Electron diffraction studying

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. 

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. 

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... 

This confirms the deductions from a gas phase electron-diffraction study P-Fo 158 pm,... 

These results, together with infrared and electron diffraction studies, indicate that gaseous HF comprises an equilibrium mixture of monomers and cyclic hexamers, though chain dimers may also occur under some conditions of temperature and pressure ... 

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.). 

Davis, M. 1. Speed, C. S. (1970) Gas-phase electron diffraction studies of some iron carbonyl complexes, J. Organomet. Chem. 21,401-413. 

Lu, Chia-Si, and J. Donohue (1944) An electron-diffraction study of some sulfur compounds. J. A mer. Chem. Soc. 66, 818-827. 

An electron-diffraction study of benzene, p-xylene, mesitylene, and hexamethylbenzene has been made recently by Jones, 0 who, on the basis of measurements extending to To = 14, reported the values Ca,. —Caj = 1.50 = = 0.01 A. and Car — Car = 1.40 = = 0.01 A. Jones t0 values are about 1.3% greater than ours, and comparison of them with our calculated values of s.4 leads to... 

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-... 

Structural data from X-ray- and electron-diffraction studies, mass spectra, and MO calculations were analyzed for R3SiCo(CO)4 (R = H, F, Cl) and Me3SiMn(CO)5 it was believed that there is partial double-bond character between Si and Co and that there may be intramolecular interaction between the axial silicon atom and the equatorial carbonyl groups 30, 212). 

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). 

Later, successful determination of the molecular structure of the free allyl radical was achieved by high-temperature electron diffraction, augmented by mass spectrometry studies (Vaida et al., 1986). The structural parameters obtained for the allyl radical were rcc 142.8 pm, rcH 106.9 pm, accc 124.6°, ccH 120.9°. This was the first electron diffraction study of an unstable organic molecule. 

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