Part I

ELECTRON DIFFRACTION

BY L. E. SUTTON

In this Volume there are three Chapters relating to electron diffraction studies of molecular structure.

The first is the usual comprehensive survey of recent work, again written by Dr D. W. H. Rankin, covering papers published up to the end of August 1975. Dr Rankin had considerable difficulty in obtaining copies of certain journals, because libraries are cutting down their subscription lists; so the need for such a review is evidently becoming even greater. He remarks that although the fall in output of papers continues, the quality has advanced markedly. There are numerous studies of structural detail in large and complex molecules, often by the now well-established techniques for combining diffraction and spectroscopic data. There are also measurements of notable accuracy on small molecules. Of unique interest are the investigations on substances that are very difficult to volatilize, which have been the special province of Russian workers.

The mass of data for bond lengths is now enormous: one can say, in current parlance, that there is a data mountain. This means that it is becoming something of an embarrassment. Our contributors have noted the relevance of many of the observations that they have reported to the concepts which have been used in rationalizing bond lengths and their variations, such as covalent radius, bond order, hybridization, bond polarity, co-ordination number, and van der Waals radii or potential functions between non-bonded atoms; but they have only been able to do this in passing. There is no lack of bases for rationalization; there may be a surfeit. A fresh, systematic, and critical review of their adequacy, usefulness, and reliability now seems due. This Senior Reporter will not be volunteering for the task,

Dr H. Oberhammer has contributed a second Chapter on Apparatus Developments, to complete the review begun in the previous Volume by R. L. Hilderbrandt. This one deals with advances made in Europe including the U.S.S.R. In view of the interest of the high-temperature work it is appropriate and timely that Dr Oberhammer should be able to include an authoritative description of the equipment used for it. He also describes the first commercially-built camera.

The third Chapter, by Dr A. G. Robiette, is on the study of large-amplitude vibrations in molecules. Investigators are increasingly concerned to elucidate the 'molecular mechanics'. This includes establishing, and if possible explaining, the potential energy functions that govern the relative motions of atoms in a molecule. These functions can become highly anharmonic and may have more than one minimum, It is with the subtleties of these difficult but very interesting problems that Dr Robiette is especially concerned.

The volume in the Landolt-Bornstein series of Tables, on interatomic distances, which was mentioned in the Introduction to Volume 1, is due to appear in September or October of this year (1976). It will list data for molecules, obtained from vapour-phase measurements by spectroscopy, electron diffraction, or the two combined; but it will not include data for diatomic molecules. The deadline for publications to be included was June 1974. Its appearance will be most welcome; and all concerned with its production are to be thanked and congratulated on completing their task.

It remains for me to thank the contributors to this Volume for their promptness and their general helpfulness, which I do with great pleasure.

1 Electron Diffraction Determination of Gas-phase Molecular Structures

BY D. W. H. RANKIN

1 Introduction

During the year from September 1974 to August 1975, about 85 papers were published reporting the results of structure determinations by electron diffraction. Thus the trend of a decreasing volume of output, noted a year ago, has continued. A crude extrapolation suggests that the last electron diffraction paper of all time will appear in 1984, and this series of chapters of results will end with Volume 13!

This decline in quantity has fortunately been accompanied by a marked advance in quality. To an ever-increasing extent, information derived from electron diffraction experiments has been combined with data from other sources, to provide solutions to problems that proved to be insoluble using one technique in isolation. At one extreme of scale, very small molecules are being studied with great care and precision, using microwave and diffraction data. By such means it has been possible to determine the effect of substituting 37Cl for 35Clo n the...