Reaction cavity concept

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... 

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]. 

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

In this section several photoreactions from the literature are examined in terms of the reaction cavity concepts outlined above. Examples have been so chosen that only the particular aspect highlighted is the major influencing factor. However, in certain cases there may be more than one factor responsible for the changes observed. To establish the generality of the proposed model, examples have been chosen from a number of different organized media. In this section features relating to enclosure and free volume are discussed. 

The reaction cavity concept, which emphasizes the shape changes that occur as the reactant guest is transformed into the product, is generally... 

Figure 7 Reaction cavity concept illustrated cavity free volume controls the product formation.

VR thanks the National Science Foundation for support of the research (CHE-9904187 and CHE-0212042). Reaction cavity concept presented in this review was originally developed by M. D. Cohen for reactions of organic crystals and fruitful collaboration between R. G. Weiss and G. S. Hammond and VR resulted in the generalization of the concept to organized media. VR thanks R. G. Weiss, N. J. Turro, R. S. H. Liu, and J. R. Scheffer for sharing their ideas on photoreactions in organized media. 

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. 

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

Our concept of a reaction cavity in organized media has been considerably modified from that of Cohen [13]. It requires the inclusion of more factors to be used effectively, but it provides a base for discussion of a myriad of reaction environments. 

B. Concept of Free Volume Stiff and Flexible Reaction Cavities... 

In 1975, Cohen introduced the concept of the reaction cavity in solid state chemistry (2. The reaction cavity was defined as the space occupied by the reacting species and bounded by the surrounding, stationary molecules. Cohen viewed the topochemical principle as resulting from the preference for chemical processes to occur with minimal distortion of the reaction cavity, either in the formation of voids within it or extrusions from it (Figure 1). 

Such crystalline-state racemizations by photoirradiation were observed in the cobaloxime complexes not only with the chiral 1-cyanoethyl group but also with the chiral l-(methoxycarbonyl)ethyl [18-20], 1-(ethoxycarbonyl)ethyl [12,21], 1,2-bis(methoxycarbonyl)ethyl [22], l,2-bis(ethoxycarbonyl)ethyl [23], and l,2-bis(allyloxycarbonyl)ethyl [24] groups as axial alkyl groups. The concept of a reaction cavity was also applicable to the racemization of the cobaloxime complexes with such bulky chiral alkyl groups. 

From the above observation, we proposed that the reaction rate should be explained by the size of the reaction cavity for the reactive group and that the chirality of the produced group should depend on the shape of the prochiral group. Using the concept of a reaction cavity, more complicated processes such as racemic-to-chiral transformation and chirality inversion will be made clear. 

The original formal theory is expressed in terms of quanttun electrodynamics with the continuum mediwn characterized by its spectnun of complex dielectric frequencies. A more recent formulation, derived from this theory, is based on the extension of the reaction field concept to a dipole subject to fluctuations exclusively electric in origin. Another procedme has been formulated starting, as for the repulsion contribution, from the theory of intermolecular forces. Following the scheme commonly exploited to derive the electrostatic contribution to the interaction energy, the molecule B is substituted by a continuum medium, the solvent S, described by a surface charge density as induced by the solute transition densities of M (the equivalent of A) and spreading on the cavity surface. 

Although the prediction of the crystal structure from the theoretical calculation has not been solved yet, it seems possible to predict the reaction process from the crystal structure of reactant molecule, since the product molecule should be made suffering from steric repulsion in the crystaUine lattice of the reactant crystal. The term of reaction cavity was first proposed by Lee and Richards to interpret the reaction region within the enzyme molecule [15]. Cohen proposed a qualitative concept of the reaction cavity, as shown in Fig. 2.11 [16]. He proposed that since the reacting molecules occupy a space of a certain size and shape in the starting crystal, the reaction cavity is surrounded by the contact surface of the surrounding molecules. The topochemical principle can be interpreted to mean that those reactions which proceed under lattice control do so with minimum distortion of the surface of the reaction cavity. Therefore, the reaction of the route (a) is more favorable than that of (b). 

Furthermore, it was made clear that the concept of reaction cavity is applicable to the usual solid-state reactions. In almost all the solid-state photoisomerization from the 2-cyanoethyl group to 1-cyanoethyl group, the reaction cavity is a good... 

The inqiortance of the three-dimensional arrangement in determining whether or not molecules are sufficiently close enough to react is clear from Section 6.2. Now we must also consider whether or not there is sufficient space around the reacting molecules within the lattice for the necessary changes in geometry to occur as we proceed from reactant to product. In addition, it is necessary to ask whether or not any required transition state can be created within the matrix. Insofar as organic solids are concerned, this is the concept of reaction cavity as discussed by Cohen. ... 

We considered in Section 6.3.1 an analogy between reactions in zeolites and how the concept of reaction cavity may be used to rationalize the course of a solid state reaction. One can, in fact, control reaction pathway by utilizing preexisting structures with desired topologies to create appropriate microreactor vessels. Such independent preexisting structures may also remain structurally intact during reaction, thweby ensuring unaffected control over reaction pathway and product selectivity. 

One of the most important beginnings in the field of solid-state photochemistry was the concept of the reaction cavity proposed by Cohen and Schmidt. Another suggestion by the same authors was that reactions take place with least motion. Both suggestions were qualitative. The first is quite valid, while the least motion idea has proven not to be general. This chapter aims at presenting the methods of most use in our research on SoHd-state photochemistry. 

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