Electron diffraction structure determination

Key words electron diffraction, structure determination, low-dose CBED, protein strueture... 

An electron-diffraction structure determination is nowadays based exclusively on comparing experimental, sME(s), and theoretical, sMT(s), molecular intensity curves. The structure parameters are adjusted until the... 

The electron diffraction structures of 3,5-dimethylpyrazole (19) and 3,5-bis(trifluoromethyl)-pyrazole (20) have been determined <93JST(29i)2l l>. The last pyrazole is a compound with a pungent odor which is too volatile to collect diffractometric data even in a sealed capillary. 

The MM4 geometry of the most stable conformation (tGg, 12) was compared with the experimental electron diffraction structure (rgf and with the MP2/B calculation (Table 9.5). Some important values cannot be uniquely determined from the diffraction pattern, and are only reported as average values with similar quantities. The MP2/B values can be compared with MM4 item by item. The agreement seems to be satisfactory. 

The electron diffraction structure analysis can accordingly be characterized as if it were the determination of the frequencies and the damping of the components of a sum of sine functions. As the molecular intensity is Fourier-transformed, another important but more descriptive function is attained, which bears, somewhat unfortunately, the name of radial distribution function, and which is indeed connected with the probability distribution function of the intramolecular inter-nuclear distances. It is a Gaussian-like distribution that... 

The structure of 3,4-bis(trifluoromethyl)-l,2-dithiet has been determined by electron diffraction. Structures (85), (86), and (87) were said to contribute to the hybrid, but (88) does not. 

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... 

These cover the following topics (a) theoretical methods, with emphasis on the utility of such methods b) molecular dimensions, as determined by X-ray, electron diffraction and microwave spectra (c) molecular spectra, covering NMR, IR, UV, mass and photoelectron spectra [d) thermodynamic aspects, such as stability, ring strain, aromaticity, shape and conformation of saturated and partially saturated rings (c) tautomerism, including prototopic and ring-chain (/) betaine and other unusual structures. 

To measure the goodness of fit, and to quantify the structural determination, a reliability (i -factor) comparison is used. In comparing the data and simulation of the experiment for many trial structures, a minimum R factor can be found corresponding to the optimal structure. In this way atomic positions can be determined in favorable cases to within a few hundredths of an A, comparable to the accuracy achieved in Low-Energy Electron Diffraction (LEED). 

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