Influence of Substituents on the Through-Space Shielding of Aromatic Rings

A series of naphthalene derivatives, bearing a methyl
group and
a substituted phenyl ring in a 1,8-relationship, have been synthesized.
The chemical shifts of the protons of the methyl group, which are
pointed toward the shielding zone of the phenyl ring, were monitored
as the phenyl substituents were varied. This work indicates that the
shielding effect of the phenyl ring is not so severely altered by
the substituents as to significantly influence the chemical shift
of the methyl group. Nonetheless, within the small changes observed
experimentally, there appears to be a tendency for electron-withdrawing
X to shift the methyl signal downfield, whereas electron-donating
X-groups cause a more upfield shift. Polarization and field effects
are discussed as possible causes for this phenomenon. Chemical shifts
computed for selected members of the series, using the recently published
procedures of Rablen and Bally, are in agreement with the experimentally
observed trends.