Proton chemical shifts in NMR .8. 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 A(Z) of 3.67 x 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. The combined scheme predicts the proton chemical shifts of a variety of fluoroalkanes over 60 data points spanning ca. 6 ppm with an rms error of 0.11 ppm, The compounds include fluoroalkanes, cyclohexanes, bornanes, norboranes and steroids, Thus fluorine SCS can be quantitatively explained on the basis of a linear electric field model without recourse to either C-F bond anisotropy or van der Waals (i.e. steric) effects.