The relationship between charge density and the chemical shifts of fluorine nuclei in aromatic compounds

Abstract

The magnitudes of the chemical shifts of fluorine nuclei para to a substituent in substituted fluorobenzenes and pentafluorobenzenes are compared with π-electron densities calculated from a semi-empirical SCF–LCAO–MO method. It is shown that the linear relationships between the fluorine chemical shift and the π-electron density on fluorine or the attached carbon atom, which are observed for π-electron densities calculated by Hückel-type methods, are also observed when the SCF theory is used. It is shown, however, that the shielding constants of fluorine nuclei calculated by the method of Karplus and Pople, with use of an average energy of 8·15 eV, have the correct order of magnitude, but the changes produced by substituents are too small. The possible reasons for this discrepancy are discussed.