Proton chemical shifts in NMR. Part 8.1 Electric field effects and fluorine substituent chemical shifts (SCS)

A calculation of the effect of a linear electric field of a polar substituent on proton chemical shifts based on partial atomic charges is shown to give a complete account of fluorine SCS in rigid molecules for all long range protons (> three bonds). A value of the linear electric field coefficient AZ of 3.67 × 10-12 esu (63 ppm au) is obtained. For vicinal protons (H·C·C·F) the electric field calculation is accurate for monofluorine substitution but considerably overestimates the effects for difluoro (CF2) and trifluoro (CF3) substituents. A model based on fluorine polarisability and correcting for di- and tri-fluoro substituents gives good agreement with the observed SCS.