Carbon-13 splittings of proton and fluorine resonance spectra in substituted fluorobenzenes

Abstract

13CH and 13CF coupling constants have been determined in 18 different fluorobenzene compounds by observing the 13C satellites in proton and fluorine resonance spectra. The dependence of the coupling constants on the type of substition and its relative position with respect to the fluorine atom has been studied. For a particular position of the substituents, the 13CH coupling constants are found to increase linearly with the electron-withdrawing power of the substituents. On the other hand, for a particular substituent, the 13C-H coupling constants are found to be highest for thepara-isomer and lowest for the meta. The 13CF coupling constants are also found to depend on the substituent and its relative position, but the variations in the case are not as regular as in the case of 13CH coupling constants. The difference in the two cases has been understood in terms of Fermi contact interactions.