Topotactic mechanism

In the case of the formation of ATLS from lanthanum oxide, the structure of lanthanum hydroxide as an intermediate allows the formation of apatite through the topotactic mechanism. Thus, the coordination number (CN) of lanthanum in lanthanum oxide is 7, while there are 7- and 9-coordinated lanthanum cations in the apatite (Table 6). The lanthanum oxide hydration leads to increase of lanthanum CN from 7 to 9. This is also accompanied by the lattice expansion in the a and b dimensions and its contraction in the c dimension (Table 6). Subsequent acid-base interaction between the lanthanum hydroxide and silica or intermediate amorphous silicate results in the silicon incorporation into the structure to form apatite structure. This leads to the additional lattice expansion in three dimensions and the decrease of CN from 9 to 7 for 64% of lanthanum cations. 

It is appropriate to emphasize again that mechanisms formulated on the basis of kinetic observations should, whenever possible, be supported by independent evidence, including, for example, (where appropriate) X-ray diffraction data (to recognize phases present and any topotactic relationships [1257]), reactivity studies of any possible (or postulated) intermediates, conductivity measurements (to determine the nature and mobilities of surface species and defects which may participate in reaction), influence on reaction rate of gaseous additives including products which may be adsorbed on active surfaces, microscopic examination (directions of interface advance, particle cracking, etc.), surface area determinations and any other relevant measurements. 

Two different pathways of formation are possible (Stanjek, 2000). One route involves aerial oxidation of lithogenic magnetite as suggested for Brazilian Oxisols on basic igneous rocks. The mechanism of this topotactic reaction is described in Chapter 14. These maghemites are usually titaniferous as are the magnetites from which they are derived (see Chap. 15) and almost free from or very low in Al (Allan et al., 1989). Their unit cell size is a function of the residual Fe" and the Ti content. 

Toda et al. reported that the topotactic and enantioselective photodimerization of coumarin and thiocoumarin takes place in single crystals without significant molecular rearrangements [49]. Molecular motion needs to be called upon to explain the photochemically activated cycloaddition reaction of 2-benzyl-5-benzylidenecyclopentanone. The dimer molecules, once formed, move smoothly in the reactant crystal to form the product crystal [50]. Harris et al. investigated the reactivity of 10-hydroxy-10,9-boroxophenanthrene in the solid state and the mechanism of the solid-state reaction was characterized by both X-ray diffraction and thermal analysis [51]. It was demonstrated that the solution chemistry of 10-hydroxy-10,9-boroxophenanthrene is different from that in the solid state, where it undergoes dimerization and dehydration to form a monohydride derivative. 

Solid-state reactions are known from thermal intracrystalline conversions (isomerizations or loss of volatile fragments), photoreactions, gas-solid reactions, and solid-solid reactions. As all of these relate strictly to the crystal packing (unifying solid-state mechanism) and are not separated in the various sections. Also, nontopotactic (normal) and topotactic (very rare) reactions are not separated in different sections. 

Thus, the solid-state photoisomerization of 10 proceeds via a topotactic reaction mechanism, while some (Z,Z)-muconic esters (e.g., 13 and 15) can pho-toisomerize to the corresponding ( , )-muconic esters (14 and 16) without any change in the space group, i.e., proceed via topochemical EZ isomerization (Scheme 13, Fig. 15) [115]. 

The initial nucleation stage of the complex-decomposition mechanism is probably similar to the simple free-anion mechanism. Either ionic or molecular metal species (ion-by-ion) or Cd(OH)2 (cluster) adsorbs on the substrate. However, instead of conversion of the hydroxide to sulphide by topotactic reaction with sulphide ions, the chalcogenide precursor (in almost all studies of this mechanism, that is thiourea) adsorbs on the Cd(OH)2 surface to form a hydroxide-thiourea complex, which then decomposes to CdS. 

Convincing evidence for phase separation was obtained from the photopolymerization behavior of 6 in the mixed 6/DSPE monolayer films. Photopolymerization of diacetylenes is a topotactic process which requires the proper alignment of the 1,3-diyne moieties [35]. Thus diacetylenes typically polymerize rapidly in the solid state but not in solution. Polymerization is triggered by ultraviolet irradiation and proceeds via a 1,4-addition mechanism yielding a conjugated ene-yne backbone (Fig. 5). The reaction can be followed by the growth of the visible absorption band of the polymer. 

Transmission of directional properties from the monomer lattice reveals that many photopolymerizations of the four-center type are topotactic, but this information is insufficient to determine whether the reactions occur via homogeneous or heterogeneous phase-separation mechanisms. Powder x-ray studies done in... 

The discussion above left aside an entire class of CPs, the polydiacetylenes (PDAs) (see Table 1). Their study has followed a path similar to that of other CPs, but with only occasional intersections. Indeed, they differ greatly from other CPs in many respects. PDAs have a feature that is almost unique in the entire polymer field, and not only among CPs single crystals of macroscopic size can be obtained. They owe that to their peculiar polymerization mechanism, which is a topotactic reaction occurring in the mon-... 

Of particular interest in the present chapter is the effect of test atmosphere on creep and creep damage mechanisms. While there are undoubtedly several factors that can promote creep cavitation and contribute to the observed changes in stress exponent and activation energy, the fact remains that the strain rates are substantially higher in air than in inert atmospheres, as shown in Fig. 8.12. This phenomenon is a direct consequence of the topotactic oxidation reaction of SiC whiskers exposed at the surface. As described by Porter and Chokshi,38 and subsequently by others,21,22 at high temperatures in air, a carbon-condensed oxidation displacement reaction occurs in which graphitic carbon and silica are formed at the whisker interface via... 

The polymerization in clathrates, which has been compared to the template polymerization of biological systems, illustrates the exact requirements which need to be fulfilled for topotactic polymerization via a free radical mechanism. The hopes to find a monomer-polymer crystal pair which meets those conditions seem rather dim indeed. 

Product structure. Dehydration may result in relatively minor changes in structure such as decreases in some or all unit cell dimensions in topotactic processes. Alternatively, reaction may result in fundamental reorganization through recrystallization, or conversion to an amorphous or zeolitic material. Dehydration. Dehydration mechanisms encompass a wide variety of routes, summarized in Figure 7.4. The following features require comment. 

The structures of metal oxides and mixed oxides are often relatively simple, so that many features of reaction, such as the properties and dispositions of extended imperfections (Section 9.4.), can be characterized more easily than for more complex sohds. The ability of these compounds to deviate from stoichiometry does, however, increase interpretational difficulties. Topotactic behaviour, arising from structures based on simple ions, is important in formulating mechanisms [87], The surface chemistry and interface reactions of oxides are also of importance in heterogeneous catalysis and metal oxidations. 

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