Cation environments

Laird DA, Barriuso E, Dowdy RH, Koskinen WC (1992) Adsorption of atrazine on smectites. Soil Sci Soc Am J 56 62-67 LeBaron PC, Wang Z, Pinnavaia TJ (1999) Polymer-layered silicate nanocomposites an overview. Appl Clay Sci 15 11-29 Lee J-F, Crum JR, Boyd SA (1989) Enhanced retention of organic contaminants by soil exchanged with organic cations. Environ Sci Technol 23 1365-1372 Lee J-F, Mortland MM, Boyd SA, Chiou CT (1989a) Shape-selective adsorption of aromatic molecules from water by tetramethylammonium-smectite. J Chem Soc Faraday Trans I 8 2953-2962... 

Lee, J.E, Crum, J.R., Boyd, S.A. (1989) Enhanced retention of organic contaminants by soils exchanged with organic cations. Environ. Sci. Technol. 23, 1365-1372. 

Figure 4.5 Structure of pyroxene minerals (a) demonstration of the end view of the single silicate chain (b) end view of the stacking arrangement of single chains, showing the position of the metal cations. There are two different cationic environments, Ml and M2. (After Putnis, 1992 Figure 6.11, by permission of Cambridge University Press.)...

In the case of LDH hosts, since only the average structure is known, it is important to consider the local structure by means of X-ray absorption spectroscopy extended X-ray absorption fine structure (EXAFS). According to an ideal model based on edge-sharing octahedra [64], it is possible to define the local environment around each type of cation in relation to the layer-charge density. Figure 11 shows the local cation environment for LDH with... 

It is clear from the above observations that pyridine molecule interacts on the catalyst surface in the following three modes (1) interaction of the N lone pair electron and the H atom of the OH group, (2) transfer of a proton from surface OH group to the pyridine forming a pyridinium ion (Bronsted acidity), and (3) pyridine coordination to an electron deficient metal atom (Lewis acidity). Predominant IR bands, vga and vigb, confirms that the major contribution of acidity is due to Lewis acid sites from all compositions. Between the above two modes of vibrations, Vsa is very sensitive with respect to the oxidation state, coordination symmetry and cationic environment [100]. A broad feature for v a band on Cu containing... 

The term secondary bond was used by Alcock (1972) to describe the longer bonds that occur in the electronically distorted cation environments described in Chapter 8, particularly those around atoms with stereoactive lone pairs. The term tertiary bond has been used here to avoid confusion with Alcock s secondary bonds. 

Cation coordination numbers using Rule 4.1 as a criterion have been determined for some 14000 cation environments (Brown 1988a). Some cations, such as are known with only one coordination number (4 in this case), but others, such as Cs+, can be found with every coordination number between 3 and 12. Whatever the total range, the frequency distribution of the coordination number for a given cation usually peaks close to the average. It is therefore convenient to take the average observed coordination number as a characteristic chemical property of the cation. 

Figure 6.4 shows the effective valence as a function of 0-0 distance for a variety of known and unknown cation environments, the unknown environments being shown in italics. The observed environments, shown in bold type, mostly lie to the right of the solid line which is given by eqn (6.2). This line can therefore be taken as the closest distance that two oxygen atoms can approach... 

Instead of MV2+ (in the photo-oxidation of leuco crystal violet (LCV)), a neutral species is sensitized by pyrene containing polymers and the Coulombic effect is not as drastic as in the case of MV2+. As shown in Figure 8, the cationic polymer is more effective than the neutral or anionic polymer. This is attributed to the Coulombic repulsion between LCV- and Py assisted by the cationic environment of the polycation. However, the Coulombic effect occurs only after forward electron transfer. 

Figures 20A and B show the PL spectra, recorded at 290 K, at 600 nm, and as a function of pressure, for Cs9(SmW10O36) and SmWi0O36-LDH, respectively (Park et al., 2002). For the sake of comparison, the line shapes are normalized and displaced along the vertical axis. In both cases, the peak position is red-shifted by 4—5 nm when the hydrostatic pressure increases from 1 bar to 61 kbar. It was shown that the red-shift from A to A lies solely in the deformation of the samarium complexes by the uniaxial stress exerted by the host layers, whereas the shift from B to B is also influenced by the change in the cation environment. Under the same conditions, B is not at the same position for the non-intercalated (HN (n -b u t y 1) 3) 9 (SmW10O3e) and Cs9(SmWi0O36) compounds (Park et al., 2002). Thus only peak A is available to measure the unixial stress. This observation can be used to determine the uniaxial stress, when the external pressure is zero. For the SmW10O36—LDH system, the uniaxial stress varies significantly from 75 at 28 K to 140 kbar at 290 K (Park et al., 2002).

In cases where there is a low concentration of cation of interest, if the cations are highly disordered in the zeolite framework, or if good crystalline samples are unavailable, atom specific or environment-specific spectroscopic probes may be preferable to determine local structures about the cation in the zeolite. NMR (4), IR ( 5, 6) ESR (7-10), optical (9,10), MSssbauer effect (11-15), and x-ray absorption studies (2,16,17,18) have been used to determine cation microenvironments. In particular, it has been shown that EXAFS (Extended X-ray Absorption Fine Structure) of the cation can often be used to give direct structure information about cation environments in zeolites, but EXAFS techniques, while giving radial distances and relative coordination numbers, are insensitive to site symmetry and cannot, in general, give both coordination numbers and relative site populations. Clearly it is desirable to use complementary spectroscopic techniques to fully elucidate the microenvironments in dilute, polycrystalline zeolite systems. 

Table 7.8. Effect of the cationic environment of water molecules on the O-H 0 hydrogen-bond lengths [441, 442]... 

Mn XANES K spectra are practically identical and independent of nature of cations in the compounds. Studying the pre-threshold structure of the spectra (Figure 3) shows that the system of non-occupied MO in the Mn clusters does not vary in the series of the manganates. It is evident that any noticeable covalent interaction between the Mn complex anion and its cation environment is absent. 

An interesting benzothiazarsolium arsenic cation (49) has been isolated from the reaction of the chlorobenzoth-iazarsole (48) with AICI3 in CH2CI2 (eqnation 30). The spectroscopic data and the X-ray structure confirm in (49) a dicoordinate cationic environment around arsenic. Part of the... 

Most of the enzymatic reactions that utilize the naturally occurring diamagnetic Mg ion still occur when the Mg is substituted by the paramagnetic Mrf ion, ofren with little or no loss of activity. Since Mn is present at much lower concentrations than Mg in the cell, Mn does not play a role in vivo. The substitution studies are nonetheless useful, since Mn can be used as a probe for cation environment. An important class of enzymes that have been studied in this way are those enzymes that utilize nucleoside triphosphates, including elongation factors and kinases. 

The reversibility of the redox cycle involving and Oj was established for Fe by Boudart and co-workers (2) using Mossbauer spectroscopic techniques. They proposed that the oxygen was held between two Fe cations. Fu et al. (3) showed that Fe-Y acted as a redox catalyst for reactions of CO with NO, CO with Oj, and N2O with CO. The ability of Fe-Y to decompose NjO into its elements was established in the work of Hall and co-workers (4), who also showed that Fe-Mordenite was as active as Fe-Y, despite containing only 16% as much Fe as its Y-zeolite counterpart. This difference in catalytic activity was thought to result from differences in the environments of the Fe within the zeolite structures. The objective of the present study was to alter the cation environment and relate that environment to the catalytic activity this was accomplished by varying the silicon-to-aluminum ratio of Y-zeolite and by coexchanging Fe with Eu. 

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