Coupled and uncoupled Hartree-Fock calculations of ring currents and proton chemical shifts: the aromatic character of five-membered heterocycles

Coupled and uncoupled Hartree-Fock perturbation theories are used within the framework of the ring current model to discuss the proton chemical shift of heterocyclic molecules. An appropriate partition of shielding constants is suggested and the calculated values support the hypothesis of a limited degree of aromaticity of this class of compounds. In addition, the results indicate that the commonly accepted scale of aromatic character based on chemical criteria is less consistent than that obtained by spectroscopical methods.