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 CDF3/CDClF2 solution

The influence of solvent polarity on the properties of hydrogen-bonded 1:1 complexes of 2,4,6-trimethylpyridine-N-15 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 CDF3/CDClF2 were employed as solvent, which is liquid down to 90 K. In order to evaluate their polarity, the static dielectric constants so of the CHF3, CHClF2 and of the binary 1: 1 mixture were measured from 160 K down to 90 K. A strong increase of so 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 F-H . . .N to a quasi-symmetric complex Fdelta-. . .H . . .Ndelta+ and eventually to a more or less zwitterionic species F-. . .H-N+. These changes are not only manifested in the scalar couplings J(H-1, F-19) and J(H-1, N-15) 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)Delta (D/H) equivalent to delta ((FHN)-H-2) - delta ((FHN)-H-1) = -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 epsilon (o) approximate to 22 and finally decreases again. The positive value of PA(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(H-1, F-19) = 30 Hz, J(H-1,N-15) = -50 Hz, J(F-19, N-15) = -96 Hz, delta ((FHN)-H-1) = 20.0 ppm, delta((FHN)-F-19) = -114.2 ppm, delta ((FHN)-N-15) = -63.5 ppm, and the one-bond H/D-isotope effects delta ((FHN)-H-2) - delta ((FHN)-H-1) = +0.27 ppm, delta((FDN)-F-19) - delta((FHN)-F-19) = 1.4 ppm and delta ((FDN)-N-15) - delta ((FHN)-N-15) = -3.4 ppm. Copyright (C) 2001 John Wiley & Sons, Ltd.