The Diamagnetic Anisotropy of Large Aromatic Systems: III Structures with Hexagonal Symmetry

The molecular orbital method is employed in calculations of the diamagnetic anisotropies of a series of highly condensed aromatic systems having hexagonal symmetry and containing up to fifty-four carbon atoms. A new method of calculation is introduced in which the π-electron susceptibility of the whole molecule is determined by an integration, no knowledge of the unperturbed energy levels being necessary: the inclusion of overlap between adjacent atomic orbitals is shown generally to have little effect on the calculations. The results are compared with those calculated using the method of Pauling: this semi-classical method is found to be quite misleading when applied to large molecules or to molecules possessing stray C-C bonds. Qualitative valence bond considerations are advanced in support of these conclusions, and certain general results are tentatively suggested.