The magnetic and optical properties of the benzene ring in aromatic compounds

Abstract

The present paper describes the third of a series of investigations undertaken at the suggestion of Sir C. V. Raman of the remarkable magnetic and optical characters exhibited by organic crystals of the aromatic class. In the earlier papers the author had described the optical and the magnetic characters of naphthalene crystals and also of anthracene and other organic compounds and discussed the significance of the results. In all these cases, however, the fact that there were two or more molecules per unit cell in the crystal lattice, and that the X-ray results available did not fix with sufficient precision either the form of the molecules or their orientation relatively to each other and to the crystal lattice prevented a complete interpretation of the results. Mrs. Lonsdale’s recent admirable investigation of the structure of hexamethylbenzene has made further and very significant progress possible. She has shown that this substance crystallises in the triclinic system with one molecule per unit cell and that the carbon atoms in the molecule form flat hexagonal rings parallel to a cleavage plane. Hence this substance forms a very suitable material for the experimental study of the magnetic and optical characters of the benzene ring in its substitution products. It has accordingly been used as the starting point in the present investigation. For convenience of reference Mrs. Lonsdale's results will be briefly recalled here. The nomenclature adopted is the same as hers. The substance crystallises out of benzene mostly in parallel plates with the be plane prominently developed. Often there are two other pairs of faces, fairly well developed, which were chosen by her as the ab and ac planes. The unit cell thus defined is a triclinic cell which was found to have the following axes and axial angles:— A. U. ° ˊ 9.010 . . . . . . . . . . . . . . . . . 44 27 8.926 . . . . . . . . . . . . . . . . . 116 43 5.344 . . . . . . . . . . . . . . . . . 119 34 The crystal is centrosymmentrical with one molecule per cell.