Reaction control molecular cages

Caging and Diffusion-Controlled Separation. Caging has been extensively studied in photolysis experiments (57). The most well-known example is the photolysis of molecular iodine in benzene (58). The successful separation of two reaction products from an elementary reaction is more complex than their... 

That steps involving atomic or molecular motion can be rate determining, even in fluids, is well known through diffusion limited reaction rates and the solvent cage effect. In solids, motion more subtle than translational diffusion can be influential, and cases of rotational diffusion control are familiar [7],... 

Shape selective reactions are typically carried out over zeolites, molecular sieves and other porous materials. There are three major classifications of shape selectivity including (1) reactant shape selectivity where reactants of sizes less than the pore size of the support are allowed to enter the pores to react over active sites, (2) product shape selectivity where products of sizes smaller than the pore dimensions can leave the catalyst and (3) transition state shape selectivity where sizes of pores can influence the types of transition states that may form. Other materials like porphyrins, vesicles, micelles, cryptands and cage complexes have been shown to control product selectivities by shape selective processes. 

This latter low quantum yield indicates that virtually none of the radical cation (DPA4) escapes out of the cage of the geminate pair TCA /DPA4, owing to an exceptionally efficient reverse electron-transfer (which is unlikely), or that the reaction of (DPA+) with molecular oxygen is too slow to compete with the diffusion-controlled recombination of (TCA ) and (DPA+). 

If we compare Eq. (XV.2.8) with Eq. (XV.2.3), we see that the latter is about twice as large. This is to be expected because the latter measures the frequency of all A-B encounters, while Eq. (XV.2.8) measures only new encounters. Collins and KimbalP have pointed out that in a diffusion-controlled bimolecular reaction between A and B, the initial rate which can be characterized by a random spatial distribution of A and B decays to the lower rate given by Eq. (XV.2.9). The reason for this is that the reaction tends to draw off the A-B pairs in close proximity and leaves a stationary distribution of A-B which approaches that given by the concentration gradient of Eq. (XV.2.6). The relaxation time for such a decay is of the order of " riB/ir AB, which for most molecular systems will be of the order of 10 sec, or the actual time of an encounter. Noyes has shown that there exist certain experimental systems in which these effects can be observed. We shall say more about them later in our discussion of cage effects in liquids. 

For reaction in solution the analysis of diffusion control is usually based on the concept of a molecular encounter. When two solute molecules come together in a solution they are effectively held within a cage of solvent molecules and make a number of collisions with each other within this cage. Such a set of repeated collisions is termed an encounter. The lifetime of each encounter is very short, 10" ° to 10" sec. 

Zeolite-supported metal clusters are a new class of catalyst made possible by syntheses involving organometallic chemistry and by precisely controlled treatment of metal complexes in zeolite cages. Elucidation of the preparation chemistry would not have been possible without the guidance of EXAFS spectroscopy. Clusters such as Ir4, Ir, and Pt (where n is about 6) are small enough to be considered quasi molecular rather than metallic. Their catalytic properties are distinct from those of metallic particles, even for structure-insensitive reactions. The zeolite pores seem to confer some properties on the clusters that are not yet well understood. 

The termination of polymer radicals occurs by various bimolecular recombination reactions. When the oxygen supply is sufficient the termination is almost exclusively via reaction (8) of Scheme 1.55. At low oxygen pressure other termination reactions may take place (Zweifel, 1998). The recombination is influenced by cage effects, steric control, mutual diffusion and the molecular dynamics of the polymer matrix. In melts the recombination of polymer peroxy radicals (POO ) is efficient due to the high mte of their encounter. 

Equation 6 indicates that the solvent strength, 6, is pressure-dependent, providing a potential route to improved selectivity and rate by "pressure-tuning the solvent. A number of attempts to demonstrate reactivity control in su rcritical CO2 for Diels-Alder (75-77) and organic photoreactions (78,79) have exhibited very small effects. Andrew and coworkers have recently demonstrated dramatic solvent cage effects on selectivity of a photo-Fries reaction close to the critical density.(80) More polar SCF s have shown more promising results control of esterification rates and polyester molecular weight distribution via enzymatic catalysis in fluoroform has been demonstrated. (81,82)... 

Molecular clusters offer an environment in which the size of the solvent cage surrounding a chromophore can be controlled to study the effect of solvation on reaction dynamics. In addition, when charged species are employed, this type of investigation can be accomplished for mass-selected clusters by using mass spectrometric techniques. 

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