Topochemical model

Phase transformations in heterogeneous catalysis have been described recently by topochemical kinetic models [111-115]. These models were taken from solid chemistry, where they had been developed for gas-solid reactions. The products of such reactions are solids. When gas is in contact with the initial solid, the reaction rate is negligible. But as nucleates of the phase 

A simple topochemical model for the growth of NiO islands on the Ni surface during the reaction of oxygen with a Ni(lll) crystal is clearly described by Holloway and Hudson [112]. They considered three cases in which the rate-determining step is, respectively (a) oxygen sorption from the gas phase (surface diffusion is fast), (b) surface diffusion of oxygen, and (c) oxygen insertion over the island boundary. 

The area of a unit circular island originating at a moment rj and then growing, will, at time t, be 

The area of all the islands, provided they do not overlap and no new nucleates are generated on the sites where the NiO phase was formed, is 

Here i is the formation rate of new phase nucleates on a unit surface. It can be shown [111] that the overall oxygen surface coverage, without taking into account the above assumptions, is 

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During work on a series of aspartyl dipeptides containing ACC 71 (vide supra, Eq. (28), Sect. 4) at the carboxyl terminus, it was reported that dispartame Asp-ACC-OMe had a distinct sweet taste [302] and that the corresponding n-propyl ester had 250-300 times the sweetness of sucrose [303]. However, replacement of phenylalanine by 2,3-methanophenylalanine gave tasteless analogues of aspartame [293, 304], and some dimethyl-ACC 214 (methanovaline) and tri-methyl-ACC 215 aspartame analogues [Asp-(Me)n-ACC-OMe] have a bitter taste. These taste properties, which depend on the number and position of the methyl substituents, have been explained on the basis of topochemical models thus, a L-shaped conformation of the dipeptide is necessary for sweet taste, Eq. (86) [3051. 

The topochemical model [112] suggests that an island can have n oxide layers. Apparently, this model can be applied in the case of the chemisorbed two-dimensional phase growth, as had been done by Boreskov et al. [116]. 

The topochemical model, however, describes the origination and growth of macrostructures. In principle one could construct kinetic models accounting for the kinetics of cluster (or nucleate) formation as a model for the system or reverse consecutive reactions [114, 121]. 

Usually phase transition in a gas-solid system is described by the topochemical models [44,194]. Such models assuming chaotic or regular configurations of the islands or nuclei of the new phase - disks or squares for the two-dimensional and sphere for the three-dimensional reaction systems. The reaction rate depends on the duration of aggregation (on the initial distribution and configuration of the islands) and that of the reaction itself. Its value is much lower than that taking place in the chaotic distribution of the adspecies. 

Dureja, H. and Madan, A.K. (2007) Topochemical models for prediction of telomerase inhibitory activity of flavonoids. Chem. Biol. Drug Des., 70, 47-52. 

Equation (4.3.78) becomes identical to Eq. (4.3.73) upon replacing by Fp, by and Kg by Kp and setting 8 = 0. Furthermore, the examination of these two equations may explain why the topochemical model with chemical reaction control is approached more readily for nonporous solid reactants, and why the diffusion-controlled shrinking-core model becomes applicable for porous solids. This behavior is readily apparent on considering that, in general, > VJA ),... 

Notes This is often called the topochemical model. The size R refers to the cone. 

The results are different for a particle of constant size with a shrinking core of unreacted material. In such a topochemical model, there are two diffusional resistances in series. First, material must diffuse from the bulk to the particle s surface second, it must diffuse from the surface through ash to the unreacted core. 

In order to rationalize such characteristic kinetic behaviour of the topochemical photoreaction, a reaction model has been proposed for constant photoirradiation conditions (Hasegawa and Shiba, 1982). In such conditions the reaction rate is assumed to be dependent solely on the thermal motion of the molecules and to be determined by the potential deviation of two olefin bonds from the optimal positions for the reaction. The distribution of the potential deviation of two olefin bonds from the most stable positions in the crystal at OK is assumed to follow a normal distribution. The reaction probability, which is assumed to be proportional to the rate constant, of a unidimensional model is illustrated as the area under the curve for temperature Tj between 8 and S -I- W in Fig. 7. 

