Cation site-concept

The volume properties of crystalline mixtures must be related to the crystal chemical properties of the various cations that occupy the nonequivalent lattice sites in variable proportions. This is particularly true for olivines, in which the relatively rigid [Si04] groups are isolated by Ml and M2 sites with distorted octahedral symmetry. To link the various interionic distances to the properties of cations, the concept of ionic radius is insufficient it is preferable to adopt the concept of crystal radius (Tosi, 1964 see section 1.9). This concept, as we have already noted, is associated with the radial extension of the ion in conjunction with its neighboring atoms. Experimental electron density maps for olivines (Fujino et al., 1981) delineate well-defined minima (cf figure 1.7) marking the maximum radial extension (rn, ,x) of the neighboring ions ... 

At very low coverages (9 < 0.1), the frequency of CO (singleton) adsorbed through the carbon end on Mg2+, Mg)t, and Mg(t is blue shifted with respect to the frequency of CO gas by 14,27, and 60 cm-1, respectively (v(CO) = 2157, 2170, and 2202 cm This shift is the typical result of the Stark effect associated with the positive electric field of the cation. Following Hush and Williams (55) and Pacchioni et al. (56), the shift is proportional to the strength of the moderate electric field sensed by CO when no d-electrons are involved, in agreement with the intuitive concept that to a first approximation the effective field sensed by CO adsorbed on a cationic site is the... 

It is thus evident from the structure and the electrostatic field calculations that the order of preference for strong cations, such as the alkali and alkaline-earth cations, is Si> Sn> Sm This also suggests that bivalent ions will replace univalent ions at the most preferred sites and that the higher the valence of a cation at a surface site the higher will be both the electron affinity of the cation and the field near the cation. And the stronger the field, the greater will be the polarization of adsorbed molecules and the tendency for reduction of the cation. Some of these questions relate more naturally to the electrostatic potential at a cation site than to the electrostatic field at points near the site. Nevertheless, we have so far dealt with the field, because it is the more important by far for our concept of the carboniogenic activity of the surface cations in catalysis. 

For analysis of the state of iron were employed EPR, FTTR, and Mossbauer spectroscopies. For structural interpretation of these results the concept of divalent transition metal cation siting was used as recently established for pentasil ring zeolites in a wide range of metal concentrations and Si/Al compositions. With help of UV-Vis and FTIR this approach evidenced three zeolite coordination of divalent cations in similar six-membered rings of framework local structures. Three cationic fiiamework sites were thus suggested, denoted as a, p and y. (For details see [7-11]). 

Figure 4-13shows the molar fractions of phenylephrine as functions of pH. Obviously in phenylephrine deprotonation of die ammonium cationic site is preferred to deprotonation of the phenolic function. Details of basic principles, hardware and software concepts and applications of NMR-controHed titrations are described in [14], [15], [16], [17], [18], [19], [20], [21], [22], Results from more laborious UVA7is [23], [24] and now the easily accessible automated NMR-controlled titrations of phenylephrine [22] are consistent The method of NMR-controlled titration is a powerful tool in analytical and structural chemistry, and was recently applied successfully to a series of biorelevant phosphorus compounds [18],[19],[20],[21],... 

The active site is viewed as an acid-base, cation-anion pair, hence, the basicity of the catalyst depends not only on the proton affinity of the oxide ion but also on the carbanion affinity of the cation. Thus, the acidity of the cation may determine the basicity of the catalyst. Specific interactions, i.e., effects of ion structure on the strength of the interaction, are likely to be evident when the carbanions differ radically in structure when this is likely the concept of catalyst basicity should be used with caution. 

In this equation rA is the radius of the cage site cation, rB is the radius of the octahedrally coordinated cation, and rx is the radius of the anion. The factor l is called the tolerance factor. Ideally, t should be equal to 1.0, and it has been found empirically that if t lies in the approximate range 0.9-1.0, a cubic perovskite structure is stable. However, some care must be exercised when using this simple concept. It is necessary to use ionic radii appropriate to the coordination geometry of the ions. Thus, rA should be appropriate to 12 coordination, rB to octahedral coordination, and rx to linear coordination. Within this limitation the tolerance factor has good predictive power. 

This section describes the nucleophilic reactions—acyl transfer reactions mostly—promoted by micelles and polysoaps. The nucleophiles are imidazoles, oxyanions and thiols, the same catalytic groups found ubiquitously in the enzyme active site. These nucleophiles are remarkably activated in the anionic form in the presence of cationic micelles and cationic polysoaps. These results are explained by the concept of the hydrophobic ion pair (Kunitake et al.,... 

Let us consider a multisite mixture of type (A, B, C,. . . ) (M, N, O,. . . ) Z, where cations of types A, B, C and M, N, O, respectively, occupy energetically distinct sites present in one mole of substance in the stoichiometric amounts Vj and V2, and Z is the common anionic group. Applying the permutability concept to each distinct site and assuming random mixing and the absence of interactions on sites, the activity of component A M Z in the mixture may be expressed as... 

Essentially, sublattice models originate from the concepts of Temkin (1945) who proposed that two separate sublattices exist in a solid-state crystal for cations and anions. The configurational entropy is then governed by the site occupation of the various cations and anions on their respective sublattices. When the valence of the cations and anions on the sublattices are equal, and electroneutrality is maintained, the model parameters can be represented as described in Section 5.4.2. However, when the valence of the cations and anions varies, the situation becomes more complex and some additional restrictions need to be made. These can be expressed by considering equivalent fractions (/) which, for a sublattice phase with the formula (/, . .. )(M"", . ..), are given by... 

Reducibility of the Cations. The removal of lattice oxygen from the active site is accompanied by the reduction of the neighboring cation. Thus another way to view the ease of removal of lattice oxygen is to look at the reduction potential of the cations at the active site. This concept was tested using a series of orthovanadates of the formula M3(V04)2, where M = Mg, Zn, Ni, and Cu, and MVO4, where M = Fe, Sm, Nd, and Eu. The cations in these two series were chosen to span a range of (aqueous) reduction potentials from -2.40 V to +0.77 V. Orthovanadates are made up of isolated VO4 units that are separated from each other by MO units. Thus there are only M-O-V bonds in these structures and no V-O-V bonds, and the difference in the ease of removal of a lattice oxygen should depend on the difference in the reduction potential of the M ion. 

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