Topotactic reactions

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

The reactions of these insertion materials, except for LiMnO, [19, 20], consist of electron and lithium-ion insertion into, or extraction from, each matrix without the destruction of its core structure this is called a topotactic reaction. A series of LiCovNi, v02 (0 < y < 1)) and... 

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. 

Keywords Crystal engineering Solid-state photoreaction Topochemical polymerization Controlled radical polymerization Dimerization Isomerization Topotactic reaction... 

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. 

One study utilized CD CdS, built up from several layers (probably a total thickness of the order of a micron) and annealed [1]. The Cn cS layer was formed by the nsnal (for this type of cell) topotactic reaction between a CuCl solution and the CdS snbstrate. While the cell performance varied over a wide range, depend-... 

When the reactivity of a solid is controlled by the crystal structure, rather than by the chemical constituents of the crystal, the reaction is said to be topochemically controlled. The nature of products obtained in a decomposition reaction is frequently decided by topochemical factors, particularly when the reaction occurs within the solid without separation of a new phase (Thomas, 1974 Manohar, 1974). A topotactic reaction is a solid state reaction where the atomic arrangement in the reactant crystal remains largely unaffected during the course of the reaction, except for changes in dimension in one or more directions. Dehydration of Mo03-2H20 is a typical example of a topotactic reaction ... 

Several reviews of early work on topotactic polymerizations and isomeriza-tions are available, and the reader is referred to the summaries of Morawetz [88] and Gougoutas [8] for a more complete account. The earliest study of a topotactic reaction appears to have been the observation, in 1932, of the polymerization of trioxane to poly-oxy-methylene [89]. Similar polymerizations of tetraoxane [90] and of trithiane [91 ] have also been reported to show retention of crystallographic axes from the monomer lattice. Other examples are discussed below. The topo-tacticity of a reaction can be determined solely by x-ray crystallographic analysis at the reactant and product endpoints. Thus a simple classification of a reaction as topotactic tells very little about how the structure of the crystal lattice changed in the course of reaction. 

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-... 

The SEM photographs for NMO-2/7 and NMO-1/2 samples are shown in Figure 4. All these samples have similar sheety crystal morphology as that of NaBir precursor [21]. This result suggests that the hydrothermal insertion of Ni(OH)2 is a topotactic reaction, in which almost no dissolution-deposition of NaBir or NiBir occurs. The sheety crystal morphology also suggests that the Ni(OH)2 inserted samples have a layered structure. 

In the weak basic solution, Ni(OH)2 is preferentially formed in the interlayer space of blmessite rather than as free Ni(OH)2 from Ni containing solution. Hydrothermal intercalation method is favored the formation of a mixed layered Ni(OH)2-manganese oxide, in which the Ni(OH)2 insertion into the bimessites is topotactic reaction. 

Reactivity in the solid-state is always connected with specific motions which allow the necessary contact between the reacting groups. In most cases solid-state reactions proceed by diffusion of reactions to centers of reactivity or by nucleation of the product phase at certain centers of disorder. This leads to the total destruction of the parent lattice. If the product is able to crystallize it is highly probable that nucleation of the crystalline product phase at the surface of the parent lattice will lead to oriented growth under the influence of surface tension. In such topotactic reactions certain crystallographic directions of parent and daughter phases will coincide. Typical examples for this behaviour are the solid-state polymerizations of oxacyclic compounds such as trioxane, tetroxane or 3-propiolactone... 

Crystalline trithiane, the sulfur containing analog of trioxane polymerizes in a topotactic reaction after irradiation on subsequent heating to 180° C 93). Again the crystal structure is twinned and differential thermal analysis has shown a higher melting point for polymers produced by solid state polymerization than for solution polymerized trithiane... 

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