Reaction cavity

In order to appreciate reactions in crystals, it is of value to consider the topo-chemical postulate and the reaction cavity concepts as the starting point. Kohlshutter proposed in 1918 that the crystal lattice plays an important role on the outcome of chemical reactions as a result of its rigidity and topology, suggesting that reactions... 

An example of a reaction cavity is illustrated by the van der Waals surface plots derived from the X-ray structures of di-2,6-dihydroxy-2,6-diphenylcyclohexanone 51 and its photodecarbonylation products, the cis- and tra i-2,6-diphenylcyclopen-tane-l,2-diols 52 and 53 (Scheme While the two products are formed in... 

A new photoactive monoclinic polymorph of 6-(2, 4 -dinitrobenzyl)-2,2 -bipyri-dine was obtained from an acetone/methanol solution, and the structure compared to the previously known photoactive orthorhombic and photoinactive monoclinic forms [94]. Correlation of these structures with those of related nitrobenzylpyridines was used to understand the relationships existing between structure and photochro-mism. The comparison of the reaction cavities around the reactive pyridyl-benzyl-nitro fragment indicated that photochromic activity required rotational freedom of the ortho-nitro group in the crystal and its accessibility from the proton-donor and proton-acceptor sites. 

In the second class of systems the reaction is such that it involves little or no change of the molecular geometry in the vicinity of the reacting sites, nor of the external shape of the crystal. The concept of the reaction cavity is useful in this context (184). This cavity is the space in the crystal containing the reactive molecule(s), and its surface defines the area of contact between this molecule and its immediate surroundings. Only if the shape of this cavity is little altered as reaction proceeds will the activation energy for the process be reasonably small and the rate of reaction nonzero. 

According to the model, a perturbation at one site is transmitted to all the other sites, but the key point is that the propagation occurs via all the other molecules as a collective process as if all the molecules were connected by a network of springs. It can be seen that the model stresses the concept, already discussed above, that chemical processes at high pressure cannot be simply considered mono- or bimolecular processes. The response function X representing the collective excitations of molecules in the lattice may be viewed as an effective mechanical susceptibility of a reaction cavity subjected to the mechanical perturbation produced by a chemical reaction. It can be related to measurable properties such as elastic constants, phonon frequencies, and Debye-Waller factors and therefore can in principle be obtained from the knowledge of the crystal structure of the system of interest. A perturbation of chemical nature introduced at one site in the crystal (product molecules of a reactive process, ionized or excited host molecules, etc.) acts on all the surrounding molecules with a distribution of forces in the reaction cavity that can be described as a chemical pressure. 

This distinction is, however, only practical. Several reaction types are not easily encompassed in the above description. For instance, Scheffer (see within this book) has provided ample examples of stereocontrolled solid-state reactions [11], while solid-state isomerizations have been studied by Coville and Levendis [12]. These processes can be explained with the reaction cavity concept, i.e. reactivity takes place in a constrained environment generated by the surrounding molecules. Relevant contributions to the field have also derived from the studies of Eckhardt [13] and those of Ohashi and collaborators [14]. 

A. Reaction cavity defined by boundary, size, and shape 91... 

B. Concept of free volume Stiff and flexible reaction cavities 96... 

V. Reaction cavities as templates highlighted with examples 132... 

A. Reaction cavities with active walls as templates ... 

B. Reaction cavities with passive walls as templates 150... 

Reaction cavities of alkanones in neat solid phases 173... 

B. Reaction cavities with very stiff walls and preformed shapes... 

C. Reaction cavities with some wall flexibility Solid inclusion... 

D. Reaction cavities with strong external medium influences ... 

E. Reaction cavities with walls of variable flexibility and... 

This description is elaborated below with an idealized model shown in Figure 17. Imagine a molecule tightly enclosed within a cube (model 10). Under such conditions, its translational mobility is restricted in all three dimensions. The extent of restrictions experienced by the molecule will decrease as the walls of the enclosure are removed one at a time, eventually reaching a situation where there is no restriction to motion in any direction (i.e., the gas phase model 1). However, other cases can be conceived for a reaction cavity which do not enforce spatial restrictions upon the shape changes suffered by a guest molecule as it proceeds to products. These correspond to various situations in isotropic solutions with low viscosities. We term all models in Figure 17 except the first as reaction cavities even... 

Can we extend the reaction cavity concept, which emphasizes the shape... 

Figure 18. The reaction cavity model as presented by Cohen. Reaction cavity before and after the reaction is shown as full lines. Transition state requirements for a reaction are shown as broken lines. Case I represents a favorable and case II an unfavorable reaction. [Reproduced with permission from M. D. Cohen, Angew. Chem. Int. Ed. Eng. 14, 386 (1975).]...

A. Reaction Cavity Defined by Boundary, Size, and Shape... 

When guest molecules are able to explore more space during their transformation to products than is available in the cavity in which they are accommodated at the time of excitation (initial reaction cavity), their behavior may depend upon the effective space explored. The effective reaction cavity, the space explored, will depend on the lifetime of the excited state, the nature of the mobility, and the structure of the guest molecule and the intermediate(s) derived therefrom. The initial and effective reaction cavity... 

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