Cage of solvent

It has been estimated (4) that in most common solvents at room temperature two reactant molecules within a cage of solvent molecules will collide from 10 to a 1000 times before they separate. The number of collisions per encounter will reflect variations in solvent viscosity, molecular separation distances, and the strength of the pertinent intermolecular forces. High viscosities, high liquid densities, and low temperatures favor many collisions per encounter. 

DY-x e dissociation energy of Y X bond probability of formed free radical pair to escape the cage of solvent or polymer kJ moU1... 

Franck-Rabinowitch hypothesis physchem The hypothesis that the decreased quantum efficiencies of certain photochemical reactions observed in the dissolved or liquid state are due to the formation of a cage of solvent molecules around the molecule which has been excited by absorption of a photon. frat)k ro bin-o.wich hT,path-3-s3s ... 

Radicals initially formed in solution by a bond homolysis will be held together briefly in a cage of solvent molecules. Because radical recombinations and disproportionations are so fast, they can compete with diffusion of the radicals through the layer of solvent molecules that surround them, with the consequence that some of the radicals formed never become available to initiate other processes in the bulk of the solution.98 These recombinations are termed geminate recombinations, and the phenomena that arise from this behavior are cage effects. 

When two radicals are in close association as a pair surrounded by a cage of solvent molecules, the two odd electrons will interact with one another just as two electrons do within a molecule. The interaction will yield either a singlet state, if the two electrons have spins paired, or a triplet, if the spins are unpaired. If, for example, the caged pair arose by thermal dissociation of an ordinary ground state molecule, in which all electrons would have been paired, the state would initially be a singlet, S, whereas if the pair arose in a photochemical reaction from dissociation of an excited molecule in a triplet state, it would be initially a triplet, T. 

It has been deduced from the order of reaction with respect to monomer that the rate of initiation is not independent of monomer (122). One could assume that initiator and monomer may form a complex from which the actual initiator is derived, or else that the AIBN and its radical products are in equilibrium within a cage of solvent. Although kinetic data are consistent with such assumptions, it appears that the results can be explained reasonably on the grounds of termination by primary radicals (21). The order of reaction with respect to AIBN initiator in nearly all of these studies is the expected 0.50 or possibly slightly higher (0.55 to 0.59). 

Fig. 10.2.4 Activated complex atoms (open circles) in a cage of solvent molecules (filled circles). The mean force (Fi) on the atoms in the activated complex from the solvent molecules is shown.

Fig. 6. A diagram showing the dynamic interconveision of solvent-separated ion pairs (SSIP, exdplexes, contact ion pairs (CIP), and free ions in solution. Electron transfer takes place within a cage of solvent molecules to generate a SSIP or more intimate charge-transfer complex, the latter being an exciplex or CIP. The nature of the charge-transfer intermediate generated may depend on the distance separating the reactants. The distance depends on the molecular structures of the reactants, i.e., their sizes, shapes, and steric features. Free ions are produced by ion dissociation from the solvent cage...

The major cause of primary radical wastage involves cage reactions. When an initiator decomposes, the primary radicals arc each other s nearest neighbors for about 10 sec. During this interval they are surrounded by a cage of. solvent and monomer molecules through which they must diffuse to escape each other. Once one or the other radical leaves the cage it is extremely unlikely that the pair... 

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. 

Solvation takes place within 100-1000 fs. Reactions in the solution phase take place in a cage of solvent molecules. Bimolecular reactions in the solvent cage take place within several hundred femtoseconds, whereas colhsions in the gas phase take place in the order of picoseconds. In the solvent cage, molecules A and B collide with each other, and a successful collision leads to reaction to give product P. Excess energy from P is transferred to solvent molecules by the subsequent collision with solvent molecules. Therefore, one of the most important roles of the solvent is removal of heat generated in the reaction. In the solution phase, the rate of a chemical reaction is determined by the activation energy. This is mostly... 

The interchange mechanism accommodates not only reactions within ion-paired precursor assemblages but also those in which the essential steps take place within a relatively immobile cage of solvent or other molecules. 

Solvent separated ion pairs will also be overall uncharged and will execute Brownian motion. They will also be enclosed in a cage of solvent molecules, but since the interactions between the ions will be considerably smaller than those between ions in contact, they will separate and escape from the cage much sooner than the contact ion pairs. On the time scale of colUsions they can be considered as separating and thus not moving as a single entity. The translational Brownian motion could then be perturbed by an external field, so that, on average, the motion of a cation could be in the direction of the external field and so be able to conduct the current, and if the ion is an anion it will move in the opposite direction. 

The factor f, called initiator efficiency, takes into account that not all the primary radicals R effectively initiate polymer chains some can be lost due to the so-called cage effect. This implies secondary reactions of the radicals within a cage of solvent surrounding the initiator [5] (the effect can be more pronounced at high conversions/viscosities due to diffusion limitations). The values of / usually lie in the range 0.3-0.8. 

The initiator radicals initially formed in solution are held together briefly in a cage of solvent molecules. This cage effect causes radical molecules to recombine and slows down their diffusion through the solvent. Therefore, the rate of initiation (R.) depends on the rate of decomposition of the initiator (k ) and on the fraction e of the initiator radicals that escape the solvent cage affecting the efficiency of the chain initiation reaction (20). 

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