Electron diffraction experiment

The 3D MoRSE code is closely related to the molecular transform. The molecular transform is a generalized scattering function. It can be used to predict the intensity of the scattered radiation i for a known molecular structure in X-ray and electron diffraction experiments. The general molecular transform is given by Eq. (22), where i(s) is the intensity of the scattered radiation caused by a collection of N atoms located at points r. ... 

The short-range order in a material is important in determining optoelectronic properties. For instance, x-ray and electron diffraction experiments performed on amorphous siHcon (i -Si) and germanium (a-Ge) have revealed that the nearest neighbor environments are approximately the same as those found in their crystalline counterparts (6) photoemission experiments performed on i -Si show that the DOS in valence and conduction bands are virtually identical to the corresponding crystal with the exception that the singularities (associated with periodicity) present in the latter are smeared out in the former. 

One important structural feature on which to focus is whether the nitrogen atom lies in the same plane as the three carbon atoms. Electron diffraction experiments have found the ground state to be slightly non-planar. You can determine the planarity of the structures you compute by examining the sum of the three C-N-C angles (for a planar molecule, the sum will be 360°) and by looking at the values of the C2-N-C4-O and C3-N-C4 Hg dihedral angles (in a planar structure, both will be 0°). 

The S-S bond between two divalent sulfur atoms plays an important role as the main stabilizer of the tertiary structure of many proteins. The simplest chemically stable compounds of this class are HSSH and CH3SSCH3. The structures of these two disulfanes have been established by microwave spectroscopy and electron diffraction experiments. 

To obtain the maximum concentration of MX2 species, the conditions for these reactions were first optimized by the pyrolytic mass spectrometry method (Kagramanov et al., 1983a). The contents of the mixture during the electron diffraction experiments were controlled by a quadrupole mass spectrometer. 

The assumption of membrane softness is supported by a theoretical argument of Nelson et al., who showed that a flexible membrane cannot have crystalline order in thermal equilibrium at nonzero temperature, because thermal fluctuations induce dislocations, which destroy this order on long length scales.188 189 The assumption is also supported by two types of experimental evidence for diacetylenic lipid tubules. First, Treanor and Pace found a distinct fluid character in NMR and electron spin resonance experiments on lipid tubules.190 Second, Brandow et al. found that tubule membranes can flow to seal up cuts from an atomic force microscope tip, suggesting that the membrane has no shear modulus on experimental time scales.191 However, conflicting evidence comes from X-ray and electron diffraction experiments on diacetylenic lipid tubules. These experiments found sharp diffraction peaks, which indicate crystalline order in tubule membranes, at least over the length scales probed by the diffraction techniques.123,192 193... 

In contrast to XRD, the background is rather high in electron diffraction experiments. The size of the background can partially be attributed to the poorer spectral resolution. Presence of electrons scattered on the apertures can not be completely neglected either. 

The g functions come from theory and three kinds of parameters are determined from an electron diffraction experiment, r (geometry), / (motion), and k (anharmonicity of vibrations). 

An adequate vapor pressure of the compound to be studied is a prerequisite for the experiment atypical vapor pressure is about 15 torrs for heavier targets lower, for molecules of light atoms higher vapor pressure is needed. If there is uncertainty in the vapor composition or it is necessary to optimize it, a simultaneous mass spectrometric investigation, ultimately, a combined mass spectrometric-electron diffraction experiment may be useful. Such a combined scheme is shown in Figure 2. 

Fig. 1. The reproduction of the first report by Mark and Wierl on a gas electron diffraction experiment ...

Then followed the report in 1930 by Mark and WierF on the first gas electron diffraction experiment and stmctural data appeared for a series of simple molecules ... 

Many inorganic molecules are present as dimers (or higher associates) in the vapor phase at the pressure necessary for the electron diffraction experiment. The reaction... 

The first electron diffraction experiment using a double effusion chamber for overheating the unsaturated vapor was carried out on aluminium(III) chloride ... 

