Cage solvent

Cage, solvent, 134 Cancellation assumption. 447 Catalysis, 263 acid, 453 buffer, 269 definitions of, 263 electrophilic, 265 general acid, 265, 268 general base, 265, 268, 271 intermolecular, 266 intramolecular, 266 nucleophilic, 266, 268, 271... 

Modeling by Pratt and Chandler [14], Zichi and Rossky [9, 15], Watanabe and Andersen [16], and Ravishanker et al. [17, 18] shows that the solvent-separated minimum can sometimes be most stable. For example. Figure 30.10 shows the krypton-krypton pair correlation function in water computed by Watanabe and Andersen [16] the second small peak represents a population of solvent-separated pairs. This state is stable because each nonpolar molecule is surrounded by a clathrate cage. Solvent-separated states are most likely to occur when the solutes are small [19]. 

An octahedral coordination capsule 560 with triangular faces covered by threefold chelating synthones has been prepared in [57], One-pot coordination-driven self-assembly of tris(2-hydroxybenzylidene)triaminoguanidinium chloride with palladium(II) chloride and sodium 5,5-diethylbarbiturate by Scheme 4.59 in the presence of triethylamine and tetraeth-ylammonium chloride gave the crystals of a cage complex of 560. This T-symmetric chiral complex contains four encapsulated sodium cations and caged solvent water molecules as well [57]. 

A special case of adsorption in cavities is that of clatherate compounds. Here, cages are present, but without access windows, so for adsorption to occur the solid usually must be crystallized in the presence of the adsorbate. Thus quinol crystallizes in such a manner that holes several angstroms in diameter occur and, if crystallization takes place in the presence of solvent or gas... 

This is no longer the case when (iii) motion along the reaction patir occurs on a time scale comparable to other relaxation times of the solute or the solvent, i.e. the system is partially non-relaxed. In this situation dynamic effects have to be taken into account explicitly, such as solvent-assisted intramolecular vibrational energy redistribution (IVR) in the solute, solvent-induced electronic surface hopping, dephasing, solute-solvent energy transfer, dynamic caging, rotational relaxation, or solvent dielectric and momentum relaxation. 

The introductory remarks about unimolecular reactions apply equivalently to bunolecular reactions in condensed phase. An essential additional phenomenon is the effect the solvent has on the rate of approach of reactants and the lifetime of the collision complex. In a dense fluid the rate of approach evidently is detennined by the mutual difhision coefficient of reactants under the given physical conditions. Once reactants have met, they are temporarily trapped in a solvent cage until they either difhisively separate again or react. It is conmron to refer to the pair of reactants trapped in the solvent cage as an encounter complex. If the unimolecular reaction of this encounter complex is much faster than diffiisive separation i.e., if the effective reaction barrier is sufficiently small or negligible, tlie rate of the overall bimolecular reaction is difhision controlled. 

For very fast reactions, the competition between geminate recombmation of a pair of initially fomied reactants and its escape from the connnon solvent cage is an important phenomenon in condensed-phase kinetics that has received considerable attention botli theoretically and experimentally. An extremely well studied example is the... 

In order to probe the importance of van der Waals interactions between reactants and solvent, experiments in the gas-liqnid transition range appear to be mandatory. Time-resolved studies of the density dependence of the cage and clnster dynamics in halogen photodissociation are needed to extend earlier quantum yield studies which clearly demonstrated the importance of van der Waals clnstering at moderate gas densities [37, 111]... 

The simple difhision model of the cage effect again can be improved by taking effects of the local solvent structure, i.e. hydrodynamic repulsion, into account in the same way as discussed above for bimolecular reactions. The consequence is that the potential of mean force tends to favour escape at larger distances > 1,5R) more than it enliances caging at small distances, leading to larger overall photodissociation quantum yields [H6, 117]. 

The analysis of recent measurements of the density dependence of has shown, however, that considering only the variation of solvent structure in the vicinity of the atom pair as a fiinction of density is entirely sufficient to understand tire observed changes in with pressure and also with size of the solvent molecules [38]. Assuming that iodine atoms colliding with a solvent molecule of the first solvation shell under an angle a less than (the value of is solvent dependent and has to be found by simulations) are reflected back onto each other in the solvent cage, is given by... 

Wang W, Nelson K A, Xiao L and Coker D F 1994 Molecular dynamics simulation studies of solvent cage effects on photodissociation in condensed phases J. Chem. Phys. 101 9663-71... 

An individual radical from die RP may be scavenged by a solvent or another chemical species to fonn diamagnetic products. Because the products are fonned following escape from the cage, they are known as escape or scavenging products. 

With heteroaromatic substrates it is possible to prepare, for example, thiazolyipyridines. It is noteworthy that basic solvents (e.g., heterocyclic nitrogen compounds) increase the yield of substitution by a cage effect (see Tables III-37 and III-38) (208). 

In discussing mechanism (5.F) in the last chapter we noted that the entrapment of two reactive species in the same solvent cage may be considered a transition state in the reaction of these species. Reactions such as the thermal homolysis of peroxides and azo compounds result in the formation of two radicals already trapped together in a cage that promotes direct recombination, as with the 2-cyanopropyl radicals from 2,2 -azobisisobutyronitrile (AIBN),... 

Once the radicals diffuse out of the solvent cage, reaction with monomer is the most probable reaction in bulk polymerizations, since monomers are the species most likely to be encountered. Reaction with polymer radicals or initiator molecules cannot be ruled out, but these are less important because of the lower concentration of the latter species. In the presence of solvent, reactions between the initiator radical and the solvent may effectively compete with polymer initiation. This depends very much on the specific chemicals involved. For example, carbon tetrachloride is quite reactive toward radicals because of the resonance stabilization of the solvent radical produced [1] ... 

The assumption that k values are constant over the entire duration of the reaction breaks down for termination reactions in bulk polymerizations. Here, as in Sec. 5.2, we can consider the termination process—whether by combination or disproportionation to depend on the rates at which polymer molecules can diffuse into (characterized by kj) or out of (characterized by k ) the same solvent cage and the rate at which chemical reaction between them (characterized by kj.) occurs in that cage. In Chap. 5 we saw that two limiting cases of Eq. (5.8) could be readily identified ... 

Aqueous Phase. In contrast to photolysis of ozone in moist air, photolysis in the aqueous phase can produce hydrogen peroxide initially because the hydroxyl radicals do not escape the solvent cage in which they are formed (36). 

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