Topotactic process

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. 

The dehydration of copper(Il) formate tetrahydrate is a topotactic process [145]. The anhydrous crystalline product of dehydration retains two-dimensional features of the reactant structure but the stacking mode is changed. This structure is different from that obtained by the direct preparation of anhydrous Cu(HCOO)2. 

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. 

It is often difficult to decide whether the described transformations are really topotactic processes or if they proceed by dissolution and recrystallization. Mixtures of water with organic solvents are useful in synthesizing new silicates from different silica sources. An example is the formation of the zeolite NasSii2028 4H2O from silica, NaOH, ethylene glycol, and water [228]. 

Topotactic processes are described more comprehensively in Mechanical Preparation of Crystalline Materials. An Oxymoron and Templated [2 + 2] Photodimerizations in the Solid State, Supramolecular Materials Chemistry. Since its original definition in 1959, the term topotaxy has evolved " to mean a solid-state chemical reaction (not necessarily reversible) that completely transforms a single crystal such that the product resembles that of the initial material both in terms of its three-dimensional structure and the orientation thereof. Topotactic is synonymous with topochemical... 

Two types of transformations can be very broadly distinguished. The first is the formation of a solid solution, in which solute atoms are inserted into vacancies (lattice sites or interstitial sites) or substitute for a solvent atom on a particular sublattice. Many types of synthetic processes can result in this type of transformation, including ion-exchange reactions, intercalation reactions, alloy solidification processes, and the high-temperature ceramic method. Of these, ion exchange, intercalation, and other so-called soft chemical (chimie douce) reactions produce no stmctural changes except, perhaps, an expansion or contraction of the lattice to accommodate the new species. They are said to be under topotactic, or topochemical, control. 

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