Concept of Reaction Cavity

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). 

We proposed the reaction cavity more precisely and quantitatively [2], 

The reaction cavities of Fig. 2.13a-c are compared in Fig. 2.14 in which the cobaloxime plane is fixed in the same position and only peripheral curves of the cavities are drawn. The solid, dotted, and dot and dashed curves indicate the peripheral ones before photoreaction at 293 and 173 K and after photoreaction at 293 K, respectively. There are two void spaces, (A) and (B), in the cavity before photoreaction at 293 K, compared with the cavity at 173 K. The void space of (B) disappears and is 

There are two void spaces, (A) and (B), in the cavity before photoreaction at 293 K, com pared with the cavity at 173 K 

Although the composite reaction cavity in Fig. 2.14 is very important in explaining the reaction mechanism, it cannot be obtained for the conventional solid-state reactions. However, the reaction cavity before reaction as shown in Fig. 2.13a can clearly indicate where the void space is and how the reactive group can move in the void space. 

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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. 

The actual field-parallel contribution of m can be calculated from Onsager s concepts of the cavity field and of the reaction field. At first, Eq. (3.15) is specified as... 

New concepts. This last point is the most important. The impact of a model is determined by the quality of the new concepts it introduces points 1-3 are just additional conditions. Onsager s model has introduced new concepts. More than 60 years have shown their validity and their capability of surviving the revolutionary changes in our methods to describe matter. We have paid attention in the preceding discussion to concepts of the cavity and of the reaction field also the concept of the internal field, essential to connect microscopic to macroscopic behavior of matter has recently been introduced into the accurate quantum mechanical realizations of the model. 

The concept of reaction field, originally formulated by Onsager [194], has been proved to be fruitful in the quantum chemical treatment of polar subsystems (solutes) embedded in polarizable environment (solvent) [195]. Simple cavity models, where the solvent is represented by a continuous dielectric medium and the solute is sitting in a cavity inside this dielectric, has numerous application in the framework of semiempirical [196-200] and ab initio [201-205] methods. The utility of this concept in the modelisation of biochemical processes was pointed out by Tapia and his coworkers [206]. 

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... 

The concept of zeolite as solid solvent has already been proposed in the literature (27), to account for the ability of zeolites to concentrate the reactants inside their cavities, in terms of partition coefficient, by favoring closer average approximation of the reactants. However, the concept as a solvent to promote ionization and solvation of ionic species seems to arise from the present results, and might be explored in other reaction systems. 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

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]. 

The alkylation of naphthalene and 2-methylnaphthalene with methanol and their ammoxidation were investigated by F r a e n k e 1 et al. [22-25] on zeolites ZSM-5, mordenite and Y. In the alkylation over HZSM-5 - unlike on H-mordenite or HY - the slim isomers, namely 2-methylnaphthalene as well as 2,6- and 2,7-dimethylnaphthalene, again clearly predominated. These authors suggest that such shape selective reactions of naphthalene derivatives occur at the external surface of zeolite ZSM-5, in so-called "half-cavities" [22, 24, 25]. D e r o u a n e et al. [26,27] went even further and generalized the concept of shape selectivity at the external surface. Based, in part, on Fraenkel s experimental results, Derouane [26] coined the term "nest effect". This whole concept, however, is by no means fully accepted and has recently been severely questioned in the light of results obtained in catalytic studies with a much broader assortment of ten-membered ring zeolites [28]. 

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 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. 

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). 

The key differences between the PCM and the Onsager s model are that the PCM makes use of molecular-shaped cavities (instead of spherical cavities) and that in the PCM the solvent-solute interaction is not simply reduced to the dipole term. In addition, the PCM is a quantum mechanical approach, i.e. the solute is described by means of its electronic wavefunction. Similarly to classical approaches, the basis of the PCM approach to the local field relies on the assumption that the effective field experienced by the molecule in the cavity can be seen as the sum of a reaction field term and a cavity field term. The reaction field is connected to the response (polarization) of the dielectric to the solute charge distribution, whereas the cavity field depends on the polarization of the dielectric induced by the applied field once the cavity has been created. In the PCM, cavity field effects are accounted for by introducing the concept of effective molecular response properties, which directly describe the response of the molecular solutes to the Maxwell field in the liquid, both static E and dynamic E, [8,47,48] (see also the contribution by Cammi and Mennucci). 

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. 

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. 

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