Electron diffraction methods

The diffraction pattern observed in LEED is one of the most connnonly used fingerprints of a surface structure. Witii XRD or other non-electron diffraction methods, there is no convenient detector tliat images in real time the corresponding diffraction pattern. Point-source methods, like PD, do not produce a convenient spot pattern, but a diffrise diffraction pattern that does not simply reflect the long-range ordermg. 

As in the case of ions we can assign values to covalent bond lengths and covalent bond radii. Interatomic distances can be measured by, for example. X-ray and electron diffraction methods. By halving the interatomic distances obtained for diatomic elements, covalent bond radii can be obtained. Other covalent bond radii can be determined by measurements of bond lengths in other covalently bonded compounds. By this method, tables of multiple as well as single covalent bond radii can be determined. A number of single covalent bond radii in nm are at the top of the next page. 

Car-C.i = 1.52 A. and C -C = 1.37 A., these distances being 0.02 A. smaller than those given by our photographs. The rings given by mesitylene and hexamethylbenzene are unusually sharp and well defined, and we estimate, by comparison with other substances for which the electron-diffraction method has been tested,81 that the error in our values of sa is not greater than about 0.5%. 

Yamaha, M. Molecular structure investigations by electron diffraction method. 

The molecular structure of the closo 12-vertex 1-SBnHn cluster (17) has been studied by electron diffraction methods augmented by ab initio calculations. Substantial distortions away from the regular icosahedron occurred by expansion of the pentagonal belt adjacent to sulfur.73 The UV-PES spectra of 17 has also been reported,71 and its microwave spectrum has been investigated and demonstrated that the molecule had C5v symmetry.74... 

The synthesis of the trismethylenemethane iron tricarbonyl complex [(CH2)3C]-Fe(CO)3 was reported by Emerson et al. in 1966 (27). The geometry of this compound in the gas phase was investigated by Almenningen et al. (28) using electron diffraction methods. These authors pointed out some structural peculiarities which were not amenable to a simple explanation, in particular, why the hypothetical planar (CH2)3C radical is distorted when bound to the Fe(CO)3 conical fragment in such a way that the carbon atoms of the CH2 groups are displaced toward — the iron atom (Fig. 9). 

Dickinson in 1923 had found that the unit of structure of tin tetraiodide is a cube containing eight molecules, with the atomic positions determined by five parameters, which he succeeded in evaluating. But the Snl molecule is tetrahedral, with its structure determined by a single parameter, so that one could predict with confidence that the investigation of the vapor by the electron diffraction method would surely permit the verification of the tetrahedral structure and the... 

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

Various electron diffraction methods are available and the choice of a pertinent method depends on the type of information which is required and also on the type of specimen. 

Many compounds, including clay minerals, form needle- or plateshaped crystals. With finely dispersed minerals, the electron diffraction method can give a special kind of diffraction pattern, the texture pattern, which contains a two dimensional distribution of a regularly arranged set of 3D reflections [2], Specimens of fine-grained lamellar or fiber minerals, prepared by sedimentation from suspensions onto supporting surfaces or films, form textures in which the component microcrystals have a preferred orientation. Texture patterns of lamellar crystals tilted with respect to the electron beam are called oblique texture electron diffraction patterns [1]. 

Mpst of the structural information about complex gas molecules has been obtained by the electron-diffraction method. Values of interatomic distances and bond angles determined by this method before 1950 are summarized in a review article by P. W. Allen and L. E. Sutton, Acta Cryst. 3, 46 (1950). ValueB of interatomic distances and bond angles for organic molecules determined by both x-ray diffraction of crystals and electron diffraction of gas molecules are summarized in a 90-page table in G. W. Wheland s book Resonance in Organic Chemistry, John Wiley and Sons, New York, 1955, and by L. E. Sutton in Tables of Interatomic Distances and Configurations in Molecules and Ions, Chemical Society, London, 1958. (Later references to the latter book will be to Sutton Interatomic Distances.)... 

As a result of the development of the x-ray method of studying the structure of crystals and the band-spectroscopic method and especially the electron-diffraction method of studying gas molecules, a large amount of information about interatomic distances in molecules and crystals has been collected. It has been found that the values of interatomic distances corresponding to covalent bonds can be correlated in a simple way in terms of a set of values of covalent bond radii of atoms, as described below.1... 

Pauling and Brockway [2] established the structure of methyl nitrate and the length and bond angles, using the electron diffraction method (p. 2). 

The Gas Electron Diffraction Method , P. Andersen and O. Hassell, in Physical Methods in Heterocyclic Chemistry , ed. A. R. Katritzky, Academic Press, New York, 1971, vol. 3, pp. 27-51. 

Many furan derivatives have been examined by X-ray diffraction methods, and natural products (especially the terpenoids) have provided numerous, interesting subjects for study. Some work has also been done by electron diffraction methods and, more recently, by microwave spectrometry. The bond lengths and angles determined for furan and assembled... 

The structure of 1,4-dioxin was first investigated by electron diffraction methods and the observed pattern was fitted to an assumed planar structure. This assumption was subsequently justified by the results of a combined IR and Raman study (Section 2.26.2.3.3) and further supported by theoretical treatments (Section 2.26.2.1). From the electron... 

Electron Diffraction and Electron Microscopy. A limited amount of information regarding graphite structure has been obtained by the use of electron beams. Grisdale (27) has measured the degree of orientation using electron diffraction methods on films of pyrolytic carbon deposited on a silica surface under a variety of conditions. Oxidation of the graphite causes an increase in the degree of orientation. 

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