The proton magnetic resonance of heterocyclic diketones

A full analysis of the 60 MHz 1H NMR spectra of heterocyclic analogues of 1,3‐indandione [N‐(2‐bromoethyl)‐phthalimide, phthalic anhydride and thiophthalic anhydride] and of 1,2‐indandione (N‐methylisatin, coumarandione and 2,3‐dihydrothionapthene‐2,3‐dione) has been carried out. The protons of the symmetric compounds resonate at lower fields than those of the asymmetric ones and the proton ortho to the CO function is more deshielded than the proton ortho to the heteroatom. Coupled Hartree‐Fock perturbation theory has been applied to estimate the ring current contributions to the proton chemical shifts and to the magnetic susceptibilities. MO calculations based on the finite perturbation theory at the CNINDO levels provide information on the relative importance of σ and π pathways for the various coupling constants.