Local chemical environment

Methyl rotors pose relatively simple, fundamental questions about the nature of noncovalent interactions within molecules. The discovery in the late 1930s1 of the 1025 cm-1 potential energy barrier to internal rotation in ethane was surprising, since no covalent chemical bonds are formed or broken as methyl rotates. By now it is clear that the methyl torsional potential depends sensitively on the local chemical environment. The barrier is 690 cm-1 in propene,2 comparable to ethane,... 

Another important feature is the twist of the DNA helix. DNA in its native B-form twists around its axis 360° in ten base pairs ( 34A in length), with the result that bases with the same neighbors have different local chemical environments. For instance, in the sequence 5 -AGGA-3, the two G s will experience different chemical environments even though they have the same neighboring bases. As a consequence the AG adduct is di-... 

Besides local geometries, the intelligent sketchpad can contain any local properties, including bond types and strengths, chromophore optical spectra, and nuclear magnetic resonance and infrared spectra characteristic of a local chemical environment. 

While XAS techniques focus on direct characterizations of the host electrode structure, nuclear magnetic resonance (NMR) spectroscopy is used to probe local chemical environments via the interactions of insertion cations that are NMR-active nuclei, for example lithium-6 or -7, within the insertion electrode. As with XAS, NMR techniques are element specific (and nuclear specific) and do not require any long-range structural order in the host material for analysis. Solid-state NMR methods are now routinely employed to characterize the various chemical components of Li ion batteries metal oxide cathodes, Li ion-conducting electrolytes, and carbonaceous anodes.Coupled to controlled electrochemical in-sertion/deinsertion of the NMR-active cations, the... 

The well-resolved imino proton resonances from base-paired uracil and guanine resonances serve as powerful probes to characterize RNA-ligand interaction by NMR. The chemical shift of imino proton resonances is responsive to perturbations in the local chemical environment that result from RNA conformational... 

It is well established [154] that MES probes the initial electronic structure while photoemission samples the final state, after ejection of the electron. Both MES I.S. and XPS b.e. shifts are sensitive to the local chemical environment of the atom [155, 156]. However, the I.S. reflects the electronic charge density, of predominantly s- character, within the nuclear volume, while XPS b.e. shifts are a measure of the overall electronic charge density from all the shells. 

For use in geochronology, the decay constant of a radioactive nuclide must be constant and must be accurately known. For a-decay and most (3-decays, the decay constant does not depend on the chemical environment, temperature, or pressure. However, for one mode of 3-decay, the electron capture (capture of K-shell electrons), the decay "constant" may vary slightly from compound to compound, or with temperature and pressure. This is because the K-shell (the innermost shell) electrons may be affected by the local chemical environment, leading to variation in the rate of electron capture into the nucleus. The effect is typically small. For example, for Be, which has a small number of electrons and hence the K-shell is easily affected by chemical environments, Huh (1999) showed that the decay constant may vary by about 1.5% relative (Figure l-4b). Among decay systems with geochronological applications, the branch decay constant of °K to °Ar may vary very slightly (<1% relative). 

The local chemical environment of the polymer-metal complex can be considered to be fairly different from ihat of the monomeric analog due to the effect of the polymer chain and the neighboring groups, even if part of the complex is the same in both. 

In order to identify the chemical nature of phosphorus and sulfur in complex matrices such as antiwear tribofilms, it is essential to compare the spectra of films with different model compounds in which the local chemical environments of phosphorus and sulfur are known. The high resolution of the technique allows characterization of the chemical nature of phosphorus and sulfur in the tribofilm. Investigators have shown that the chain length of polyphosphate is related to the length of alkyl groups in ZDDP. By comparison of the L-edge XANES spectra of the tribofilms with the spectra of model compounds with known structures, it has been possible to speciate the chemical nature of phosphorus and sulfur in the antiwear tribofilms. 

In the XANES technique, it is essential to compare spectra of films with those of different model compounds in which the local chemical environments of the phosphorus and the sulfur are known (Brown et al., 1992 Fuller et al., 1998). 

Next, we investigated the dependence of the chlorine atom concentration in the growing film on the process conditions. To take into account the effect of the local chemical environment of a surface chlorine atom on the probability of its reaction with a water molecule, we used the following dependence of the activation energy of this reaction on the number of the nearest neighboring Zr atoms in the chlorine coordination shell ... 

High resolution Si NMR spectroscopy can provide considerable insights into the structure and distribution of silicate and aluminosilicate anions present in solutions and gels from which zeolites are synthesized. The narrowness of individual lines and the sensitivity of the chemical shift to details of the local chemical environment make it possible in many instances to identify exact chemical structures. Studies using Si NMR have shown that the distribution of anionic structures is sensitive to pH and the nature of the cations in solution. Alkali metal cation NMR has demonstrated the formation of cation-anion pairs the formation of which is postulated to affect the dynamics of silicate and aluminosilicate formation and the equilibium distribution of these species. NMR has proven useful in identifying the connectivity of A1 to Si,... 

The absorption edges in Fig. 10.10 are not perfectly sharp, but have a delicate fine structure ("Kossel35 lines") that was first explored in the 1930s. Since about 1970, this fine structure is now used in EXAFS (extended X-ray absorption edge fine structure spectroscopy) and in XANES (X-ray absorption near edge spectroscopy) the oscillations are due, again, to a chemical shift, which can be used to identify the local chemical environment of the emitting element in the sample. 

Solid-state NMR is one of the most powerful spectroscopic techniques for the characterisation of molecular structures and dynamics.1 This is because NMR parameters are highly sensitive to local chemical environments and molecular properties. One advantage of solid-state NMR is that it enables dealing with quadrupolar nuclei, which most of the NMR-accessible nuclei are in the periodic table. Moreover, it provides an opportunity to obtain information regarding the orientation dependence of the fundamental NMR parameters. In principle, such NMR parameters are expressed by second-rank tensors and it is the anisotropy that is capable of yielding more detailed information concerning the molecular properties. 

Mossbauer spectroscopy has been quite successful in identifying catalyst components. Mossbauer spectroscopy provides quantitative site populations, easily discriminating between various metal oxidation states and anion coordinations, and it can lead to phase compositions or distributions between phases of the isotope under investigation. It also gives quantitative population distributions of local distortion environments and local chemical environments, via extracted quadrupolar splitting distributions. 

The electronic structures of silicate minerals of polymerization intermediate between nesosilicates and tektosilicates have been studied to a lesser extent than have SiOj or the olivines. The complexity of their crystal structures makes calculation difficult, and their diversity in terms of local chemical environment makes phenomenological assignment of their spectra difficult. Nonetheless, some recent comparative studies have given valuable electronic structure information on such materials. 

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