In electron diffraction studies

One way to 3D molecular descriptors is deriving them from molecular transforms, which are generalized scattering functions used to create theoretical scattering curves in electron diffraction studies. A molecular transform serves as the functional basis for deriving the relationship between a known molecular structure and x-ray and electron diffraction data. The general molecular transform is... 

Electron diffraction is the other of the two important sources of gas-phase structural data. As discussed by Hedberg (this volume), the intensity of electrons scattered by molecules is modulated by the interatomic distances, both bonded and nonbonded. Since interatomic distances enter explicitly into electron diffraction determinations, the method is in some ways more direct than spectroscopy. Moments of inertia are functions of Cartesian coordinates of individual atoms rather than distances between atoms. On the other hand, electron diffraction is much more susceptible to experimental error than spectroscopic techniques.15 Problems with structural determinations by spectroscopic methods often stem almost entirely from model error, whereas in electron diffraction both experimental and model error are important. Experimental and model error in electron diffraction are discussed elsewhere in this volume by Hedberg, and we shall confine ourselves here to definitions of the various structural parameters that arise in electron diffraction studies and the relationships among them and spectroscopic quantities. 

The rather limited crystallinity of PPP produced by the Kovacic route, which can be increased to 60% by annealing above 400°C, has been an obstacle to the determination of the unit cell. Well-oriented diffraction patterns have only been obtained in electron diffraction studies. Nonetheless, Kovacic et al. [169] have laid down the basic aspects of the structure and its problems in their very original work, which included an x-ray study. Their indexing scheme for the reflections has been verified in several later studies. The authors propose a pseudo-orthogonal cell with two chains passing through, and dimensions o = 7.81 A, h =... 

Several review papers (see ref. 9 in Section 1) also give discussions of error sources and accuracy in electron-diffraction studies. Some earlier works dealing with these topics are given in ref. 95. 

Results based on factor ratios must be used with caution in electron-diffraction studies, since the assumptions for the test are never strictly valid and sometimes are far from being so, for example because of systematic errors or errors in assumed parameters. Si ratio plots may, however, be quite useful if the significance levels are not used uncritically. One should, for example, always inspect the refined parameters in all the calculations to see if any unreasonable values have been obtained. 

These three major topics— mean amplitudes of vibration, the interrelation of molecular structures obtained from spectroscopic and electron diffraction data, and the shrinkage effect in electron diffraction studies— comprise the main areas of interplay between spectroscopy and electron diffraction. Cyvin s book treats the first and third topics at length. This chapter concentrates on the second topic, which had until very recently not been reviewed at all. An article by Kuchitsu " now deals with molecular structure determination by spectroscopic and electron diffraction methods Kuchitsu and Cyvin have summarized the notation and definitions of the varied bond-length parameters employed in gas-phase molecular structure determination. 

Gasteiger and co-authors recently suggested a new approach. They developed a molecular transform, derived from an equation used in electron diffraction studies, that allows the presentation of the 3D structure of a molecule by a fixed (constant) number of variables. On the other hand, a generator of 3D structures of organic compounds was created (see Three-dimensional Structure Generation Automation). This made it possible to study the correlations between 3D structures and infrared spectra using ANN. 

Electron diffraction studies are usually limited to transferred films (see Chapter XV), One study on Langmuir films of fatty acids has used cryoelectron microscopy to fix the structures on vitrified water [179], Electron diffraction from these layers showed highly twinned structures in the form of faceted crystals. 

Bartell and co-workers have made significant progress by combining electron diffraction studies from beams of molecular clusters with molecular dynamics simulations [14, 51, 52]. Due to their small volumes, deep supercoolings can be attained in cluster beams however, the temperature is not easily controlled. The rapid nucleation that ensues can produce new phases not observed in the bulk [14]. Despite the concern about the appropriateness of the classic model for small clusters, its application appears to be valid in several cases [51]. 

As noted earlier, most electron diffraction studies are perfonned in a mode of operation of a transmission electron microscope. The electrons are emitted themiionically from a hot cathode and accelerated by the electric field of a conventional electron gun. Because of the very strong interactions between electrons and matter, significant diffracted intensities can also be observed from the molecules of a gas. Again, the source of electrons is a conventional electron gun. 

