Rigid cage

This scheme is very simple if compared with the jungle of hypotheses formulated to justify the photochemical isomerization of pentaatomic heterocycles. I do not know if it is true or if it represents a constriction of the nature in a rigid cage. I hope my work will be useful to direct future research efforts in this field. It is not important if future data will confirm or destroy my hypothesis The most important thing is to elicit a discussion. 

Hence, the phenomena of the low reaction rate in the polymer matrix cannot be explained by the limiting rate of reactant orientation (rotational diffusion) in the cage. This result becomes the impetus to formulate the conception of the rigid cage of polymer matrix [16-20]. In addition to the experiments with comparison of the rate constants in the liquid phase and polymer matrix, experiments on the kinetic study of radical reactions in polymers with different amounts of introduced plasticizer were carried out [7,9,15,21], A correlation between the rate constant of the reaction k and the frequency of rotation vOT of the nitroxyl radical (2,2,6,6-tetramethyl-4-benzoyloxypiperidine-/Y-oxyI) was found. The values of the rate constants for the reaction... 

Experimental data are in good agreement with this equation (see Table 19.4 and Figure 19.2). As was shown, the parameter m is constant in the very line of experiments with one reaction in the same polymeric matrix and is always less than unity. The latter means that Erot > Eor. This agrees with the conception of a rigid cage. 

The Values of Er and Eor for Bimolecular Reactions of Nitroxyl Radicals with Phenols Calculated According to the Rigid Cage Model for Reaction in a Polymer Matrix (Equation (19.7)) [7,9,14,15,21]... 

Fourth, the model of a rigid cage for a bimolecular reaction in the polymer matrix helps to explain another specific feature. This model explains the simultaneous increase in activation energy and preexponential factor on transferring the reaction from the liquid (Eh At) to solid polymer matrix (Es, As). In the nonpolar liquid phase / obs = E = gas but in the polymer matrix [3,21] it is... 

The mechanism of antioxidant action on the oxidation of carbon-chain polymers is practically the same as that of hydrocarbon oxidation (see Chapters 14 and 15 and monographs [29 10]). The peculiarities lie in the specificity of diffusion and the cage effect in polymers. As described earlier, the reaction of peroxyl radicals with phenol occurs more slowly in the polymer matrix than in the liquid phase. This is due to the influence of the polymeric rigid cage on a bimolecular reaction (see earlier). The values of rate constants of macromolecular peroxyl radicals with phenols are collected in Table 19.7. 

The azaadamantanes with nitrogen atoms in bridgehead positions present an interesting series of compounds with 1,3-n/n interactions in a rigid cage. The PE spectrum of... 

Due to the pyramidalization of the C atoms and the rigid cage structure of Cjq the outer convex surface is very reactive towards addition reactions but at the same time the inner concave surface is inert (chemical Faraday cage). This allows the encapsulation, observation and tuning of the wavefimction of extremely reactive species that otherwise would immediately form covalent bonds with the outer surface. 

An interesting example of less rigid cages is also provided by molecular self-assembling capsules obtained in the Rebek group [56]. 81 was found to form dimers held together by hydrogen bonds between the donors at its ends and the acceptors in the middle. 

The source of the strain in adamantane is not readily apparent but appears to be due to features present in the rigid, cage structure of the molecule. Less rigid molecules, e.g. acyclic alkanes, cyclohexane and /nms-decalin, are free to relax and to adopt conformations in which the best balance between angle, nonbonded and torsional strain is achieved. Thus, C-C-C bond angles of 112.4... 

Pfizer and others have continued to explore novel bicyclic diamines. This approach has been used successfully by a number of groups [23, 24], although the resulting highly rigid cage structures appear not to show any advantage over the simpler diamines. 

In one specific model the potential well arises from the interaction of the molecule with its nearest ndghbours. These are taken to form a more or less rigid cage which rotates without surface slip in a continuum whose viscous, or dscoelastic, properties are those of the bulk fluid. In this sort of model we note that the exchange of momentum between molecule and cage does not permit the random motions, of molecule with respect to cage and of cage with respect to bulk medium, to be imcorrelated. 

Fig. 18. Bond distances (see legend) vs. time, in fs, for the old and new bonds in the N2 + O2 —> 2N0 reaction in a 125 atom cluster. The figure illustrates the higher efficiency of the heavier rare gas atoms in providing a more rigid cage for the outcome of the first bimolecular collision. The inset shows the hyperspherical radius [see Eq. (10)] p vs. time in fs. The hyperspherical radius is a measure of how near the four atoms that take part in the reaction are to one another. Top panel A Xei25 cluster at an impact velocity of 7 km/s. Note how the atoms are almost as compressed in their second as in the first bimolecular collisions. (The times of these collisions are indicated by arrows.) Bottom panel A Nei25 cluster at an impact velocity of 12 km/s. The second collision is not very eflfective in bringing the four atoms together. In both panels only one stable NO molecule is formed.

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