Substituent effects aryl fluoride chemical

Adcock, W. and Kok, G.B., Polar substituent effects on 19F chemical shifts of aryl and vinyl fluorides a fluorine-19 nuclear magnetic resonance study of some l,l-difluoro-2-(4-substituted-bicyclo[2,2,2]oct-l-yl)ethenes, J. Org. Chem., 50, 1079, 1985. 

Substituent Effects of Aryl Fluorides versus Benzylic Fluorides. A remarkable difference is observed for the effect of para substituents on the chemical shifts of aryl fluorides versus benzyl fluorides. In the case of aryl fluorides, electron-donor substituents cause shielding of the fluorine nucleus, whereas such substituents give rise to deshielding of the fluorine nuclei of benzylic fluorides (Scheme 2.2).2... 

Scheme 2.2 Contrasting effects of substituents on fluorine chemicals shifts of aryl and benzyl fluorids...

---