Experimental Molecular Structure

Abstract

The principal sources of information about the structure of molecules in the free state are electron diffraction of gases, spectroscopy in the infrared, Raman and microwave regions, and x-ray diffraction of crystals. The past year has seen a continuation of the trends noted in the previous review by Beach (1). The general structural principles for most small molecules are reasonably well understood, and interest now is directed toward the under­ standing of variations of bond distances and angles in different molecules and the study of more complicated systems, such as larger molecules and the nature of intramolecular motions. The advances in experimental technique in electron and x-ray diffraction and the striking interpretations of pure rotational spectra in the microwave region are beginning to provide a body of information relating to interatomic parameters where the distances are reliable to 0.01 'A or better and the angles reliable to 30'. Some success is obtained in accounting for the observed inter­ atomic distances by modifying the sum of the covalent radii (14) with cor­ rections for ionic or multiple bond character (18), but it is apparent that there is a need for precise data from a largernumberof molecules. Accordingly, much of the work at the present time is concerned with improving experi­ mental techniques or with redetermining the structures of many molecules by the newer methods. Electron diffraction.-The need for the determination of a larger number of parameters has led many workers in the field of electron diffraction to search for increased sensitivity and objectivity. The early electron diffraction investigations were accomplished by the visual correlation technique, where calculated intensity patterns based on assumed models were compared with the visually observed diffraction pattern. As the complexity of the molecules studied increased and it became necessary to determine a larger number of parameters, it became increasingly difficult to arrive at a unique model. At the present time, many molecules are being reinvestigated and extensive work is being done to increase the accuracy and sensitivity of the experimen­ tal techniques. Recent papers by .the Karles (2, 3, 4) and Hastings & Bauer (5, 8) con­ tain detailed discussions of the effects of various experimental factors on the diffraction patterns: the proper design of a gas nozzle for introducing the sample into the electron beam, the effect of finite sampJe size and divergence of the beam, mUltiple scattering, the use of beam stops and rotating sectors, and the response characteristics of photographic emulsions.