X-ray analysis of the dibenzyl series IV—detailed structure of stilbene

Abstract

A preliminary account of the crystal structure of stilbene in relation to other compounds in the dibenzyl series has already been given in Part III of this work (Robertson, Prasad and Woodward 1936). The general arrangement of the four molecules in the unit cell was deduced from certain absolute intensity measurements, the physical properties of the crystal, and a comparison with the accurately known structure of dibenzyl (Robertson 1934, 1935 a ). Although the approximate structure was established beyond any doubt, the precise determination of the atomic positions has proved to be a matter of very great difficulty. This is due to the presence of four molecules in the unit cell and to the fact that it is impossible to formulate a definite model of the molecule from the chemical structure owing to the possibility of free rotation about the single bands. For the accurate analysis it was necessary to determine the phase constants of all the weaker reflexions, and in the course of this work over twenty trial structures were calculated and compared, In this connexion the mechanical sorting method of structure factor calculation previously described (Robertson 1936) proved of great assistance. The repeated application of a double Fourier synthesis to the ( h 0 l ) set of structure factors then gradually refined the values for the atomic co-ordinates, and we believe that the final results given below are now comparable in accuracy to those obtained for the other structures in this series. It is unnecessary to describe in detail the various stages by which this structure has been reaches, and the results of the final Fourier synthesis only are given in the following pages. The relative orientations of the molecules in the crystal now recorded should various physical properties of the crystal to be expressed as functions of the individual molecules. In particular, the results now obtained will justify more accurate measurements being made on the magnetic susceptibilities and the refractive indices of the crystal than are available at present.