The magnetic anisotropy of naphthalene crystals

Abstract

In a recent paper Raman and Krishnan have shown how the phenomenon of birefringence exhibited by liquids in a magnetic field can be utilised to determine the degree of magnetic anisotropy of the molecules concerned. Since magnetic double refraction is an effect arising from the molecules being both magnetically and optically anisotropic, and the optical anisotropy is known from studies on light scattering, the magnetic anisotropy can be evaluated from the known value of the Cotton-Mouton constant. In this way, they showed that the benzene molecule has a high degree of magnetic anisotropy, the susceptibility in a direction perpendicular to the benzene ring being about twice as great as that in any direction parallel to the ring. In another papers they showed how this magnetic anisotropy may reasonably be explained in terms of the special structure of the benzene molecule. Considerations of the same kind suggest that the higher members of the aromatic series ( e. g. , naphthalene and anthracene) should exhibit an even more striking degree of anisotropy. Some calculations, as yet unpublished, by Krishnan indicate that the naphthalene molecule should have, in directions perpendicular to the ring, a susceptibility about four times as large as in directions parallel to the ring. It is known that diamagnetism of crystal is in general anisotropic. In a recent paper Raman and Krishnan have discussed the case of nitrates and carbonates and shown how the anisotropy of the crystals is in each case connected with the structure of the individual ions. If, as stated above, the naphthalene molecule possesses a large magnetic anisotropy in the liquid state, it is reasonable to expect that naphthalene crystals in the solid state should also exhibit a very pronounced magnetic anisotropy. In fact, Bragg and Oxley have made some qualitative observations on the magnetic behaviour of naphthalene crystals which support this inference, but no quantitative results are available. The present work was undertaken with a view to obtain some measurements of the susceptibility of the crystals in different directions and with the hope that such data would be of assistance in determining the orientation of the molecules in the crystal lattice.