Electron-diffraction studies

The value of the bond length is that measured by Akishin and Spiridonov (7) In a high-temperature electron diffraction study. Electron diffraction patterns (7) for SrBr were satisfactorily explained on the basis of a linear model (180 10 ). Later studies by Wharton et al. (8 ), using electric deflection of molecular beams to detect dipole moments, showed no polarity In the SrBtg molecule this Is most reasonably explained by a linear and centrosymmetrlc configuration. 

The fundamental vibrations are those observed in the gas-phase Raman spectra by Monostorl and Weber ( ) and the gas-phase infrared spectra by Makl et al. (6) and Chalmers and McKean (7 ). The frequencies are essentially the same as those selected by Shimanouchi (8) who also lists earlier spectral studies. Electron-diffraction studies of the gas were reported by Thornton (9) and Hoffman and Livingston ( 0). The adopted bond length is the average of these two results which differ by only 0.006 A. 

Materiak scientists study electron diffraction patterns to learn about the surfaces of solids. 

Later studies by Sugiyama et were based on electron diffraction and were directed at addressing questions concerning the nature of the differences between the Iq, and I forms of cellulose. In a landmark study, electron diffraction patterns were recorded from V. macrophysa both in its native state, wherein the Iq, and Ifj forms occur in their natural relative proportions, and after annealing using the process first reported by Horii and coworkers, which converts the Iq, form into the I form. The native material, which is predominantly the Iq, form, was shown to produce a complex electron diffraction pattern similar to that which had earlier led Honjo and Watanabe to propose an eight-chain unit cell. In sharp contrast, the annealed sample, which is essentially all of the I form, produced a more simple and symmetric pattern that could be indexed approximately in terms of a two-chain monoclinic unit cell. 

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. 

Transmission electron microscopy (TEM) can resolve features down to about 1 nm and allows the use of electron diffraction to characterize the structure. Since electrons must pass through the sample however, the technique is limited to thin films. One cryoelectron microscopic study of fatty-acid Langmuir films on vitrified water [13] showed faceted crystals. The application of TEM to Langmuir-Blodgett films is discussed in Chapter XV. 

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. 

For bulk structural detemiination (see chapter B 1.9). the main teclmique used has been x-ray diffraction (XRD). Several other teclmiques are also available for more specialized applications, including electron diffraction (ED) for thin film structures and gas-phase molecules neutron diffraction (ND) and nuclear magnetic resonance (NMR) for magnetic studies (see chapter B1.12 and chapter B1.13) x-ray absorption fine structure (XAFS) for local structures in small or unstable samples and other spectroscopies to examine local structures in molecules. Electron microscopy also plays an important role, primarily tlirough unaging (see chapter B1.17). 

Chapman J A and Tabor D 1957 An electron diffraction study of retracted monolayers Proc. R. See. A 242 96-107... 

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). 

Electron diffraction studies indicate that phosphoms pentoxide vapor consists of P O q molecules. The vapor usually condenses to the hexagonal crystalline modification but under rapid cooling can be condensed to an amorphous soHd (P-form). The Hquid obtained by melting the stable orthorhombic modification cools to form a glass which is the P-form. The Hquid obtained from the H modification also can be supercooled to a glass. 

Vitreous siUca is considered the model glass-forming material and as a result has been the subject of a large number of x-ray, neutron, and electron diffraction studies (12—16). These iavestigations provide a detailed picture of the short-range stmcture ia vitreous siUca, but questioas about the longer-range stmcture remain. 

Structural parameters and interatomic distances derived from electron diffraction (7) (77JST(42)l2i) and X-ray diffraction (8) studies (76AX(B)3178) provide unequivocal evidence that pyrazine is planar with >2a symmetry. There is an increased localization of electron density in the carbon-nitrogen bonds, with carbon-carbon bonds being similar in length to those in benzene. ... 

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... 

Azetidine itself has been studied by electron diffraction, which reveals a non-planar structure (Figure 1) (73CC772). The enhanced length of the bonds reflects the strain in the ring and the angle between the CCC and CNC planes of 37° is similar to that found for cyclobutane (35°), but quite different from that for oxetane (4°). 

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