Part I

BY L. E. SUTTON

The part which electron diffraction studies on vapours can play in the elucidation of molecular structure is commonly regarded as much more restricted than that of X-ray crystallographic studies. This view needs a great deal of qualification. It is true that much more complex structures can be tackled by the latter method because of the three-dimensional information which can be obtained from a crystal. Thermally unstable species must be examined at low temperatures and therefore often in crystalline form. Ionic species can usually be examined only in crystals. However, for compounds which are not too complex and which can be vapourized satisfactorily, electron diffraction can and, broadly speaking, usually does give more accurate determinations of interatomic distances than X-ray crystallography. Hydrogen atoms can be more readily placed by electron diffraction. For the examination of species which exist only in the vapour phase, for finding the shape and size of a free molecule not constrained by packing, or for the study of gas-phase equilibrium, e.g. of conformational isomers, and the evaluation of ΔH and ΔS values, it is necessarily the method to use.

I have been vividly reminded of the progress in techniques by sorting through some old electron diffraction photographs taken in the 1930s. The great potential increase in accuracy and reliability which stemmed from the development of rotating sector cameras just after the War has been brilliantly realized. This has come about by combining instrumental observations of scattering intensity, which the new cameras made possible, with more rigorous theoretical treatments and the resources of high-speed computing. Error analysis has become vastly more sophisticated. The accuracy in favourable cases is as high as that of microwave spectroscopy, indeed the limiting factors are now seen to be the same in both methods, viz. the correct treatment of the effects of vibration.

The further hopes which once may have been entertained, that electron diffraction could be used independently of spectroscopy to derive the complete vibrational characteristics of molecules, have not been realized. Instead, it has become apparent that the best way to determine a structure precisely is to combine electron diffraction and spectroscopy.

There has been no shortage of excellent reviews of methodology; and there have been extensive reviews of results, but these have usually been restricted to some particular fields or interests. In the present Report the aim has been to have a fresh look at the fundamentals of the scattering process and then a very critical look at accuracy. This has been done by Dr. H. M. Seip of the Oslo school. It has proved impossible this year to review the development of apparatus adequately so this is being left for a future volume. A search of the literature of the past six to seven years has shown that the amount of interesting structural information which has come from electron diffraction studies is far greater than had been realized; so a serious attempt has been made by Dr. B. Beagley to present a comprehensive survey of it all, emphasizing the general structural principles which emerge. This has been a major task, but it has brought together a wealth of chemically significant facts and ideas. Dr. A. G. Robiette has reviewed the present state of the interplay between electron diffraction and spectroscopy which has been one of the growing points in structural studies and which promises to be of even greater importance. Here also it has proved impossible to cover the whole field adequately this year; but a general survey has been given. Most of the major topics have thus been covered, but there is one task which ought to be attempted when the time is ripe. This is to make a systematic comparison of structures of molecules as free molecules and in the crystal so that more understanding of the effects of interaction, in the most general sense, of a molecule with its environment may be developed.

I should like to thank the contributors, Dr. Seip, Dr. Beagley and Dr. Robiette for their spirited and willing help. The number of people working in this field is still relatively limited and there is a good deal of personal co-operation which has, as always,...