Ring currents and proton magnetic resonance in aromatic molecules

The molecular orbital theory is adapted to the calculation of magnetic shielding constants in aromatic molecules. The usual lcao perturbation theory is generalized to take account of the imaginary perturbation due to an external magnetic field. The induced field at a point is then calculated by inserting a test dipole, adopting approximations due to London, and using the perturbation theory to evaluate a coupling energy. The results differ somewhat from those obtained in a rather different manner by Pople.