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

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• 作者：Ilja G. Shenderovich ; Peter M. Tolstoy ; Nikolai S. Golubev ; Sergei N. Smirnov ; Gleb S. Denisov ; and Hans-Heinrich Limbach
• 刊名：Journal of the American Chemical Society
• 出版年：2003
• 出版时间：September 24, 2003
• 年：2003
• 卷：125
• 期：38
• 页码：11710 - 11720
• 全文大小：220K
• 年卷期：v.125,no.38(September 24, 2003)
• ISSN：1520-5126

文摘

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 temperaturerange 94-170 K. Below 140 K, the slow proton and hydrogen bond exchange regime is reached wherefour hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants acrossthe ¹⁹F¹H¹⁹F and ¹⁹F¹H¹⁵N hydrogen bridges have been measured which allowed us to determine thechemical composition of the complexes. The simplest complex, collidine hydrofluoride ColHF, is characterizedat low temperatures by a structure intermediate between a molecular and a zwitterionic complex. Its NMRparameters 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 saltexhibiting 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 scalewith 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), thenew complexes represent an interesting model system where the evolution of scalar couplings betweenthe heavy atoms and between the proton and the heavy atoms of hydrogen bonds can be studied. As inthe related FHF case, we observe also for the FHN case a sign change of the coupling constant ¹J_FH whenthe F···H distance is increased and the proton shifted to nitrogen. When the sign change occurs, that is,¹J_FH = 0, the heavy atom coupling constant ²J_FN remains very large, of the order of 95 Hz. Using thevalence bond order model and hydrogen bond correlations, we describe the dependence of the hydrogenbond coupling constants, of hydrogen bond chemical shifts, and of some H/D isotope effects on the latteras a function of the hydrogen bond geometries.