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Influence of the temperature‐dependent dielectric constant on the H/D isotope effects on the NMR chemical shifts and the hydrogen bond geometry of the collidine–HF complex in CDF 3 /CDClF 2 solution
Author(s) -
Shenderovich Ilja G.,
Burtsev Andrej P.,
Denisov Gleb S.,
Golubev Nikolai S.,
Limbach HansHeinrich
Publication year - 2001
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.938
Subject(s) - chemistry , kinetic isotope effect , chemical shift , hydrogen bond , dielectric , solvent , hydrogen , nuclear magnetic resonance spectroscopy , analytical chemistry (journal) , solvent effects , deuterium , computational chemistry , molecule , stereochemistry , organic chemistry , atomic physics , physics , optoelectronics
The influence of solvent polarity on the properties of hydrogen‐bonded 1 : 1 complexes of 2,4,6‐trimethylpyridine‐ 15 N with HF and DF, labeled below as FHN and FDN, has been studied by multinuclear magnetic resonance spectroscopy in the slow hydrogen bond exchange regime reached below 190 K. Mixtures of CDF 3 /CDClF 2 were employed as solvent, which is liquid down to 90 K. In order to evaluate their polarity, the static dielectric constants ε o of the CHF 3 , CHClF 2 and of the binary 1 : 1 mixture were measured from 160 K down to 90 K. A strong increase of ε o from 14 at 190 K to 38 at 103 K is observed for the mixtures used in the NMR measurements. Upon cooling, i.e. increase of the dielectric constant, the NMR spectra indicate a gradual transformation of an asymmetric molecular complex FH···N to a quasi‐symmetric complex F δ− ···H···N δ+ and eventually to a more or less zwitterionic species F − ···H N + . These changes are not only manifested in the scalar couplings J ( 1 H, 19 F) and J ( 1 H, 15 N) but also lead to characteristic primary and secondary H/D isotope effects on the chemical shifts of the hydrogen bonded nuclei. Whereas the primary isotope chemical shift effect p Δ( D / H ) ≡ δ(F 2 HN) − δ(F 1 HN) = −0.2 ppm is negative at 190 K and in agreement with an asymmetric hydrogen bond in the molecular complex, it changes its sign when the temperature is lowered, goes through a maximum of +0.27 ppm at ε o ≈ 22 and finally decreases again. The positive value of p Δ(D/H) is in agreement with D more confined to the hydrogen bond center compared with H, which constitutes a fingerprint of a quasi‐symmetric hydrogen bond involving a single well potential for the proton motion. The quasi‐symmetric complex is further characterized by the following NMR parameters, J ( 1 H, 19 F) = 30 Hz, J ( 1 H, 15 N) = −50 Hz, J ( 19 F, 15 N) = −96 Hz, δ(F 1 HN) = 20.0 ppm, δ( 19 FHN) = −114.2 ppm, δ(FH 15 N) = −63.5 ppm, and the one‐bond H/D‐isotope effects δ(F 2 HN) − δ(F 1 HN) = +0.27 ppm, δ( 19 FDN) − δ( 19 FHN) = 1.4 ppm and δ(FD 15 N) − δ(FH 15 N) = −3.4 ppm. Copyright © 2001 John Wiley & Sons, Ltd.