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^†

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

¹³C Detected Scalar Nitrogen−Nitrogen Couplings Across the Intramolecular Symmetric NHN Hydrogen Bond of Proton Sponge

13C Detected Scalar Nitrogen−Nitrogen Couplings Across the Intramolecular Symmetric NHN Hydrogen Bond of Proton Spong

Geometries and Tautomerism of OHN Hydrogen Bonds in Aprotic Solution Probed by H/D Isotope Effects on ¹³C NMR Chemical Shifts

The 1H and 13C NMR spectra of 17 OHN hydrogen-bonded complexes formed by CH313COOH(D) with 14 substituted pyridines, 2 amines, and N-methylimidazole have been measured in the temperature region between 110 and 150 K using CDF3/CDF2Cl mixture as solvent. The slow proton and hydrogen bond exchange regime was reached, and the H/D isotope effects on the 13C chemical shifts of the carboxyl group were measured. In combination with the analysis of the corresponding 1H chemical shifts, it was possible to distinguish between OHN hydrogen bonds exhibiting a single proton position and those exhibiting a fast proton tautomerism between molecular and zwitterionic forms. Using H-bond correlations, we relate the H/D isotope effects on the 13C chemical shifts of the carboxyl group with the OHN hydrogen bond geometries

Nuclear Scalar Spin−Spin Couplings and Geometries of Hydrogen Bonds

Ab initio calculations of the scalar coupling constants 1J15N-1H ≡ JNH and 2J15N···15N ≡ JNN of the N−H···N hydrogen bonds in the anion [C⋮15N···L···15N⋮C]- (1), L = H, D, and of the cyclic hydrogen-bonded formamidine dimer (HCNHNH2)2 (2) have been performed using the density functional formalism as a function of the hydrogen bond and molecular geometries. The coupling constants are discussed in comparison with the experimental and calculated constants 1J19F-1H ≡ JFH and 2J19F-19F ≡ JFF reported previously as first set of examples of scalar couplings across hydrogen bonds for the hydrogen-bonded clusters of [F(HF)n]-, n = 1−4 by Shenderovich, I. G.; Smirnov, S. N.; Denisov, G. S.; Gindin, V. A.; Golubev, N. S.; Dunger, A.; Reibke, R.; Kirpekar, S.; Malkina, O. L.; Limbach, H. H. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 422. Using the valence bond order model, which has been successfully applied previously to explain hydrogen bond correlations in crystallography and solid-state NMR of hydrogen-bonded systems, the coupling constants are related to the hydrogen bond geometries and NMR chemical shifts. In terms of this model, there is no principal difference between FHF- and NHN hydrogen-bonded systems. Whereas the coupling constant values calculated using the DFT method for the fluorine case only reproduce the experimental trends, the agreement between theory and experiment is much better in the nitrogen cases, which allows one to determine the hydrogen bond geometries including the hydrogen bond angle from a full set of experimental coupling constants. It is found that the coupling constants JAB in A−H···B are proportional to the product of valence bond orders (pAHpHB)m, where m is an empirical parameter equal to 2 in the case of fluorine bridge atoms and close to 1 in the case of nitrogen bridge atoms. The coupling constants JAH depend on two terms, a positive term proportional to pAH and a negative term proportional to pAH(pHB)2 leading to vanishing or even negative values of JAH at larger A···H distances; in this region the constants JAB are larger than the absolute values of JAH. As a consequence, vanishing couplings between a hydrogen-bonded proton to a heavy nucleus across the hydrogen bond cannot be taken as an indication for a noncovalent character of this hydrogen bond. The existence of JAB is taken as a strong evidence for the covalent character of the hydrogen bonds studied. This is corroborated by a analysis of the molecular orbitals of (1) and their individual contributions to the coupling constants