In neutral and alkaline environments, the magnesium hydroxide product can form a surface film which offers considerable protection to the pure metal or its common alloys. Electron diffraction studies of the film formed ia humid air iadicate that it is amorphous, with the oxidation rate reported to be less than 0.01 /rni/yr. If the humidity level is sufficiently high, so that condensation occurs on the surface of the sample, the amorphous film is found to contain at least some crystalline magnesium hydroxide (bmcite). The crystalline magnesium hydroxide is also protective ia deionized water at room temperature. The aeration of the water has Httie or no measurable effect on the corrosion resistance. However, as the water temperature is iacreased to 100°C, the protective capacity of the film begias to erode, particularly ia the presence of certain cathodic contaminants ia either the metal or the water (121,122). 

The geometries of oxiranes have been determined mainly by X-ray diffraction on crystalline natural products, the oxirane ring being widespread in nature (Section 5.05.5.3). However, the detailed structure of the parent compound (Figure 1) has been secured by microwave spectroscopy and electron diffraction studies (64HC(l9-l)l). The strain in this... 

Ketones also favor eclipsed conformations. The preference is for the rotamer in which the alkyl group, rather than a hydrogen, is eclipsed with the carbonyl group because this conformation allows the two alkyl groups to be anti rather than gauche. Electron diffraction studies of 3-pentanone indicate the conformation shown to be the most stable rotamer, in accord with this generalization. ... 

The most stable conformation of cyclohexane is the chair. Electron diffraction studies in the gas phase reveal a slight flattening of the chair compared with the geometry obtained when tetrahedral molecular models are used. The torsion angles are 55.9°, compared with 60° for the ideal chair conformation, and the axial C—H bonds are not perfectly parallel but are oriented outward by about 7°. The length of the C—C bonds is 1.528 A, the length of the C—H bonds is 1.119 A, and the C—C—C angles are 111.05°. ... 

Electron diffraction studies [3] have revealed that hexagons within the sheets are helically wrapped along the axis of the nanotubes. The interlayer spacing between sheets is 0.34 nm which is slightly larger than that of graphite (0.3354 nm). It was dso reported [2] that the helicity aspect may vary from one nanotube to another. Ijima et al. [2] also reported that in addition to nanotubes, polyhedral particles consisting of concentric carbon sheets were also observed. 

Electron diffraction studies provide valuable information about structures in the gas phase. Consequently, this method is important for chalcogen-nitrogen compounds that are liquids or gases at room temperature. The application of this technique has provided evidence for the monomeric structures of the 1,2,3,5-dithiadiazolyl radical [CEsCNSSN] (3.3) and the 1,3,2-dithiazolyl [CEsCSNSCCEs] (3.4), a... 

Photoelectron spectra have confirmed the expected trends in the frontier orbitals.The tetrafiuoro derivative 12.12 (R = F) is prepared by treatment of C6F5SNSNSiMc3 with CsF in acetonitrile (Scheme 12.2). Several difiuoro- and trifiuoro-benzodithiadiazines have also been prepared by these methods.In contrast to 12.12 (R = H), which has an essentially planar structure in the solid state,the dithiadiazine ring in the tetrafiuoro derivative is somewhat twisted. In the gas phase, on the other hand, electron diffraction studies show that 12.12 (R = F) is planar whereas 12.12 (R = H) is non-planar. ... 

Electron diffraction studies in the gas phase reveal an unusual structure in which the 5-coordinate Al atom has square-pyramidal... 

TABLE 3. Sulfonyl molecular geometries not yet considered in Reference 5 all gas-phase electron diffraction studies... 

Preliminary result of electron diffraction study in this Laboratory. b L. W. Pickett, Proc. Roy. Soc. (London), A142, 333 (1933). J. Dhar, Indian J. Phys., 7, 43... 

There is evidence from electron diffraction studies of pyrazine and pyridine that this value should be increased by 0.01 or 0.02 A because of the electronegativity of nitrogen (V. Schomaker and L. Pauling, paper to be published in Jour. Am. Chem. Soc.). 

The most extensive application which was made of the resonance curve was to the carbon-chlorine bond in phosgene and the chloroethylenes. In the electron-diffraction study of these substances2 the carbon-carbon and carbon-oxygen doublebond values 1.38 and 1.28 A. were assumed the question accordingly arises as to what effect the new double bond values would have on the carbon-... 

The values of bond angles (around 100°) are in accordance with a singlet ground state. The bond angles in the dichlorides SiCl2 (Hargittai et al., 1983) and GeCl2 (Schultz et al., 1982) which were determined by direct electron diffraction studies of these unstable species are close to those from the IR matrix technique (Table 3). 

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