Cation site

The superacid-catalyzed cracking of hydrocarbons (a significant practical application) involves not only formation of trivalent carbo-cationic sites leading to subsequent /3-cleavage but also direct C-C bond protolysis. 

Single-Stack Acceptor. Simple charge-transfer salts formed from the planar acceptor TCNQ have a stacked arrangement with the TCNQ units facing each other (intermolecular distances of ca 0.3 nm (- 3). Complex salts of TCNQ such as TEA(TCNQ)2 consist of stacks of parallel TCNQ molecules, with cation sites between the stacks (17). The interatomic distance between TCNQ units is not always uniform in these salts, and formation of TCNQ dimers (as in TEA(TCNQ)2) and trimers (as in Cs2(TCNQ)Q can lead to complex crystal stmctures for the chainlike salts. 

Fig. 2. Stmcture of the mineral 2eohte chaba2ite is depicted by packing model, left, and skeletal model, right. The sihcon and aluminum atoms He at the corners of the framework depicted by soHd lines. In this figure, and Figure 1, the soHd lines do not depict chemical bonds. Oxygen atoms He near the midpoint of the lines connecting framework corners. Cation sites are shown in three different locations referred to as sites I, II, and III.

W, X, Y each represent cationic sites of different geometries (12). From this generic representation, the chemical composition of the amphibole asbestos can be given as foUows ... 

Amphiboles. The crystalline stmcture common to amphibole minerals consists of two ribbons of siUcate tetrahedra placed back to back as shown in Figure 5. The plane of anionic valency sites created by this double ribbon arrangement is neutralized by the metal cations. The unit cell has seven cationic sites of three different types these sites can host a large variety of metal cations without substantial dismption of the lattice. 

ACID DYES Commercial acid dyes contain one or more sulfonate groups, thereby providing solubility in aqueous media. These dyes are apphed in the presence of organic or mineral acids (pH 2—6). Such acids protonate any available cationic sites on the fiber, thereby making possible bonding between the fiber and the anionic dye molecule. Wool, an animal fiber, is an amphoteric coUoid, possessing both basic and acidic properties because of the amino and carboxylic groups of the protein stmcture. In order to dye such a system, coulombic interactions between the dye molecule and the fiber must take place ie, H2N" -wool-COO + H2N" -wool-COOH. The term acid dye is appHed to those that are capable of such interactions. Acid dyes... 

Copper(I) oxide [1317-39-1] is 2lp-ty e semiconductor, Cu2 0, in which proper vacancies act as acceptors to create electron holes that conduct within a narrow band in the Cu i7-orbitals. Nickel monoxide [1313-99-17, NiO, forms a deficient semiconductor in which vacancies occur in cation sites similar to those for cuprous oxide. For each cation vacancy two electron holes must be formed, the latter assumed to be associated with regular cations ([Ni " h = Semiconduction results from the transfer of positive charges from cation to cation through the lattice. Conduction of this type is similar... 

The vacant sites will be distributed among the N lattice sites, and the interstitial defects on the N interstitial sites in the lattice, leaving a conesponding number of vacancies on die N lattice sites. In the case of ionic species, it is necessaty to differentiate between cationic sites and anionic sites, because in any particular substance tire defects will occur mainly on one of the sublattices that are formed by each of these species. In the case of vacant-site point defects in a metal, Schottky defects, if the number of these is n, tire random distribution of the n vacancies on the N lattice sites cair be achieved in... 

In all of these oxide phases it is possible that departures from the simple stoichiometric composition occur dirough variation of the charges of some of the cationic species. Furthermore, if a cation is raised to a higher oxidation state, by the addition of oxygen to tire lattice, a conesponding number of vacant cation sites must be formed to compensate tire structure. Thus in nickel oxide NiO, which at stoichiomen ic composition has only Ni + cations, oxidation leads to Ni + ion formation to counterbalance the addition of extra oxide ions. At the same time vacant sites must be added to the cation lattice to retain dre NaCl sUmcture. This balanced process can be described by a normal chemical equation thus... 

Here Oq represents the oxide ion which is incorporated, IT represents the Ni + ion which is a positive hole, and an exU a negative charge being indicated by the superscript dot, thus V/,j is the vacant cation site where tire double dot represents the absence of two positive charges at that site. 

