The crystalline structure of anthracene. A quantitative X-ray investigation

Abstract

The aromatic hydrocarbons naphthalene and anthracene were among the first organic compounds to be investigated by the X-ray method. The early work on these crystals afforded evidence that the long axes of the molecules lay along the c direction in the crystal, but less definite indication was obtained regarding the lateral disposition of the molecules. An attempt was made to settle this point by a study of the intensities of the principal X-ray reflections by visual estimate, and this led to a distorted “tetrahedral” structure being advanced. Somewhat later it was shown by Sir William Bragg that this structure was untenable, the discrepancies between the calculated and the measured values of certain reflections being too great. In this work absolute measurements of intensity were obtained for the more important planes, and the evaluation of the atomic positions carried out by Fourier analysis. This led to a structure consisting of molecules with flat or slightly distorted carbon rings, whose planes made an angle of about 25° with the bc plane, and with the long axis of the molecule tilted about 6° away from the direction of the c axis towards a more upright position. Almost at the same time Banerjee independently arrived at a structure, based upon the measurement of some intensities, together with the results of optical and magnetic measurements, which agreed very closely with Bragg’s structure. At the commencement of the present work, then, it could be taken that the structure of anthracene was approximately known. It was very important, however, to confirm these results by more accurate and more extensive intensity measurements, and also to attempt to work out the finer details of the structure. For example, it was not known if the carbon rings were quite flat like the graphite structure, or slightly puckered, perhaps intermediate between the diamond and graphite type: the evidence seemed to point to the latter possibility. The carbon to carbon distance was not accurately known. The only estimate obtained was 1⋅48 A., which is again intermediate between the diamond (1⋅54) and the graphite (1⋅42) distance.