High-resolution Fluorine Magnetic Resonance Spectra of Some Perfluoroalkyl Derivatives of Sulfur Hexafluoride

Abstract

The high-resolution fluorine magnetic resonance spectra of a group of perfluoroalkyl derivatives of sulfur hexafluoride were analyzed and the chemical shifts and spin-spin coupling constants determined. The fluorine chemical shifts are found to vary with the electronegativity of the substituents. Fluorine- fluorine coupling constants are observed between nuclei separated by three, four, and five bonds, but the coupling constants do not vary in a coherent manner with a change in the number of bonds separating interacting nuclei. The results are in agreement with the idea that coupling between fluorine nuclei separated by three or four single bonds is due to both through-space and through-bond interactions but that the importance of the through-bond interaction falls off rapidly with the number of intervening bonds and the coupling is largely a throughspace interaction when the nuclei are separated by five single bonds. Finally, the very small coupling constants observed for interaction between nuclei separated by three bonds in the CF3--CF2 and -CF2--CF2- fragments would still appear to be anomalies possibly to be explained as resulting from rotational averaging of trans and gauche coupling constants of opposite signs.