Electron diffraction, general discussion

Other techniques previously described for general investigation of tautomeric equilibria (76AHC(S1)1> involve heats of combustion, relaxation times, polarography, refractive index, molar refractivity, optical rotation, X-ray diffraction, electron diffraction, neutron diffraction, Raman, fluorescence, phosphorescence and photoelectron spectroscopy, and mass spectrometry. The application of several of these techniques to tautomeric studies has been discussed in previous sections. Other results from the more important of these will be referred to later in this section. 

Here we discuss structures that have been established at the atomic level revealing the exact conformation of the polypeptide chain. All were determined by X-ray diffraction analysis of a tridimensional protein crystal. Some o -helical membrane protein structures have been analyzed by electron diffraction of o-dimensional crystals, although generally with a lower accuracy. For a long time structural analyses by NMR... 

X-ray and electron diffraction techniques have been used to obtain the data on epitaxy. X-ray diffraction methods are particularly useful for thick oxide films and have the advantage of giving diffraction patterns from both the oxide film and the metal suhstrate. For oxide films less than several hundred Angstroms thick, electron diffraction techniques are necessary in most cases, hi general, an electron diffraction pattern is not obtained from the metal substrate unless the oxide film is extremely thin, the surface is only partially covered with oxide, or the metal surface is rough. Reflection type diffraction techniques have been used with bulk specimens and transmission techniques with thin specimens and stripped oxide films from bulk metal specimens. Bach technique has its special advantages and limitations, but these will not be discussed here. 

A (1x1) LEED pattern is generally observed upon sputtering and annealing in UHV. To my knowledge no quantitative LEED study has been reported, probably because of the difficulty of creating defects when the sample is bombarded with electrons (see section 3.1.4). A medium-energy electron diffraction study (MEED) study of Ti02(l 10) employed an ESDI AD optics with a channelplate this setup is more sensitive than a conventional LEED apparatus, and allows for very small electron currents to be used. The results of this study are basically consistent with the (1x1) structure depicted in Fig. 3b. X-ray photoelectron diffraction (XPD) spectra also fit the expected (1x1) termination [33], as do STM and AFM results (which are discussed in section 3.1.3 below). 

The difficulty in using mean square amplitudes as a source of information on the force field is that the precision with which the quantities can be determined experimentally is, in general, barely sufficient to make the information useful, even with the most precise data currently available. Exceptions are the molecules PC13 [ref. 23] and some C, Si, and Ge tetrahalides,24 for which the electron diffraction patterns have been particularly well analyzed and useful information on the force field has been obtained. Mean square amplitudes may also sometimes be used to distinguish between two alternative discrete solutions to the force field of the type discussed earlier for the B2u species of benzene such examples have been discussed recently by Hoy, Stone, and Watson.4... 

General discussion of intra- and intermolecular interactions 3 van der Waals interactions 3 Coulombic interactions 5 Medium effects on conformational equilibria 5 Quantum mechanical interpretations of intramolecular interactions 7 Methods of study 8 Introduction 8 Nmr and esr spectroscopy 8 Microwave spectroscopy (MW) 12 Gas-phase electron diffraction (ED) 12 X-ray crystallographic methods 13 Circular-dichroism spectroscopy and optical rotation 14 Infrared and Raman spectroscopy 18 Supersonic molecular jet technique 20 Ultrasonic relaxation 22 Dipole moments and Kerr constants 22 Molecular mechanic calculations 23 Quantum mechanical calculations 25 Conformations with respect to rotation about sp —sp bonds 27 Carbon-carbon and carbon-silicon bonds 28 Carbon-nitrogen and carbon-phosphorus bonds 42 Carbon-oxygen and carbon-sulphur bonds 48 Conformations with respect to rotation about sp —sp bonds Alkenes and carbonyl derivatives 53 Aromatic and heteroaromatic compounds 60 Amides, thioamides and analogues 75 Conclusions 83 References 84... 

There are numerous physical methods that can be used to determine partial molecular structures. But here we will limit our discussion to the four methods that are in wide current use and that may be utilized to accurately determine a total molecular structure. These are three diffraction methods, namely electron diffraction, neutfon diffraction, and X-ray diffraction, and one spectfoscopic method, microwave spectfoscopy. (There are, of course, many other methods for studying molecular stfucture, some of which are extremely powerful, although usually in limited areas. But, not all of chemistfy can be discussed here, so we will limit the topics to those mentioned.) Each of these methods measures something that is a little different, sometimes quite a bit different, from what the other methods measure. The calculational methods, described in the next chapter, generally calculate something that is still different from any of the above. The structures of the same molecule determined by these different methods are not, in general, identical. Hence, we need to understand how the structures obtained from each of these methods are interrelated. 

The difficulties mentioned in the general discussion in connection with the electron diffraction investigation of sulphone molecular geometries are of course relevant for the polysulphuryl fluorides as well. It is possible, however, to determine the bond angle at the bridging oxygen atom very accurately. 

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