Hydrogen bonds are a key feature of chemical structure and reactivity. Recently there has been much interest in a special class of hydrogen bonds called "strong" or "low-barrier" and characterized by great strength, short distances, a low or vanishing barrier to hydrogen transfer, and distinctive features in the NMR spectrum. Although the energy of an ordinary hydrogen bond is ca 5 kcal mol-1, the strength of these hydrogen bonds may be > or = 10 kcal mol-1. The properties of these hydrogen bonds have been investigated by many experimental techniques, as well as by calculation and by correlations among those properties. Although it has been proposed that strong, short, low-barrier hydrogen bonds are important in enzymatic reactions, it is concluded that the evidence for them in small molecules and in biomolecules is inconclusive.

Strong hydrogen bonds in chemistry and biology

Publisher Summary In energy-filtering transmission electron microscopy (EFTEM), the zero-loss electrons or electrons passing an energy-loss window of the electron energy-loss spectroscopy (EELS) are used for image formation. This can be achieved by using the scanning mode in a dedicated scanning transmission electron microscope (STEM) or in a TEM with a spectrometer behind the camera chamber or by using an imaging filter lens in the column of a TEM. The conventional TEM and STEM modes can be combined in this way with the mode of electron spectroscopic imaging (ESI) and electron spectroscopic diffraction (ESD), and different modes can be used to record an EELS spectrum. An EFTEM can therefore make full use of elastic and inelastic electron-specimen interactions. This chapter provides an overview of the physical background and the possibilities of EFTEM. The relevant physics of elastic and inelastic scattering is also discussed followed by the instrumentation of EFTEM. The theoretical approaches for understanding the contrast and examples of application are presented for ESI and for ESD.

Energy-filtering transmission electron microscopy

https://escholarship.org/content/qt8d33n38m/qt8d33n38m.pdf?t=p0t0iy

Limitations to significant information in biological electron microscopy as a result of radiation damage

A condensed description is given of the fundamental processes involved in radiation damage and the effects of radiation on the physical and chemical properties of organic materials, particularly polymers It is shown that the radiation doses received by specimens in the electron microscope are extremely high, very much greater than those used in radiation chemistry experiments Because of this, only qualitative predictions of behaviour in the electron microscope can be made A number of authors have described the changes in the image or diffraction pattern of particular specimen types during observation in the electron microscope and their work is reviewed here In general, contrast features in the image may disappear, due to loss of mass or crystallinity, or new features may appear due to distortion of ordered regions The effects of radiation damage on attainable resolution, and possible methods of improving the resolution are then discussed

Radiation damage and electron microscopy of organic polymers

Electron diffraction and microscopy are among the most important techniques for studying the structures of solids. This review aims to give a comprehensive introduction to the basic principles of the scattering of fast electrons and to highlight selected applications of importance. It begins by discussing electron scattering by single atoms and describes how single atoms may be imaged. The geometry of diffraction from perfect single crystals is then considered and a simple kinematical theory which yields approximate values for diffracted intensities is given. It is shown how simple principles have been used to image monatomic steps on the surface of crystals. More accurate methods of calculating diffracted intensities are given and particular attention is paid to describing concepts often found to be difficult, for example the dispersion surface. Principles of various recent electron scattering techniques are outlined including high-voltage electron microscopy, scanning electron microscopy, convergent-beam electron diffraction and the critical voltage effect. Applications described range from measuring bonding electron charge densities to the imaging of dislocations. Finally some recent theoretical developments on the problem of imaging imperfect crystals at atomic resolution are discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/42/11/002/pdf

The scattering of fast electrons by crystals

The crystal orientations which give maximum electron transmission have been determined for a wide variety of materials at 100 to 1000 kv. It is found both experimentally and theoretically that the diffraction conditions for best transmission change considerably as the accelerating voltage is raised above 100 kv. These conditions are tabulated for materials of low, medium and high atomic numbers. The changes are predicted by many-beam dynamical diffraction theory and may be understood in terms of the channelling properties of individual Bloch waves which represent the fast electrons inside crystals. The nature of defect contrast under these conditions is considered, as is the sensitivity of theoretical calculations to absorption and extinction parameters. Penetration of heavy materials at 1000 kv using the crystal orientations given in this paper is considerably greater than previously expected. For example, defects were observed in a gold foil well over one micron in thickness.

