ABSTRACT
This chapter concentrates on O nuclear magnetic resonance studies of rigid, planar systems, work that was ongoing in our laboratories concurrent with our torsion angle studies. A theory was developed that suggested, in part, that the net deshielding effect was the result of a contraction of the orbitals on the resonant nucleus, due to repulsive van der Waals interactions, leading to a corresponding increase in the r–3 term of the paramagnetic expression of the Karplus-Pople equation. The analysis of the effects of structural variation on single bonded oxygen atoms is not well-documented. The effects appear to be larger than earlier thought. It also seems probable that the oxygen of the donor for intramolecular hydrogen-bonding experiences deshielding. The effect of substituents on the O chemical shifts for the 3-substituted phthalic anhydrides and 7-substituted phthalides were similar, both experiencing a comparable magnitude of deshielding in relationship to the respective parent molecules.
