Reaction cavity free volume

Studies by Nishiyama and Fujihara [149] utilizing azobenzene derivative (27) as isomerizable chromophores have demonstrated the importance of reaction cavity free volume in L-B films. The L-B films of amphiphilic derivative 4-octyl-4 -(3-carboxytrimethyleneoxy)-azobenzene (27) upon irradiation was found to be stable, no geometric isomerization of the azobennzene moiety occurred. This compound forms L-B films with water soluble polyallylamine 28 at an air-water interface. Reversible cis-trans photoisomerization occurs in the film containing 28. The reversible photoisomerization reaction in polyion complexed films is thought to occur because of the increased area per molecule provided in the film. The cross sections of molecule 27 in the pure film and in film containing 28 were estimated to be 0.28 and 0.39 nm2. Such an increased area per molecule... 

Clearly, homogeneous single crystal—>single crystal phototransformation can be achieved only if the reaction cavity free volume is of sufficient size and shape to accommodate the product. 

Where the dimer yields are not very high (<50%), it is reasonable to conclude that the presence of the unfavorably oriented second molecule (B) does not affect the reaction taking place in other words, the reaction cavity free volume of the favourably oriented molecules (A) is not interfered by that of B. 

Rather phase-insensitive Norrish II photoproduct ratios are reported from irradiation of p-chloroacetophenones with a-cyclobutyl, a-cyclopentyl, a-cycloheptyl, a-cyclooctyl, and a-norbonyl groups [282], In each case, the E/C and cyclobutanol photoproduct ratios are nearly the same in neat crystals as measured in benzene or acetonitrile solutions. On this basis, we conclude that the reaction cavity plays a passive role in directing the shape changes of these hydroxy-1,4-biradicals. As long as the initial ketone conformation within the cavity permits -/-hydrogen abstraction (and these ketones may be able to explore many conformations even within their triplet excited state lifetime), the cavity free volume and flexibility allow intramolecular constraints to mandate product yields. 

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

Figure 9 Difference in the nature of the reaction cavity in soft and hard reaction media. Top Hard reaction cavity, cavity free volume fixed. Bottom Soft reaction cavity, flexible cavity free volume.

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

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

Thus the boundaries of the enclosures in organized media may be of two types they may be stiff (i.e, none of the guest molecules can diffuse out and the walls do not bend), as in the case of crystals and some inclusion complexes, or flexible (i.e., some of the guest molecules may exit the cavity and the walls of the cavity are sufficiently mobile to allow considerable internal motion of the enclosed molecules), as in the case of micelles and liquid crystals. In these two extremes, free volume needed for a reaction is intrinsic (built into the reaction cavity) and latent (can be provided on demand). 

In discussions to this point, no significant interaction between a guest and its medium has been considered. This is probably the case in the reaction cavity model of Cohen [13] as well, since product selectivity was attributed mainly to the presence or absence of free volume within the cavity. The analogy of guests in hosts to balls in boxes is very deficient, but is really not different from the situation in the kinds of crystal systems which first inspired the Cohen nomenclature. Interatomic attraction and repulsion was important in analyzing those systems and was even critical to the crystal engineering used to assemble some of the systems used in the studies by Schmidt and his co-workers [1,48,89]. In addition to being stiff or flexible, cavity walls must... 

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. 

Over the last decade, a large number of examples (in the crystalline state) have corroborated the reaction cavity model due to Cohen and have brought out elegantly the need to have free volume within reaction cavities for the occurrence of solid state reactions. Even quantitative correlations have been attempted. Scheffer, Trotter, and co-workers have examined free volumes within reaction cavities to gain insight into the mechanism of intramolecular photorearrangements of enones [126,127]. They have shown that the course of a solid state reaction is influenced profoundly by certain specific... 

We illustrate in this section with a number of examples how the presence or absence of free volume within a reaction cavity determines the feasibility of a reaction in organized media. Presence of free volume alone may not be sufficient to effect a reaction within a reaction cavity. Its location and its directionality (presence of free volume in the critical dimension) are extremely important, as revealed by a number of examples discussed in Section D. 

The importance of free volume within the reaction cavities in the case of inclusion complexes has also been shown by several examples. Lahav, Leiserowitz et al. have observed that irradiation of the inclusion complexes of acetophenones with deoxycholic acid yields an addition product enantio-merically pure in each case (Scheme 10) [136]. Supported by a detailed... 

Consideration of overall free volume within a reaction cavity may not always help in understanding or predicting the photobehavior of guest reactant... 

Similar studies in zeolites having different sizes of channels/cages also reveal the importance of the directionality of free volume within the reaction cavity [159]. Both the photochemical and the photophysical behavior of trans-stilbene and longer all trans-a,w-diphenyl polyenes critically depend on the zeolite in which they are included. In pentasil zeolites (ZSM-5, -8, and... 

Consider reactant molecules or intermediates being caged within a reaction cavity with limited free volume. A preference might be envisioned in the reactions these reactant molecules or intermediates undergo, if the competing reactions require different amounts of free volume for shape changes that take... 

C. The increasing order of preference for disproportionation follows the same increasing trend in restriction experienced by the guests/intermediates and the available free volume within the reaction cavity isotropic solution, silica surface, and zeolite, the last being the medium providing very high restriction and smallest free volume. 

These results suggest that tight fit is needed to orient molecules within reaction cavities having passive walls. Too tight a fit will leave no free volume within a reaction cavity that would be needed to accommodate displacement of atoms during the course of a reaction. This limits the number of transformations that can be achieved within a reaction cavity wherein the reactants are held tightly. 

As discerned from Table 9, the photoproduct ratios from irradiation of neat 77 vary drastically among homologues and with temperature for the n = 7 member, Unlike the n-alkanones discussed previously, the cyclic diones must be intrinsically bent as shown in the ORTEP drawings in Figure 43. Since the C=0- H, distance in each case is <3.0 A, excitation of the molecules should result in formation of hydroxy-1,4-biradicals. The fate of those radicals depends upon the flexibility of the reaction cavity and its free volume. The results indicate that sufficient free volume is present in the reaction cavities to allow topochemical conversion to photoproducts, but not... 

The solid phases of 81 are also well ordered macroscopically and their higher E/C ratios require that the hydroxy-1,4-biradical be in rather inflexible reaction cages with little excess free volume. Hydrogen bonding to neighboring ketone molecules may be partially responsible for the high photoproduct ratios found upon collapse of the biradicals in the solid phases, but the size, shape, and flexibility of the reaction cavity are clearly the more important factors. The highest E/C ratios observed in the second solid phase of 81a... 

Figures 53 and 54 show the structure of the 3/98d complex as it exists in the unit cell [154, 303], Unlike the complexes with 98a-c, the 98d complex has both hydroxyl groups of one 3 hydrogen bonded to both carbonyl groups of one molecule of 98d. As a result, the diyne backbone is curved (Figure 53) [154, 303], There is no reason to believe that the walls of the reaction cavity experienced by 98d or by transients, lOld and 102d derived from it, in optically active 3 complexes are any more rigid or contain less free volume than do the other complexes. The enantiomeric purity of the product must result from specific attractive host-guest interactions retained along the...

Exploration of the modes of solubilization of 2- and s-105 in the solid phases of C21 has led to many unexpected and complicating observations [321], First and foremost among these is that only guest alkanone molecules whose lengths are very near that of C21 can be incorporated isomorphously into its solid phases. Thus, C21 is a much more demanding host matrix than BS the dimensions and free volume content of the reaction cavities it provides have a very narrow distribution and their walls are probably less flexible than those in many neat crystals. 

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