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

Abstract

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 A_Z 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.