Topochemical process

We have found a rare reaction where two topochemical processes occur competitively in a single crystal, that is, a competitive photocyclo-dimerization and -polymerization in the crystal of 1,4-dicinnamoylbenzene 3 (15). On photoirradiation with a mercury lamp (100W) at 20°C for 9 h, the crystals of 3 (2.00g), dispersed in heptane (400 ml), are transformed into amorphous substances consisting of a tricyclic dimer, 21,22,23,24-tetraphenyl-1,4,11,14-tetraoxo-2 (3), 12 (13) -diethano(4,4Jparacyclophane, 4 (isolated yield 57%) (16), a mixture of oligomers (ca. 30%) and unreacted 3 (7%). 

A correlation between surface and volume processes is described in Section 5. The atomic-molecular kinetic theory of surface processes is discussed, including processes that change the solid states at the expense of reactions with atoms and molecules of a gas or liquid phase. The approach reflects the multistage character of the surface and volume processes, each stage of which is described using the theory of chemical kinetics of non-ideal reactive systems. The constructed equations are also described on the atomic level description of diffusion of gases through polymers and topochemical processes. 

As a whole, considerable experimental data have been accumulated in the literature concerning the influence of the gas phase on the surface state of adsorbents and catalysts. The surface state is used specially to preliminarily produce the required surface composition of a catalyst. At the same time, quantitative characteristics of these changes are mainly available only for binary alloys contacting with molecules of H2, CO, and 02 [41,42]. Investigations conducted in recent years reveal that the influence of adsorbed particles on the state of a solid surface is apparently more significant than is customarily considered at present. The structural transformations in the surface layers (an example is the rearrangement of the surface layer of platinum in the adsorption of CO and 02 molecules [43]) and the processes of formation of new phases in them, which are similar to three-dimensional topochemical processes [44], may be of a major significance. 

It should be noted that reduction of the high-temperature superconductor YBa2Cu307 to YBa2Cu3-06 (Fig. 7) is a topochemical process. 

X-ray crystallographic studies during the course of the reaction have demonstrated that the reaction is a typical topochemical process involving a direct rearrangement of the monomer crystal to the polymer crystal having an extended rigid rod-like structure. By x-ray analysis and DSC on the thermal depolymerization of the polymer crystal, a reversible topochemical process has been demonstrated for monomer and polymer crystals. 

This is a striking example of a matrix effect originating in the topochemical process of a substance which is chemically the same (oligomer) but behaves in three different ways photodepolymerization, photopolymerization, and no photoreaction, depending on its physical state (in solution, in as-prepared and recrystallized crystalline states)33. Further discussions on the matrix effect have been made in correlation with crystallographic studies (see Sect. IV.b. and VII.). 

From the viewpoint of the topochemical process, the diolefinic compounds prepared so far are classified into three groups photopolymerizable, photooligomerizable1, and photostable crystals. Among these, nine kinds of monomer crystals and their polymers have been subjected to structure analysis so far. The obtained crystal data are compiled in Table 4. 

Morphological changes, which are classified into four groups, have been correlated to the degree of topotactic control. Thermal analysis has been studied on DSP poly-DSP in some detail. Two main endothermic peaks of as-polymerized poly-DSP crystals are characterized as thermal depolymerization in the crystalline state and crystal melting point followed by thermal depolymerization in the molten state. From the results of the studies on the heat treatment of as-polymerized polymer crystals, a reversible topochemical processe has been established. 

Based on the results of crystalline-state depolymerization, a reversible topochemical process, which is a monomer crystal lattice-controlled photopolymerization and a polymer crystal lattice-controlled thermal depolymerization, is established65. ... 

Apart from the uniqueness of the reaction the morphology of the product depends substantially on the distribution of the reaction centers in the crystal. Two limiting cases can be considered. In the case of a heterogeneous topochemical process the reaction starts preferentially at specific defect sites and proceeds with nucleation of product phases. This mechanism eventually leads to the destruction of the mother crystal since the coherence between the various nuclei is lost under the influence of... 

Shlyakhtin, O.A., Topochemical processes in the freeze drying synthesis of nickel-zinc ferrites, Ph.D. Thesis, Moscow State University, Moscow, 1985. 

As can be seen from Scheme 2, only one of the two possible enantiomeric cyclobutane polymers should be formed in any given chiral crystal, by a topochemical process within this motif. 

Ingraham and Marier [88] identified the intermediate Zn0.2ZnS04 in the decomposition of ZnS04. This is a topochemical process, following linear kinetics and the rate is decreased by the adsorption of SO3 on the product oxide. The reverse reaction (ZnO + SO3 ZnS04) is very slow if the solid is not finely divided. 

The crystal structure of the monomer sufficiently supports the structure of the photodimer expected from the spectral studies if the topochemical process is assumed in this photoreaction. The highly photostable behavior of the dimer is quite reasonable because, after one pair of double bond reacts, then the residual pair will be rearranged far outside of the photoreactive arrangement in the crystal. 

As indicated by Equation (1), intercalation reactions are usually reversible, and they may also be characterized as topochemical processes, since the structural integrity of the host lattice is formally conserved in the course of the forward and reverse reactions. Typically, these reactions occur near room temperature, but this is in sharp contrast with most conventional solid-state synthetic procedures which often require temperatures in excess of 600 °C, the term Chemie Douce has been coined to describe this type of low-temperature reaction. Remarkably, a wide range of host lattices has been found to undergo these low-temperature reactions, including framework (3D), layer (2D), and linear chain (ID) lattices. 

This diffusion can occur or along the phase boundaries, when the C-S-H nuclei do not cover the whole C3S surface, but they are formed on the active centres only, or can proceed through the continuous C-S-H layer (Fig. 3.11). This would result in the epitaxial growth of C-S-H on the nuclei formed earlier. This mechanism will correspond obviously to the topochemical process. 

In the later period of hydration the topochemical process cannot be rejected, in the light of collected experimental data. However, its role in hydration process seems to be significantly limited, if a veiy low volume diffusion rate is considered. Moreover, the variable ions Ca and H SiO " concentrations during the lydration cannot be explained basing on the topochemical mechanism. These concentrations correspond to the C3S congment dissolution, occurring simultaneously (sections B-C, D-E, F-G) and C-S-H crystallization (sections C-D... for C/S = 1.5) (see Fig. 3.6). 

However, Fierens [9] has shown, that the hydroxylation of the slag glass surface layer, as a result of water molecules chemisorption, which is enhanced by the surface electron defects (trapped electrons), can be considered as a topochemical process. The topochemical processes became important also at later hydration stage, when the pozzolanic reaction of slag glass network, impoverishes of the majority of alkaline elements, with calcium ions is occtrrring, and calcium ions are chemisorbed on the active sites of solid phase. 

Some authors are of the opinion that the pozzolanic reaction consists in the binding of calciitm ions with the amorphous silicate-alirminate gel and thus it is a to-pochemical process [30], As a resirlt, a C-S-H shell, poorly permeable for iorts, is formed on the surface of pozzolana. It is referred to the former hypothesis of the mechanism of CjS reaction with water. C3S imdergoes the hydrolysis with the release of calciitm ions, which ate diffusing to the liquid phase. These ions ate adsorbed on the formed acidic layer, emiched in sihca, and react / situ with the formation of C-S-H, as a result of topochemical process [31],... 

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