Magnetic anisotropy and electronic structure of aromatic molecules

Abstract In a recent paper Pauling (1936) calculates the diamagnetic anisotropy of benzene and other aromatic hydrocarbon molecules on “the assumption that the 2pz electrons are free to move under the influence of the impressed fields from carbon atom to adjacent carbon atom”. He points out that this is a quantitative extension of previous and generally accepted ex­planations of magnetic anisotropy given by Ehrenfest (1925, 1929) for Bi and Sb, and by Raman and Krishnan (1927) and Raman (1929a, b) for benzene and its derivatives. The idea that certain electrons can move in this way is implicit in the theory of molecular orbitals, as applied by Hückel (1931a, b, 1932, 1933) to aromatic and unsaturated molecules. Hückel (1934), in fact, pointed out that the application of the theory to such problems as magnetism and light absorption would be a task for further research. With the object of applying the theory of molecular orbitals to the quantitative explanation of diamagnetic anisotropy, I have for some time been engaged on a determination of the diamagnetic constants of substances which can be used for this purpose, including some which had been pre­viously examined and which then gave the results used by Pauling (Krishnan, Guha and Banerjee 1933; Krishnan and Banerjee 1935). It was clear, almost from the first, that the original measurements on naphthalene and anthracene were out of harmony with the rest; these measurements were repeated independently by Krishnan and myself, and corrected results were published in a joint paper (Lonsdale and Krishnan 1936). These new results remove a difficulty commented upon by Pauling. I have also determined the dia­magnetic constants of certain new compounds, especially cyanuric triazide, cyanuric trichloride and metal-free phthalocyanine. This work was begun and completed and the following paper written, in ignorance that Pauling was making the calculations described in his recent paper, and it therefore adds independent evidence in favour of the essential principle.