Fluorine deshielding in the proximity of a methyl group. An experimental and theoretical study

Abstract 19F chemical shifts are reported for a series of alkyl‐substituted fluoronaphthalenes and related compounds that exhibit steric crowding around the fluorine atom due to the proximity of methyl groups. Comparisons of these chemical shifts with others measured in similar compounds but without the corresponding methyl groups show that this crowding effect produces a large low‐field shift of the 19F signal. Similar effects, but with an even larger sensitivity to the steric crowding, were recently reported for 17O chemical shifts. The electronic origin for this proximity effect on 19F chemical shifts is analyzed within the molecular orbital theory, at the INDO level, using the inner projections of the polarization propagator technique. It is found that the presence of the methyl group produces an increase in the absolute value of the paramagnetic term of the magnetic shielding constant.