Maximizing the penetration in high voltage electron microscopy

The degradation of polyethylene and polyoxymethylene crystals in the high voltage electron microscope was determined from 100–1000 kV. As in previous work at lower electron energies, the dose needed to change the initial crystalline diffraction patterns into amorphous ring patterns was measured. It was found that the dose required to destroy the crystal order increases by a factor of about three in going from 100 to 1000 kV. The result agrees roughly with the theoretically predicted ionization rate, assuming that the damage rate is proportional to the ionization rate.

Radiation damage of polymers in the million volt electron microscope

In a high voltage electron microscope, the Kikuchi patterns from low index planes change considerably as the accelerating voltage is raised above 100 kV. These changes, which result from many-beam interactions among the systematic reflections, are studied in detail at 100 to 1000 kV in f.c.c, b.c.c., and h.c.p, metals having a wide range of atomic numbers. In addition, a new method for calculating both conventional and high voltage Kikuchi patterns is given. The method is based on a simple theoretical relationship between Kikuchi patterns and many-beam rocking curves, and thus avoids the mathematical complications of previous theories of Kikuchi patterns. This method also leads to considerable physical insight into Kikuchi patterns and into their relation to the diffraction channelling behavior of electrons in crystals, and explains, for the first time, the anomalous 100 kV Kikuchi patterns from the (111) planes of diamond cubic materials.



In einem Hochspannungselektronenmikroskop andern sich die Kikuchidiagramme von niedrig-indizierten Ebenen betrachtlich, wenn die Beschleunigungsspannung uber 100 kV gesteigert wird. Diese Anderungen, die von den Mehrstrahl-Wechselwirkungen zwischen den systematischen Reflexionen herruhren, werden ausfuhrlich zwischen 100 und 1000 kV in k.f.z., k.r.z. and h.d.p. Metallen in einem weiten Bereich der Atom-Ordnungszahlen untersucht. Zusatzlich wird eine neue Methode fur die Berechnung sowohl der konventionellen als auch der Hochspannungs-Kikuchidiagramme angegeben. Diese Methode beruht auf einer einfachen theoretischen Beziehung zwischen Kikuchidiagrammen und Vielstrahl-“rocking”-Kurven und vermeidet somit mathematische Komplikationen fruherer Theorien der Kikuchidiagramme. Diese Methode fuhrt auserdem zu bedeutenden physikalischen Einsichten in die Kikuchidiagramme und in deren Beziehungen zum Verhalten der Elektronen bei Beugungskanalbildung in Kristallen und erklart zum erstenmal die anomalen 100 kV-Kikuchidiagramme von den (111)-Ebenen kubischer Materialien mit Diamantstruktur.

Kikuchi patterns in a high voltage electron microscope

The critical-voltage effect of high-voltage electron diffraction has been used to measure the X-ray Debye temperatures of Cr, α-Fe and their disordered alloys. The 220 critical voltages were measured at room and elevated temperatures for the pure metals and at room temperature for five intermediate compositions. The data for the pure metals were sufficient to determine both the Debye temperature and the deviation from the free-atom value of the atomic scattering factor at the first-order reflection position. The results agree with those of other workers. The scattering factor deviations were assumed to be the same in the alloys as in the pure metals, and this made it possible to determine the alloy Debye temperatures from a single room-temperature measurement of the critical voltage at each intermediate composition. The Debye temperatures are analyzed successfully in terms of a simple one-parameter theory, and are correlated with the alloy melting-point data through Lindemann's formula.

/pdf/high-voltage-electron-diffraction-measurement-of-the-debye-1vek5ilpn1.pdf

High-voltage electron diffraction measurement of the Debye temperatures of Cr, α-Fe and their disordered alloys

Resolution, Contrast and Interpretation of HVEM Images of Crystals

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