Ground States of Conjugated Molecules. X. 19F NMR Chemical Shifts in Aryl Fluorides

Abstract

The procedures developed in previous papers of this series have been extended to aromatic fluorides, using ¹⁹F NMR chemical shifts as a guide to the selection of parameters for fluorine. Attempts to correlate chemical shifts with the local π‐electron density on fluorine proved unsatisfactory, the correlation depending on the number of atoms ortho to fluorine that carry $p$ or $\pi$ electrons. Similar results followed for charge densities calculated by a recently developed semiempirical SCF MO treatment in which all the valence electrons are included. Analysis of the Karplus–Das–Prosser–Goodman treatment of chemical shifts suggested that this “ortho effect” arose from neglected long‐range interactions. A treatment including these was developed and shown to account well for the ¹⁹F chemical shifts of nearly 100 aryl fluorides.