Part I

BY L. E. SUTTON

In this Volume there are only two Chapters about electron diffraction studies of molecular structure, but the second of them is unusually long.

The first is the customary, comprehensive survey of recent work: the period covered is August 1975 to August 1976. Our thanks are due to Dr D. W. H. Rankin for undertaking this task in three successive years. In introducing his detailed report he has made some interesting and pertinent general remarks. He draws attention to the increasing complexity of the molecules now being studied and, what in part arises from this, to the increasing tendency of investigators to use a variety of ancillary information in the analyses. He stresses the need for very clear statements of what such information is introduced and how it is used. This point has already been made by the Commission on Electron Diffraction of the International Union of Crystallography (see Acta Crystallogruphica, 1976, A32, 1013) but it is of increasing importance and can properly be emphasized more strongly. The analyses reported include some very high temperature work, a fair amount of looking for unusual bond lengths or bond angles, and a large number of conformational studies.

Because of the current interest in conformation it is appropriate that we have a Chapter, by Professor Robert K. Bohn, surveying the results obtained in recent years. It includes a series of massive tables of data which in themselves represent a very substantial work of scholarship and which should be of great value. The quantity of work came as a surprise to us all and, indeed, proved quite disconcerting. Professor Bohn is mainly concerned with systematizing the known facts and with the broad picture, so he does not give much space to detailed discussions of causes of conformation; but he draws attention to one important generalization which works for the great majority of cases. This is that a double bond, such as the carbonyl bond (C=O), can be regarded as two equal but opposite bent bonds. The description of the bond given by photo-electron spectroscopy is, however, that it is a σ-bond plus a π-bond, i.e. two different component elements. This further illustrates the dichotomy which is frequently found between the geometrical (or geographical) and the energetic descriptions of molecules. Neither one is universally right or wrong. Each is appropriate for one aspect of structure. Perhaps we need to understand better how to predict which is required for a particular purpose, and why.

My remaining task is the pleasant one of thanking the two contributors to this Part for their perseverence and cheerful co-operation which, as in previous years, have made the lot of the Senior Reporter a relatively happy one.

1

Electron Diffraction Determinations of Gas-phase Molecular Structures

1 Introduction

In this chapter the results are reported of nearly 100 structure determinations of molecules in the gas phase by electron diffraction, published in 81 papers, between August 1975 and August 1976. There are about 20 gas diffraction instruments active in the world, so the average output is only five published structures per unit. As there are ten or more structures reported for each of three instruments (EG100 in Moscow, KD.G2 in Oslo, and the 'Oslo apparatus') it is clear that there is considerable unused capacity. Why is this so? Is it that equipment is often out of commission for technical reasons? Is there a shortage of people interested in studying gas-phase structures? Or is there a lack of molecules suitable for study by this method? From my own experience as a preparative chemist with no diffraction apparatus of my own, it is not simply any one of these. The problem seems to be one of bringing together men, molecules, and machines.

There is evidence from papers reviewed in this chapter that it is becoming increasingly difficult to find simple molecules for study for electron diffraction. The Figure shows the distribution of compounds investigated in terms of number of atoms per molecule, compared with the corresponding data for two years previously. It is clear that the peak of the smoothed distribution has moved from about eight atoms per molecule to around 14 and that there has been a dramatic drop in the number of very small molecules studied.

This tendency to use more complex molecules may be demonstrated in two other ways. In the first place, larger and heavier species tend to have lower vapour pressures, and so to an increasing extent high nozzle temperatures are being used. This year, temperatures above room temperature were used for more than 65% of all the compounds investigated, with over 25% requiring more than 100 °C. Secondly, the diffraction experiments very often cannot give sufficient information to enable a full structure determination for a complex molecule to be carried out. Thus in many cases additional experimental data, such as rotational constants, are used, while in others heavy reliance is placed on vibrational amplitudes calculated...