Anisotropies diamagnétiques moléculaires expérimentales des composés aromatiques condensés

For explaining diamagnetic effects observed in condensed aromatic compounds, their molecular diamagnetic anisotropies must be known. A method is proposed which, on the basis of relatively accurate structural data and of two experimental crystal anisotropies, allows the determination of molecular anisotropies in the plane and perpendicular to it. For monoclinic compounds whose magnetic tensor has no revolution symmetry, it is necessary to correct to first order the orientation of the principal magnetic axes of the crystal with respect to revolution symmetry. The proposed method is applied and discussed in the case of ten condensed aromatic compounds which crystallize in the orthorhombic and monoclinic systems. It yields, in some cases, molecular anisotropies very different from those suggested by different authors. One very simplified model produces results which fit in with the molecular anisotropies obtained as long as the arrangement of the rings is not too complex.