Electron Diffraction Data

As-deposited carbonaceous films were studied by Kakinoki et al. [55,56], Dove [57], and Poltavtsev et al. [58] using electron diffraction (without lenses). These authors describe the as-deposited films as strictly amorphous. The relevant experimental data are presented in Table 1.1 the values of 5 = 2 sin 9 / 7. A correspond to successive maxima of /( ), and graphite values of i and hkl are given for comparison. 

In the same manner, the calculated P(x) are represented by values of x A of each peak in Table 1.2. Here P x) of graphite, calculated for a single crystal, is given with separate values for intralayer pair of atoms and interlayer ones [11]. 

Surprisingly the calculated P(x) are different for the three studies. Those of Kakinoki et al. are the most different from graphite these authors decomposed the first peak at 1.50 A into 1.42 A (sp bond) and 1.54 A (sp bond) and did the same for all peaks to conclude finally that a mixture of sp and sp bonds is arranged following the random network model. 

The preparation of as-deposited films is expected to introduce severe strain due to rapid quenching, and this is probably responsible for the occurrence of a metastable disorder (amorphous state) easy to cure by heating. 

Therefore, the crux of the problem is how to select a convenient model for the occurrence of such a metastable transformation. The answer lies in the appropriate selection of the imaging technique. 

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Contributions in this section are important because they provide structural information (geometries, dipole moments, and rotational constants) of individual tautomers in the gas phase. The molecular structure and tautomer equilibrium of 1,2,3-triazole (20) has been determined by MW spectroscopy [88ACSA(A)500].This case is paradigmatic since it illustrates one of the limitations of this technique the sensitivity depends on the dipole moment and compounds without a permanent dipole are invisible for MW. In the case of 1,2,3-triazole, the dipole moments are 4.38 and 0.218 D for 20b and 20a, respectively. Hence the signals for 20a are very weak. Nevertheless, the relative abundance of the tautomers, estimated from intensity measurements, is 20b/20a 1 1000 at room temperature. The structural refinement of 20a was carried out based upon the electron diffraction data (Section V,D,4). 

Carvalho, C A M., Hashizume, H., Stevenson, A.W. and Robinson, I K. (1996) Electron-density maps for the Si( 111) 7x7 surface calculated with the maximum-entropy technique using X-ray and electron-diffraction data, Physica B, 221,469 186. 

Fig. 5.2 Radial distribution curves, Pv

During the last decades, a large body of structural information has been derived from gas-electron diffraction studies. The corresponding results are nearly exclusively reported in the literature in terms of r distances, or the equivalent thermal average intemuclear distances, which are denoted r. The r distances are defined by the relation, r = r — If. Alternative methods for interpreting gas-electron diffraction data are possible, for example, in terms of -geometries5, but they are currently too complex to apply in routine stmctural analyses, because they require detailed information on the molecular potential energy surface which is not usually available. 

Iijima, K., K. Tanaka, and S. Onuma. 1991. Main Conformer of Gaseous Glycine Molecular Structure and Rotational Barrier from Electron Diffraction Data and Rotational Constants. J. Mol. Struct. 246, 257-266. 

The structure of a molecule is given by the three-dimensional distribution of atomic cores and valence electrons. This structure has been elucidated for many molecules with the use of X-ray or electron diffraction data. Chemical properties of molecules are observed under conditions which permit internal motions. Such observations yield views which may differ markedly as a function of time. Thus, observable properties are determined from equilibrated ensembles of species differing in geometry and energy. 

Electron diffraction data on gaseous cyclopentasilane (61) fit both the C2 and C, models. Rustad s simple MM method calculates virtually equal energies for these conformers, and 61 is likely to undergo pseudorotation, as does cyclopentane (200). 

In HREM images of inorganic crystals, phase information of structure factors is preserved. However, because of the effects of the contrast transfer function (CTF), the quality of the amplitudes is not very high and the resolution is relatively low. Electron diffraction is not affected by the CTF and extends to much higher resolution (often better than lA), but on the other hand no phase information is available. Thus, the best way of determining structures by electron crystallography is to combine HREM images with electron diffraction data. This was applied by Unwin and Henderson (1975) to determine and then compensate for the CTF in the study of the purple membrane. 

Table 4. Results of the multipole model refinement of germanium (single crystal) with electron diffraction data...

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