridges have been measured which allowed us to determine the chemical composition of the complexes. The simplest complex, collidine hydrofluoride ColHF, is characterized at low temperatures by a structure intermediate between a molecular and a zwitterionic complex. Its NMR parameters depend strongly on temperature and the polarity of the solvent. The 2:3 complex [ColHFHCol]+[FHF]- is a contact ion pair. Collidinium hydrogen difluoride [ColH]+[FHF]- is an ionic salt exhibiting a strong hydrogen bond between collidinium and the [FHF]- anion. In this complex, the anion [FHF]- is subject to a fast reorientation rendering both fluorine atoms equivalent in the NMR time scale with an activation energy of about 5 kcal mol-1 for the reorientation. Finally, collidinium dihydrogen trifluoride [ColH]+[F(HF)2]- is an ionic pair exhibiting one FHN and two FHF hydrogen bonds. Together with the [F(HF)n]- clusters studied previously (Shenderovich et al., Phys. Chem. Chem. Phys. 2002, 4, 5488), the new complexes represent an interesting model system where the evolution of scalar couplings between the heavy atoms and between the proton and the heavy atoms of hydrogen bonds can be studied. As in the related FHF case, we observe also for the FHN case a sign change of the coupling constant 1JFH when the F···H distance is increased and the proton shifted to nitrogen. When the sign change occurs, that is, 1JFH = 0, the heavy atom coupling constant 2JFN remains very large, of the order of 95 Hz. Using the valence bond order model and hydrogen bond correlations, we describe the dependence of the hydrogen bond coupling constants, of hydrogen bond chemical shifts, and of some H/D isotope effects on the latter as a function of the hydrogen bond geometries