Flexible reaction cavity

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

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

Closer examination of the cyclobutanol t/c ratios from homologous and isomeric n-alkanones in the same zeolites reveals some interesting trends (Table 12). The t/c ratios from all of the alkanones in the Na-X and Na-Y zeolites are as expected from reaction in a large or flexible reaction cavity which is very polar (like that provided by an alcohol solution). The t/c ratios in the ZSM zeolites indicate that selectivity depends upon the total ketone length and the position of the odd electron centers of the BR along the chain. Ratios of 60 or more are reported from 4-alkanones with 9 or more carbon atoms the t/c ratios from 4-octanone in ZSM-5 and ZSM-11, 15 and 18 respectively, mark 8 carbon atom chains as being critical to a c-BR occupying... 

Soft or Flexible Reaction Cavity Solution, Micelles, Liquid crystals,... 

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

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

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... 

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... 

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. 

C. Reaction Cavities with Some Wall Flexibility ... 

E. Reaction Cavities with Walls of Variable Flexibility and Strong External Medium Influences Aqueous Microheterogeneous... 

The highly flexible nature of the micellar reaction cavities has been... 

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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