In the course of our studies of iron-chloride systems, the stmcture determination of both monomeric and dimeric iron(III) chlorides are scheduled. Prior to the electron diffraction experiments, a mass spectrometric investigation of the vapor composition of iron(III) chloride was carried out as a function of the temperature under the electron diffraction experimental conditions ). It was establi ed that a nozzle temperature of 190 °C provides sufficient vapor pressure and the vapor consists of only dimeric molecules. In order to produce monomeric molecules whose relative abundance is an order of magnitude greater as regards the dimers, the chamber containing the nc e had to be heated to 450 °C while the other chamber, containing the sample was kept at about 200 °C. The analysis of the dimeric stmcture has been completed ) while that of the monomer is in progress. 

Electron diffraction experiments provide valuable information about structures in the gas phase. Consequently, this method of structural determination is important for inorganic ring systems that are volatile liquids or gases at room temperature. For example, the essentially planar structures of borazine (3.1, E = NH) and the isoelectronic boroxin ring (3.1, E = 0), ° the monomeric structure of the radical [CFgCNSSN] (3.2), and the all-cw arrangement of the... 

M.A. Van Hove, W.H. Weinberg, C.-M. Chan Low-energy electron diffraction experiment, theory and surface structure. (Springer, Berlin, 1986). 

Compounds with six fluorine ligands have provided structural difficulties. Xenon hexafluoride exists as the monomer in the vapour state and has been shown largely by electron diffraction experiments to be non-octahedral, and also fluxional.44 Although VSEPR theory might have... 

Fig. 1. Experimental layout of the pump-probe electron diffraction experiment. 2a> = 2 harmonic, and 3oj = 3ri harmonic of the Titanium-Sapphire laser operating at 800 nm.

In conventional gas electron diffraction experiments, an effusive beam is used in which vibrational levels of molecules are thermally populated and the width of a peak in a radial distribution curve is determined by thermally averaged mean amplitudes. When a molecular beam or a free jet is used, mean amplitudes could become small, since the contribution from the vibrationally excited levels is reduced significantly. As a consequence, sharper peaks are expected in the radial distribution curve, and the spatial resolution of the snapshot could be improved. However, it seems that the observed peaks in the radial distribution curve are considerably broad even though a molecular beam is used. There could be some reasons to have such broadened peaks in the radial distribution curve. 

In the search for a presumed, but not conclusively detected conformer, the natural approach is to use a high a temperature as possible in the electron-diffraction experiment. Several such attempts have been made, so far in vain169d The negative results in the case of 1,3-butadiene are not in contrast with the theroretical findings, since no rigid second conformer is to be expected. 

An electron diffraction experiment was performed with a beam of electrons accelerated by a potential difference of 10 keV. What was the wavelength of the electron beam in nm ... 

Structural parameters for SCBs obtained from different methods of quantum chemistry in comparison with the results from electron diffraction experiment have been reported (Table 1) <2006JST(800)146>. 

The pinwheel structure is not only observed for chiral adsorbates. An early example was reported for small molecules at low temperatures on graphite in UHV (Fig. 19). Neutron and electron diffraction experiments as well as... 

Van Hove M.A., Weinberg W.H., Chan C.M., Low-Energy Electron Diffraction Experiment, Theory and Surface Structural Determination, Springer, Berlin, 1986. 

Figure 1-12) [30], This pattern was extended by Alan Mackay into the third dimension and he even produced a simulated diffraction pattern that showed 10-foldedness (Figure 1-13) [31], It was about the same time that Dan Shechtman was experimenting with metallic phases of various alloys cooled with different speeds and observed 10-foldedness in an actual electron diffraction experiment (Figure 1-14) for the first time. The discovery of quasicrystals has added new perspective to crystallography and the utilization of symmetry considerations. 

The unique moment of discovery came in April 1982 when Dan Shechtman was doing some electron diffraction experiments on alloys, produced by very rapid cooling of molten metals. In the experiments with molten aluminum with added magnesium, cooled rapidly, he observed an electron diffraction pattern with tenfold symmetry (see, the pattern in the Introduction). It was as great a surprise as it could have been imagined for any well-trained crystallographer. Shechtman s surprise was recorded with three question marks in his lab notebook, 10-fold [140],... 

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