Similar to macroscopic polymer networks, microgels have a more or less fixed surface. Due to the large value of their surface/mass ratio, microgels may be used as models for studying topochemical reactions at polymer surfaces. 

Fig. 6 Stacking model for the muconate derivatives in the crystalline state and the definition of stacking parameters used for the prediction of the topochemical polymerization reactivity, d c the intermolecular distance between the 2 and 5 carbons, is the stacking distance between the adjacent monomers in a column. 6 and 02 are the angles between the stacking direction and the molecular plane in orthogonally different directions [59]...

It should be said that at present the available literature concerning the kinetic models which account for the topochemical character of catalyst surface processes is limited, but reference can be made to refs. 119 and 120. In ref. 119, a kinetic model for the oxidation of hydrogen on platinum is... 

Jezierska et al. [67] applied the Kohonen neural network to select the most relevant descriptors. Here a Kohonen network is built with the transposed matrix, i.e., with the matrix where the roles of descriptors and molecules are exchanged. From the map of descriptors, 36 descriptors were selected. This number of descriptors was further reduced to six, five, four, or three descriptors. Statistical parameters of compared models are reported in Table 3 of [67]. It is evident from the table that the model built with four selected descriptors show comparable parameters to the model built with 36 descriptors. The selected descriptors belong to topostructural and topochemical classes. 

Some important aspects of topochemical polymerizations can be understood by inspection of Eq. (1), All reactivity comes about by very specific rotations of the monomers and by 1,4-addition of adjacent units and an extended, fully conjugated polymer chain is formed. The unique feature of the topochemical polymerization of diacetylenes is the fact that in many cases the reaction can be carried out as a single phase process. This leads to macroscopic, defect-free polymer single crystals which cannot be obtained, in principle, by crystallization of ready-made polymers by conventional methods. Thus, polydiacetylenes are ideal models for the investigation of the behaviour of macromolecules in their perfect three dimensional crystal lattice. 

The diacetylenic acids (9,10) have also been widely investigated because of their polymerizability. Here the interesting diacetylenic entity is normally incorporated into the structure of an alkanoic acid to give a compound such as CH3(CH2)ioC=C-C=C(CH2)7COOH. The topochemical polymerization proceeds within the LB layer but results in an array of two-dimensional domains whose size is influenced by material purity. Diacetylenes have also been incorporated into lipidlike molecules and polymerized as model membranes 11, 12). Because of the structural flexibility of LB films, there is likely to be continued interest in polymer LB films research, some of which will involve preformed polymers (13). However, the rigidity of many polymer films prevents this approach being used generally. 

In Figure 5 the phase diagram of the system (6) and the 3-pentyl homologue (7) is given. Three distinct ranges can be detected A and C represent solid solutions of the two monomers in the triclinic centrosymmetric structures of pure (6) and (7), respectively, but range B clearly indicates the formation of a new phase. The cell constants of a crystal of 50(6) 50(7) composition, as well as the stereochemistry of the topochemical products, indicated that indeed this phase is isomorphous to enantiomeric and racemic (2) (Table 1), thus confirming the validity of the model. 

The model used in the treatment of topochemical decomposition is that of a cylinder or sphere (Fig. 4.4), in which it is assumed that the radius of the intact chemical substance decreases linearly with time. For the contracting cylinder model, the mole fraction x decomposed at time t is given by... 

Figure 4.4 Model of a sphere or a cylinder used in theoretical treatment of topochemical reactions.

Roy K, Ghosh G. QSTR with extended topochemical atom (ETA) indices. VI. Acute toxicity of benzene derivatives to tadpoles. J Mol Model 2006 12(3) 306-16. 

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