The measurement of bond lengths in conjugated molecules of Carbon Centres

I believe there are at least two good reasons for choosing this subject for my contribution. In the now fairly long history of the science of X -ray crystal analysis the aromatic hydrocarbon structures play quite an interesting part. Naphthalene and anthracene, for example, were among the very first organic crystals to be examined by the X -ray method, although they were certainly not the first organic structures to be fully determined. The first reasonably complete organic structure determination was probably that of hexamethylenetetramine, carried out by Dickinson & Raymond (1923). However, the measurement and comparison of the unit cells of naphthalene and anthracene by Bragg (1921) opened the chapter on the study of molecular crystals and foreshadowed much of what was to follow. Later, the full analysis of another aromatic structure, hexamethylbenzene, by Lonsdale (1929), revealed the beautiful simplicity of these structures, and established the planar arrangement of the atoms in the aromatic ring. More recent X -ray work has taken advantage of this simple two-dimensional arrangement of the atoms in most aromatic molecules, and because of this has been able to achieve an accuracy in bond length measurement which is probably much higher and more reliable than that attained for other complex molecules. My second reason for choosing this particular group of structures is that, again on account of the simple atomic arrangements, theoretical study of these molecules has reached a considerable degree of refinement. The peculiar and characteristic properties of aromatic compounds have long been know n to chemists. In a remarkably short time after the discovery of the wave equation by Schrödinger these aromatic properties received a theoretical explanation. This was first accomplished by the work of Slater, Hückel and Pauling, in the development of the conception of resonance of molecules among several valence bond structures. Besides other properties, it is now possible to calculate bond orders and bond lengths, either by the Slater-Pauling valence-bond or pairing method, or by the later molecular orbital treatments of Hund, Mulliken, Hückel and Lennard-Jones.