Shapes of molecules obtained in crystals are compared with those in the free state. Examples are shown for large, conformational changes, for small changes due to the hydrogen bond formation, and for subtle changes caused by intermolecular repulsion forces. Also discussed are the experimental and theoretical aspects to be considered in studying and interpreting the shapes of simple molecules in crystals.
The recent investigations on the structure of gaseous molecules are reviewed. The results covered by this review are limited especially for those obtained by means of gas electron diffraction. In many of them, however, spectroscopic information is cooperatively used in the structure determination. Special emphasis is placed upon the regularity of structure parameters observed in several series of molecules. The following topics are also briefly described; unstable molecules and radicals, high temperature diffraction studies, intramolecular multiple scattering, large amplitude motion and the ab-initio calculation of structure parameters.
The structure of liquids studied by diffraction experiments is described in this review. The methods of obtaining molecular correlation functions are discussed for both spherical and non-spherical molecules and straightforward analysis of the diffraction data in polyatomic liquids for obtaining the molecular correlation function is very difficult. The result of liquid water is described in detail, which is only one example studied by the computer simulation and dif f racf ion experiments in polyatomic liquids. Since the electron density distribution of water molecule is very nearly spherical, correlation functions for molecular scattering center is derived from X-ray diffraction data. It will be possible that diffraction intensities in polyatomic liquids are calculated by the use of the technique of molecular dynamics with the advent of large electronic computers and more powerful experimental data than usual results are obtained by advanced experimental techniques, for example, by using solid state detectors for X-ray.
A survey is made of recent developments in the study of bonding electron distribution in molecular crystals. X-ray diffraction is a unique tool to directly observe electron densities in molecules. The high accuracy in the observed density (±0.1 e Å-3 or better), now attainable by careful experiments and appropriate corrections for systematic errors, has made it possible to study bonding electrons. Various information of chemical interest has been extracted from difference electron density maps. The X-N map, which is obtained by a combined use of X-ray and neutron diffraction, is most powerful to reveal detailed bonding features around atoms such as lone pair deformation. Physically meaningful densities have been obtained even for d electrons. Methods of electron population analysis of accurate diffraction data are now being established. The interplay between diffraction experiment and molecular theory is increasing.
An important role which x-ray diffraction method plays in an investigation of chemical reaction mechanism is reviewed. When an intermediate is isolated as a crystal in the course of a given reaction, the geometrical structure, as well as the electronic structure, of the intermediate can be analyzed by x-ray method. The reactivity of a substrate, the activity of a catalyst, etc. can be elucidated based on the obtained structure. A remarkable example on the dioxygen adducts of Vaska complex, IrCI (CO) (O2) (PPh3) 2 and InI (CO) (O2) (PPh3) 2, is described. In this case the difference in the O-O distances rationalizes the difference in the catalytic activities of the parent complexes. Other examples are also described.
To date extensive studies on solid-state polymerization of many monomers have been reported mainly covering kinetics and reaction mechanism. However, the lack of crystal data for monomers is a hindrance for clarifying the role of structures of original monomer crystals on their solid-state polymerization. In this review a few examples on structural investigation of solid-state polymerization are given. The examples include photopolymerization, y-irradiated addition polymerization, ring opening polymerization and polymerization in urea- and thiourea-adducts, and illustrate the variety of solidstate polymerizations. The characteristics of resulting polymers are also mentioned briefly in terms of stereospecificity of molecular, chain, preferred orientation of polymer crystal, fine texture etc.
A survey is made for recent development in X-ray diffraction studies on the excitable tissues, with special emphasis upon biological and lipid membranes. The exposure or counting time needed to observe diffraction patterns from muscles and membranes has been very much shortened recently by the combined use of the total reflection mirror, the curved monochrometer and the position sensitive proportional counter. There are two approaches in determining ρ (z) of membranes from diffraction data, where z is the coordinate normal to the membrane and ρ (z) is the projection of the electron density distribution to the z axis. One of them is to use Bragg reflections from the lamellar phase of membrane-water system and the other is to use continuous diffraction patterns from membranes dispersed in water, where relative positions of membranes have no correlation. These two approaches are discussed referring to results on lecithin (dipalmitoyl phosphatidylcholine) membranes obtained recently by Y. Inoko and by T. Yamaguchi and K. Furuya. The former has investigated the lamellar phase of the lecithin-water system and the latter have obtained continuous diffraction patterns in good contrast from UO2++ -decorated lecithin double layers dispersed in water. Calculated ρ (z) in these two cases are presented.
Graphic display systems are steadily becoming a tool in the field of molecular structural (conformational) analysis. Comparison of the features of many working systems in U. S. A. and UK made by G. R. Marshall et al. of Washington University, U. S. A. (in Computer Representation and Manipulation of Chemical Information, p. 203 (1974) John Wiley & Sons) is shown to understand the present status of the method. From the viewoint of application to X-ray crystallography, presentation of electron density maps made by T. H. Gossling at the Medical Research Council Laboratory of Molecular Biology, UK, (Acta Cryst. 22, 465 (1967) ) is also very important and useful. Besides, as a subsidiary tool of packing analysis, the system at the Princeton University Computer Graphics Laboratory, U. S. A., has been used to solve the structure of Guanosine-3', 5'-cytidine monophosphate by R. Langridge and his coworkers (Biopolymers, 12, 2731 (1974) ) . Using the accumulated information of protein crystallography and chemical sequence of proteins, the system at the Electrotechnical Laboratory in Tokyo, Japan, has been used to challenge the prediction of the tertiary structure of a protein by the present author.
From the structural-chemical point of view, the ‘non-benzenoid aromatic compounds’ are reviewed. Hückel's (4n+2) rule has been proved by the X-ray analysis of several annulenes. It was pointed out, however, that resonance energies of non-alternant hydrocarbons are overestimated. Structural studies on the conjugated odd-membered ring systems have shown that these systems are very sensitive to various kinds of interactions, electronic, steric and intermolecular hydrogen bonding effects. The characteristic chemical and physical behaviours can be reasonably related to the structures.