Organized media cavity

We noted earlier (Section III.D) that there can be more than one type of reaction cavity in an organized medium. If the interconversion between molecules experiencing different environments of sites of an organized medium is slow on the timescale of excited state processes, then the excited state behavior of reactant molecules must be considered in terms of several reaction cavities accessed. Studies from several laboratories have shown that site inhomogeneity in organized media is more common than site homogeneity. We highlight this point with a few illustrative examples below. 

Unlike isotropic media, where molecules have equal mobility and conformational flexibility in all dimensions, in an organized medium their mobility and flexibility are restricted or constrained in at least one dimension. For example, the reaction cavities of a micelle and cyclodextrins are made up of a hydrophobic core and a hydrophilic exterior (Fig. 11). A highly polar boundary separates the hydrophobic core from the aqueous exterior. Such a boundary provides unique features to these media that are absent in isotropic solution. Translational motion of a guest present within the reaction cavity is hindered by the well-defined boundary. 

A nonpolar neutral species in a polar medium such as water experiences interfacial tension. Solvophobic theory is a general statement of hydrophobic theory, which has been developed to explain the tendency of neutral organic species to flee the water phase. It has been reported that the solution of nonelectrolytes in water is attended by a net decrease in entropy [65,158]. This has been attributed to an increased structuring of water molecules in the vicinity of the solute. The process may be conceptually rationalized by considering that a solute must occupy space in a cohesive medium. The solute must create a cavity in the water milieu and then occupy that cavity [19,65,158]. The very high cohesive density of water creates considerable interfacial tension in the... 

The cage effects measured in various media are compiled in Table 2. Results clearly show that all of the organized media listed in the table have a cage effect larger than is observed in benzene ( 0%). Also note that the magnitudes of the cage effect and the yields of the rearrangement product depend on the medium (probably a reflection of the differences in the characteristics of the reaction cavity in various media). 

The ESIPT of 2-(2 -hydroxyphenyl)-4-methyloxazole (HPMO) (27) has been explored by Douhal and co-workers [166] for its probe characteristics in a variety of organized media which include cyclodextrin, calixarene, micelle, and HSA. The incorporation of HPMO into hydrophobic cavities in an aqueous medium involves the rupture of its intermolecular hydrogen bond to water and formation of an intramolecular hydrogen bond in the sequestered molecule. Upon excitation (280-330 nm) of this entity, a fast intramolecular proton-transfer reaction of the excited state produces a phototautomer (28), the fluorescence of which (Xm = 450 170 nm) shows a largely Stokes-shifted band. Because of the existence of a twisting motion around the C2—C bond of this phototautomer, the absorption and emission properties of the probe depend on the size of the host cav-... 

From the above examples it is clear that reaction cavity provided by an organized or confining medium has unique features that mimics some of the features of proteins. While crystals and zeolites provide reaction cavities that are inflexible, there is a whole spectrum of organized and confined media (e.g., micelles, host-guest complexes, monolayers and bilayers, liquid crystals etc.,) that allow different degrees of freedom to the reactant molecules. These systems demonstrate clever usage of favorable entropy that is so important in natural systems. One should keep in mind... 

A chemical reaction can be viewed as a phenomenon dealing with molecular shape (topology) changes. Whether a particular reaction will take place will depend upon whether the product can fit within the space occupied by the reactant. The space occupied by the reactant is the reaction cavity. Since the boundaries of a reaction cavity are undefined in an isotropic solution, size matching of the reactant, products and the reaction cavity is not important in this medium. On the other hand, when the reaction cavity has a well-defined boundary, as in most organized assemblies (especially in solid state), size matching can become important and occasionally may even become the sole factor controlling the feasibility of a reaction (Fig. 8). 

Expressions 2 and 3 show that, in order to overcome this energy difference, the micropore cavities should be largely filled with adsorbed molecules. As mentioned earlier, in cases where low-alumina-content materials have been directly synthesized, high values for 6 are invariably found. This confirms Barrer s postulation. The dominant interaction that governs narrow- and medium-pore zeolite synthesis is the strong interaction of the occluded organic molecule with the micropore wall. 

The observed photobehavior of the benzaldehyde-CDx complexes in the solid state is unique and completely different from that of these complexes in aqueous solution and also from that of benzaldehyde 36 in organic solvents. The substantial formation of 4-benzoylbenzaldehyde 38 upon irradiation in (3- and y-CDx cavities indicates that these medium-sized CDx s provide the radical pair within a fairly spacious supercage environment, thus allowing the para-rearrangement (Scheme 13). The formation of practically racemic 37 upon irradiation of the y-CDx complex may also be attributed to the looser orientation of benzaldehyde 36 in the y-CDx cavity than in the (3-CDx cavity. It was thus demonstrated that the chiral hydrophobic cavity of native cyclodextrins not only modifies the photoreactivity of the included guest but also functions as a chiral supramolecular environment for photochirogenesis, albeit resulting in only modest ee%. 

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