Electron density modification

The second type of electron-density modification is noncrystallo-graphic symmetry averaging. If there is more than one copy of the molecule in the asymmetric unit, and the relationship between them is known (for example, from the rotation function), constraints can be placed on the electron density map to make the two molecules look approximately the same. This information can then be used to improve the electron density, and hence the relative phases of the structure, and... 

Often the least-squares refinement is combined with an EDM (electron density modification) procedure the advantage is that it is not necessary to interpret electron density peaks in terms of atomic species. In practice the map is modified by a suitable function g to obtain a better representation p of the structure ... 

It was previously shown (15) that the selectivity for O-xylene hydrogenation (cis and trans dimethylcyclohexane) is sensitive to electronic density modification. In order to estimate electronic transfer on Pt catalysts induced by Au addition, Table 4 shows that it was not observed any change in selectivity for the 0-xylene reaction on Pt-Au catalysts. 

Such change in selectivity could be explained by assuming that gold, by a ligand effect, could change the electronic properties of platinum. The selectivity of o-xylene hydrogenation reaction (cis and trans dimethylcyclohexane) has shown to be sensitive to electronic density modifications... 

On the other hand the electronic density modifications due to the coordination of the monomer, as well as substitution (in ohain... 

This modification of the charge interaction is responsible for shifts in the electron density as permitted by the polarizability of the molecule. 

Summarizing, the introduction of nitrogen at the place of C-3 in thiophene does not deeply disturb the electronic environment of the sulfur atom, but it induces in the rest of the molecule some alternating modification of the electronic density (Figs. 1-3 and 1-4). The perturbations induced by the nitrogen in the tt bond order of thiophene are... 

The atom and bond concepts dominate chemistry. Dalton postulated that atoms retained their identities even when in chemical combinations with other atoms. We know that their properties are sometimes transferable from one molecule to another for example, the incremental increase in the standard enthalpy of formation of a normal hydrocarbon per CHj group is —20.6 1.3 kJmol . We also know that more often there are subtle modifications to the electron density. 

The molecular chemisorption of CO on various alkali-modified metal surfaces has been studied extensively in the literature. It is well established that alkali modification of the metal surface enhances both the strength of molecular chemisorption and the tendency towards dissociative chemisorption. This effect can be attributed to the strongly electropositive character of the alkali, which results in donation of electron density from the alkali to the metal and then to the adsorbed CO, via increased backdonation into the... 

The expressions (4.22)-(4.23) found in chap. 4 for the isomer shift 5 in nonrelativ-istic form may be applied to lighter elements up to iron without causing too much of an error. In heavier elements, however, the wave function j/ is subject to considerable modification by relativistic effects, particularly near the nucleus (remember that the spin-orbit coupling coefficient increases with Z ). Therefore, the electron density at the nucleus l /(o)P will be modified as well and the aforementioned equations for the isomer shift require relativistic correction. This has been considered [1] in a somewhat restricted approach by using Dirac wave functions and first-order perturbation theory in this approximation the relativistic correction simply consists of a dimensionless factor S (Z), which is introduced in the above equations for S,... 

The extra oxygen decreased the activation energy of C-H bond scission of the methoxy, which is the rate-limiting step of the selective methanol oxidation. TPD spectra of CO indicate that extra oxygen species reduce the electron density of Mo atoms in MoNC rows. This modification causes the decrease of the activation energy for the methoxy dehydrogenation. The extra oxygen is... 

The crystallinity of organic pigment powders makes X-ray diffraction analysis the single most important technique to determine crystal modifications. The reflexions that are recorded at various angles from the direction of the incident beam are a function of the unit cell dimensions and are expected to reflect the symmetry and the geometry of the crystal lattice. The intensity of the reflected beam, on the other hand, is largely controlled by the content of the unit cell in other words, since it is indicative of the structural amplitudes and parameters and the electron density distribution, it provides the basis for true structural determination [32],... 

Further inspection of Fig. 4.5 demonstrates that the O atoms are surrounded by dark zones. The STM technique probes not only the atomic topography but also the electronic structure, and the dark zones reflect the modification of the local electronic density in the vicinity of the adsorbates, this modification being responsible for the operation of indirect interactions (which may be either repulsive or attractive) between adsorbed particles mediated through the substrate. 

In the following section, we describe the case of adsorption of a Sn complex onto a palladium oxide suspension. In an alkaline medium (a basic PdO hydrosol), chlorides in the SnCL complex are substituted in the coordination sphere of tin(IV) by hydroxo anions, which are in excess, yielding the stannate Sn(OH)g complex. The Sn Mossbauer spectroscopy spectrum of a bimetallic sol (frozen in liquid nitrogen) is compared with a true stannic solution. At the same tin concentration, it shows the changes in the Sn environment due to adsorption onto the PdO surface (Fig. 13.27). The isomer shift S is found to be close to zero for the stannate solution and increases when contacted with the PdO suspension, indicating a modification of the coordination sphere of tin. The increase in 5 can be correlated to an increase in the core level electronic density of tin. The quadrupole splitting A, is related to a modification of the symmetry of the close environment of tin, due to adsorption of Sn(OH)g complexes onto the PdO colloidal nanoparticles. 

Figure 7. Crystalline (LT) and plastic (HT) phases of MjPy and MjPu. Key a, structure of the orthorhombic LT-NajPj as an example for the ordered crystalline low-temperature modifications (See Table I. The relation of the LT-phase to the HT-phase is indicated by a dotted line.) b, structures of the cubic HT-MjPj and HT-M3P11 as examples for the plastically crystalline phases c, temperature-dependent Raman spectra of Na Py and d, typical electron density in the plastically...

Several steps were needed to determine the structure of the core particle to higher resolution (Fig. Id). The X-ray phases of the low-resolution models were insufficient to extend the structure to higher resolution, since the resolution of the early models of the NCP was severely limited by disorder in the crystals. The disorder was presumed to derive from both the random sequences of the DNA and from heterogeneity of the histone proteins caused by variability in post-translational modification of the native proteins. One strategy for developing an atomic position model of the NCP was to develop a high-resolution structure of the histone core. This structure could then be used with molecular replacement techniques to determine the histone core within the NCP and subsequently identify the DNA in difference Fourier electron density maps. 

ESR methods unambiguously establishes the presence of species bearing unpaired electrons (ion-radicals and radicals). The ESR spectrum quantitatively characterizes the distribution of electron density within the paramagnetic particle by a hyperfine structure of ESR spectra. This establishes the nature and electronic configuration of the particle. A review by Davies (2001) is highly recommended as a guide to current practice for ESR spectroscopic studies (this quotation is from the title of the review). The ESR method dominates in ion-radical studies. Its modern modifications, namely, ENDOR and electron-nuclear-nuclear triple resonance (TRIPLE) and special methods to observe ion-radicals by swiftness or stealth are described in special literatures (Moebius and Biehl 1979, Kurreck et al. 1988, Werst and Trifunac 1998). 

The ability of polypyridyl ligands to accept electron density from electron-rich rhenium centers and, thus, to contribute to the stabilization of rhenium complexes with the metal in low oxidation states has already been discussed for rhenium(II) compounds. Only small modifications to the polypyridyl ligand or the metal center can create dramatic differences in the properties of the resulting complexes. Generally, the starting materials which have been introduced as precursors for rhenium(II) polypyridyl complexes in Section 5.3.2.6.2, are also appropriate for the synthesis of related rhenium(l) compounds. 

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