Relationship between Pi-Electronic Distributions and F19 NMR Shielding in Para-Substituted Fluorobenzenes

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Abstract
Experimental results on F19 shielding in p‐substituted fluorobenzenes are combined with calculations of pi‐electronic charge distribution from MO theory and are used in several procedures devised to test shielding theories. It is concluded that both —R and +R p‐substituted fluorobenzenes represent a special case for which there is a reasonable degree of theoretical justification for relating the F19 shielding with the pi charge density of the fluorine atom. The tests devised for p‐substituted fluorobenzenes do not clearly distinguish between the shielding equations of Prosser and Goodman and those of Karplus and Das. Also, the tests do not critically distinguish which of the following may be taken as the better estimator of the pi charge density of the fluorine atom: the shielding parameter based upon unsubstituted fluorobenzene as the standard of comparison or the shielding parameter based upon the meta isomer of the p‐substituted fluorobenzene. Data are presented on F19 shielding of the acid fluoride atom of p‐substituted benzoyl fluorides and benzenesulfonyl fluorides. These results show that the special shielding relationship derived for p‐substituted fluorobenzenes does not apply.