Diamagnetic Anisotropy and Electronic Structure of Aromatic Molecules

Abstract

Previous theories of the diamagnetic anisotropy of aromatic molecules, based on the method of molecular orbitals, neglect the correlation effects between electrons. In the present theory the anisotropy is treated from the standpoint of the Heitler-London-Pauling-Slater approximation by the inclusion of ionic terms in the secular equation. By means of an approximate treatment it is shown that correlation effects tend to reduce the diamagnetic susceptibility, the effect being relatively greater in complex molecules than in benzene. The contribution of higher order permutation integrals is also discussed and found to be negligible. The correct approximation lies intermediate between the pure H-L-S-P and pure molecular orbital approximations, but in order to explain the high value of the anisotropy, one is forced to assume that the true situation lies much closer to the molecular orbital viewpoint. The variation of the anisotropy of benzene through all intermediate stages of approximation is discussed by means of a more exact model. All calculations were carried out by treating the permutational degeneracy by means of the Dirac vector model.