RECENT WORK ON THE STRUCTURE OF MOLECULES

By Linus Pauling

Southern California Section, American Chemical Society.

Athenaeum, Friday, February 7, 1936.

I. Introduction.

It is hard for me to believe that nearly eight years have gone by since I spoke before this section on "The Nature of the Chemical Bond." At that time I was enthusiastic about the newly-developed quantum mechanics, and I said that the whole of chemistry depends essentially on two fundamental phenomena: the Pauli Exclusion Principle and the Heisenberg-Dirac Resonance Phenomenon. Now I have become somewhat more cautious; I am not sure that I would make exactly this statement this evening. It is true that these phenomena underlie chemistry, in that they are responsible for the variation of chemical properties from element to element, for the formation of chemical bonds in general between atoms, and for many general chemical phenomena. But there is more to chemistry than an understanding of general principles. The chemist is also, perhaps even more, interested in the characteristics of individual substances - that is, of individual molecules. During the last ten years great progress has been made in the development and application of methods of studying the structure of molecules. We now have detailed information regarding the arrangement of atoms and the distribution of valence bonds in many molecules, and every year our information increases a great deal. With this knowledge of the structure of molecules as a basis, we can begin to understand the chemical properties of substances - we can see why an atom of chlorine attached to carbon in one molecule reacts in a different way from an atom of chlorine in another molecule.

But perhaps I am getting too enthusiastic about a new development in chemistry. If so, it seems to be a natural thing for a chemist to do - to attempt to make every new discovery into a general chemical theory. Thus no sooner had Volta discovered the electric cell than Berzelius began electrolyzing solutions, and soon he had developed his electrochemical theory. This is a case where the new theory had a great deal of good in it; as a qualitative guide, Berzelius' electrochemical series is perfectly satisfactory. His extension of the theory to organic chemistry was not so satisfactory; but nevertheless it was valuable in leading to the formulation of the idea of radicals. The pessimist, however, may prefer to compare the situation now with that in 1874, in which year, shortly after the development of Darwin's theory, there appeared in the Journal für praktriche Chemie a paper by Pflaundler on "The Struggle for Existence and Natural Selection among Molecules." This attempt at a chemical theory has not been of much value.

Ten years ago there were available several powerful physical methods for investigating the structure of molecules, such as the interpretation of band spectra, the study of the structure of crystals with x-rays, the determination of the electric dipole moments of molecules, and so on. Now these methods are still available, and in general have become much more powerful. For example, new methods of interpretation of x-ray data have been discovered, leading more directly than was possible before to the structure of the crystal. The band spectroscopist is now able to attack more complicated molecules than before. By studying corresponding molecules containing light and heavy hydrogen he is able to learn additional facts about interatomic distances and valence bond force constants; indeed, I feel that we are justified in considering heavy hydrogen itself an important tool for studying the structure of molecules, in view of the large amount of information that can be obtained with its use.

In addition to these older methods, there have been recently discovered several new methods of attack on the problems of structural chemistry. Electron waves as well as x-rays are now used in studying the structure of crystals; electron waves are especially valuable for the study of surfaces, absorbed gas layers, etc. In addition both electron waves and x-rays are used for the study of the structure of liquids and of gas molecules. Magnetic methods of investigation have been found to be very useful. It has even been shown that the fine structure of x-ray absorption edges can be used to determine accurately the distances between atoms. Of these methods, I shall restrict myself to the discussion of one, the determination of the structure of gas molecules by the diffraction of electron waves, feeling that it is more satisfactory to give a thorough treatment of this one than a superficial treatment of many methods.