Chemical surrounding

Extended X-ray absorption fine structure (EXAFS) measurements based on the photoeffect caused by collision of an inner shell electron with an X-ray photon of sufficient energy may also be used. The spectrum, starting from the absorption edge, exhibits a sinusoidal fine structure caused by interferences between the outgoing and the backscattered waves of the photoelectron which is the product of the collision. Since the intensity of the backscattering decreases rapidly over the distances to the next neighbor atoms, information about the chemical surroundings of the excited atom can be deduced. 

As a result of different chemical surroundings the resonance frequency absorption will show definite peaks registered by Fourier transform technique. Compared to an internal standard the intensity of the signals is used for quantitative determination of different phosphorus-containing compounds in a given sample. 

NEP and PVP. In the polymer where monomers are linked in a chain, hydrophobic parts are largely screened from interactions with the solvent. For free monomers such screening is not possible so that they experience more unfavourable interactions with the solvent. The adsorption energy parameter xs is not affected by the different chemical surrounding of free monomer and polymer segments, since the mechanism for interaction with the surface is hydrogen bonding in both cases (8).)... 

In order to dissipate the recoil energy Mossbauer was the first to use atoms in solid crystal lattices as emitters and also to cool both emitter and absorber. In this way it could be shown that the 7-ray emission from radioactive cobalt metal was absorbed by metallic iron. However, it was also found that if the iron sample were in any other chemical state, the different chemical surroundings of the iron nucleus produce a sufficient effect on the nuclear energy levels for absorption no longer to occur. To enable a search for the precisely required absorption frequency, a scan based on the Doppler effect was developed. It was noted that a velocity of 102 ms-1 produced an enormous Doppler shift and using the same equation (7) it follows that a readily attainable displacement of the source at a velocity of 1 cms-1 produces a shift of 108 Hz. This shift corresponds to about 100 line-widths and provides a reasonable scan width. 

Fatty acids are clearly larger in size and show markedly slower diffusion velocity than the small water (or creatine) molecules which have been examined so far by diffusion weighted NMR spectroscopy. However, assessment of diffusion properties of lipids could be a key step for further experimental studies of skeletal muscle lipid metabolism. Diffusion properties of FFA and triglycerides are likely different due to differences in molecular weight. In addition, effects of temperature, chemical surroundings, and the mobility of small lipid droplets in the cytosol may also lead to measurable differences in the diffusion characteristics. 

In a study with 3 EMS patients the relative signal intensities in proton spectra recorded at an echo time of 50 ms with a repetition time of 2 s were quantitatively compared with healthy controls. It was evident that the muscle specific signals from TMA and Cr are relatively low in EMS patients (TMA 0.20-0.43% in the patients and 0.47-0.68% in the controls creatine 0.32-0.60% in the patients and 0.48-0.77% in the controls) probably indicating changes of concentrations and altered chemical surroundings of TMA and Cr in the myoplasm of the diseased cells. The overall lipid signal was found to be... 

We intend in this chapter to consider the manner in which the symmetry of the chemical surroundings of an ion determines the effect of this environment on the energy levels of the ion. In the crystal field and ligand field theories we often wish to regard the effect of the environment as a small perturbation on the states of the free ion. For the benefit of readers not acquainted with certain general features of the electronic structures of free atoms and ions, a brief resume of the subject is given in this section. 

If the Mossbauer isotope in the source and absorber is situated in identical chemical surroundings, the nuclear energy levels in both will be identical and the observed resonance will occur at zero relative velocity (5E = 0). Different chemical environments, however, result in different perturbations... 

X-ray photoelectron spectroscopy of atomic core levels (XPS or ESCA) is a very powerful tool for characterization of the chemical surrounding of atoms in molecules. In particular, since the method is very surface sensitive, it is possible to monitor the first stages of the interface formation, i.e., in our case the interaction between individual metal atoms and the polymer. Standard core level bonding energies are well known for common materials. However, in our case, we are studying new combinations of atoms and new types of structures for which there are no reference data available. In order to interpret the experimental chemical shifts it is useful to compare with theoretical estimates of the shifts. 

Nitroxide radicals are widely used as spin labels in biology, biochemistry and biophysics to gain information about the structure and the dynamics of biomolecules, membranes, and different nanostructures. Their widespread use is related to an unusual stability, which allows researchers to label specific sites and to detect the most informative EPR parameters (g and hyperfme tensors) that are very sensitive to interactions with the chemical surroundings. Figure 2.1 collects all the radicals used in the following to illustrate the different aspects mentioned in the preceding section. 

The fluorescence lifetime of a fluorophore is highly sensitive to its molecular environment. Many macromolecular events, such as rotational diffusion, resonance-energy transfer, and dynamic quenching, occur on the same timescale as the fluorescence decay. Thus, time-resolved fluorescence spectroscopy can be used to investigate these processes and gain insight into the chemical surroundings of the fluorophore. 

Behrens, P. Voids in variable chemical surroundings Mesoporous metal oxides, Angew. Chem. Int. Ed, 1996, 35, 515-518. 

Sol-gel matrices can also provide a chemical surrounding that favors enzymatic reactions. Lipases act on ester bonds and are able to hydrolyze fats and oils into fatty acids and glycerol. These are interphase-active enzymes with lipophilic domains and the catalytic times reaction occurs at the water-lipid interface. Entrapped lipases can be almost 100 times more active when a chemically modified silica matrix is used. The cohydrolysis of Si(OMe)4 and RSi(OMe)3 precursors provides alkyl groups that offer a lipophihc environment that can interact with the active site of Upases and increase their catalytic activity. Such entrapped lipases are now commercially available and offer new possibilities for organic syntheses, food industry, and oil processing. ... 

Effect of excitation mode and chemical surroundings on single-hole filling... 

The sensitivity and selectivity of catalytic and photocatalytic reactions to small electronic and structural perturbations in the chemical surroundings of the active sites are crucial to understanding the mechanisms involved. 

NMR spectroscopy is a very useful tool for determining the local chemical surroundings of various atoms. Komin et al studied theoretically this for the adenine molecule of Fig. 20 both in vacuum and in an aqueous solution using different computational approaches. In all cases, density-functional calculations were used for the adenine molecule, but as basis functions they used either a set of localized functions (marked loc in Table 45) or plane waves (marked pw). Furthermore, in order to include the effects of the solvent they used either the polarizable continuum approach (marked PCM) or an explicit QM/MM model with a force field for the solvent and a molecular-dynamics approach for optimizing the structure (marked MD). In that case, the chemical shifts were calculated as averages over 40 snapshots from the molecular-dynamics simulations. Finally, in one case, an extra external potential from the solvent acting on the solute was included, too, marked by the asterisk in the table. 

It is well known that /C-x-ray spectra depend on the chemical surroundings of the atom (13) and extensive experimental studies for the chemical effects on the KjSiKoc x-ray intensity ratios of 3d transition elements have been reported (14). Recently we have measured the K/3 /K a ratios for some chemical compounds of Cr and Mn with a high-energy resolution spectrometer and compared the experimental results with the theoretical calculations (15,16). Several theoretical estimations of the chemical effects on the /C-x-ray intensity ratios have also been made (ISIS). 

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