Kinetics X-rays

Mechanisms in Organic Reactions Molecular Interactions Reaction Kinetics X-ray Crystallography Lanthanide and Actinide Elements Maths for Chemists Bioinorganic Chemistry Chemistry of Solid Surfaces Biology for Chemists Multi-element NMR Peptides and Proteins Biophysical Chemistry Natural Product The Secondary Metabolites... 

Overall, this volume shows how many different mechanistic tools—biochemical kinetics. X-ray crystallography, state-of-the-art computations—have been brought to bear in parallel and occasionally in concert on the problem of understanding the origins of catalysis by ODCase. Still, it is clear that a consensus has not yet been reached as to the catalytic mechanism. In fact, while very few mechanistic proposals have been disproven, many new mechanistic proposals have arisen. For example, as pointed out by several of our authors, dynamic effects may prove to be essential for catalysis in this peculiar enzyme. We hope that the diverse chemistry discussed in this volume will inspire not only additional calculations and experiments, but also new viewpoints from which the mystery of ODCase s catalytic mechanism may be unraveled. 

Photoelectron spectroscopy provides a direct measure of the filled density of states of a solid. The kinetic energy distribution of the electrons that are emitted via the photoelectric effect when a sample is exposed to a monocluomatic ultraviolet (UV) or x-ray beam yields a photoelectron spectrum. Photoelectron spectroscopy not only provides the atomic composition, but also infonnation conceming the chemical enviromnent of the atoms in the near-surface region. Thus, it is probably the most popular and usefiil surface analysis teclmique. There are a number of fonus of photoelectron spectroscopy in conuuon use. 

XPS is also often perfonned employing syncln-otron radiation as the excitation source [59]. This technique is sometimes called soft x-ray photoelectron spectroscopy (SXPS) to distinguish it from laboratory XPS. The use of syncluotron radiation has two major advantages (1) a much higher spectral resolution can be achieved and (2) the photon energy of the excitation can be adjusted which, in turn, allows for a particular electron kinetic energy to be selected. 

X-ray photoelectron spectroscopy (XPS) is among the most frequently used surface chemical characterization teclmiques. Several excellent books on XPS are available [1, 2, 3, 4, 5, 6 and 7], XPS is based on the photoelectric effect an atom absorbs a photon of energy hv from an x-ray source next, a core or valence electron with bindmg energy is ejected with kinetic energy (figure Bl.25.1) ... 

Figure Bl.25.1. Photoemission and Auger decay an atom absorbs an incident x-ray photon with energy hv and emits a photoelectron with kinetic energy E = hv - Ej. The excited ion decays either by the indicated Auger process or by x-ray fluorescence.

Aiiger peaks also appear in XPS spectra. In this case, the x-ray ionized atom relaxes by emitting an electron with a specific kinetic energy E. One should bear in mind that in XPS the intensity is plotted against the bindmg energy, so one uses ( Bl.25.1) to convert to kinetic energy. 

New metliods appear regularly. The principal challenges to the ingenuity of the spectroscopist are availability of appropriate radiation sources, absorption or distortion of the radiation by the windows and other components of the high-pressure cells, and small samples. Lasers and synchrotron radiation sources are especially valuable, and use of beryllium gaskets for diamond-anvil cells will open new applications. Impulse-stimulated Brillouin [75], coherent anti-Stokes Raman [76, 77], picosecond kinetics of shocked materials [78], visible circular and x-ray magnetic circular dicliroism [79, 80] and x-ray emission [72] are but a few recent spectroscopic developments in static and dynamic high-pressure research. 

If monochromatic X-rays are used as the ionizing radiation the experimental technique is very similar to that for XPS (Section 8.1.1) except that it is the kinetic energy of the Auger electrons which is to be measured. Alternatively, a monochromatic electron beam may be used to eject an electron. The energy E of an electron in such a beam is given by... 

If the energy of the iacident x-rays and the spectrometer work function are known, the measured kinetic energy can be used to determine the binding energy E from... 

Auger Electrons. The fraction of the holes in an atomic shell that do not result in the emission of an x-ray produce Auger electrons. In this process a hole in the 4h shell is filled by an electron from theyth shell, and the available energy is transferred to a kth shell electron, which in turn is ejected from the atom with a kinetic energy = E — Ej —. Usually, the most intense Auger electron lines are those from holes in the K shell and involve two... 

The use of water as a co-catalyst in Ziegler-type polymerizations was first introduced in 1962 (47). The reaction kinetics and crystallinity of the resulting polymers measured by x-ray scattering has been studied (48—51). 

The results of determination of the form of presence of As, Se, Nb, Mo, Ni, Cu in different solid compounds ai e given. The application of RII LEL for the study of stmctural transformations in chalkogenid glasses is shown. The X-ray spectral determination of crystal water, the possibility of studying of dissolution-crystallization processes and kinetics of some chemical reactions ai e discussed. 

The serine proteinases have been very extensively studied, both by kinetic methods in solution and by x-ray structural studies to high resolution. From all these studies the following reaction mechanism has emerged. 

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