The cationic ring opening polymerization of oxolane (THF) or of N-substituted aziridines can be initiated by oxocarbenium salts [42]. The methacrylic ester unsaturation is insensitive to cationic sites, and polyoxolanes (poly-THF) macromonomers are obtained in good yields. 

Variations in interlayer cation sites in clay minerals as studied by Cs MAS nuclear magnetic resonance spectroscopy. Am. Mineral. 75 970 (1990). ... 

The kinetics outlined above, first observed empirically by Giintherschulze and Betz, were modelled by Verwey" with the rate-controlling energy barrier being that between to adjacent cation sites within the oxide film. The same basic form can be derived if the rate-controlling energy barrier is that between a metal atom on the metal surface and an adjacent cation site in the film. The rate is then limited by ion injection into the film rather than... 

ZnO contauns excess metal which is accommodated interstitially, i.e. at positions in the lattice which are unoccupied in the perfect crystal. The process by which ZnO in oxygen gas acquires excess metal may be pictured as follows. The outer layers of the crystal are removed, oxygen is evolved, and zinc atoms go into interstitial positions in the oxide. We represent interstitial zinc by (ZnO). However, the interstitial zinc atoms may ionise to give (Zn O) or even (Zn O). The extra electrons produced in this way must occupy electron levels which would be vacant in the perfect crystal. We represent them by the symbol (eo), and refer to them as free electrons. They can be pictured as Zn ions at normal cation sites. We see therefore that three reactions can be written, each giving non-stoichiometric ZnO ... 

The rate of isotopic exchange in the solid state, between cobalt in the cation and in the anion of [60Co(H2O)6] [Co(edta)]2 4 H20, was increased [1144] by irradiation (100 Mrad) of the reactant. It was concluded that exchange occurred via vacancies, rather than through motion of a ring of cobalt atoms, one from a cationic site and the other from a neighbouring anionic site. 

The lower yield for olefin than for S02 was explained by scavenging of the formed free olefin on the cationic sites of the irradiated polymer in a homopolymerization reaction, thus reducing G(olefin). Adding cation scavengers it was found that the overall product yield was reduced with concurrent reduction of the S02/olefm ratio towards unity73. Thus it can be concluded that the homopolymerization of the olefin is occurring by a cationic mechanism. 

Fig. 5.18 View of the structure of Cs2[(Zr6B)ClT 1.9813.02] along a showing the Cs cation sites as located in channels along the view direction. Thermal ellipsoids are draws at the 50% level.

The endogenous activity of antithrombin III is gready potentiated by the presence of acidic proteoglycans such as heparin (Chapter 48). These bind to a specific cationic site of antithrombin III, inducing a conformational change and promoting its binding to... 

PMo 12-polymer composite film catalyst [9]. This demonstrates that PM012 catalyst was not in a crystal state but in an amorphous-like state, indicating that PM012 catalyst was molecularly dispersed on the PS support via chemical interaction. As attempted in this work, it is believed that heteropolyanions (PMoi204o ) were strongly immobilized on the cationic sites of the PS bead as charge-compensating components. 

These, then, cire the set of possible defects for the Plane Net, and the following summarizes the types of intrinsic defects expected. Note that we have used the labelling V = vacancy i = interstitial M = cation site X = anion site and s = surface site. We have already stated that surface sites are special. Hence, they are included in our listing of intrinsic defects. 

The latter deals with an impurity aoioa on a cation site coupled with an impurity cation on an anion site, both with the proper charge. We have mentioned interstices but not in detail. They appear as a function of structure (Refer back to Chapter 1 - Diagram 1.3.2.). There is one in a tetreihedron, four in a body-centered cube, and six in a simple cube. Thus, a in aVi is 1, 4 or 6, respectively. We shall need this symbol later, as well as Vi, the unoccupied interstitial. 

The explanation lies in the defect reactions controlling the formation of the phosphor itself. The defect reactions occurring were found to be the substitution of a trlvalent cation on a divalent site and the defects reactions thereby associated. This is shown in the following table which compares these two methods of preparing such materials. In this case, the increase in brightness was found to be related to the amount of activator actually being incorporated into the lattice. It is well known that phosphor brightness is proportional to the numbers of Sb3+ ions (activators) actually incorporated into cation sites of the pyrophosphate structure. 

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