And solvent cage

Because of its tunable density and low viscosity, synthetic organic chemists are beginning to utilize supercritical C02 as a medium for exploring reaction mechanisms and solvent cage effects [10,11]. Asymmetric catalysis represents an area in which supercritical C02 may be useful as a solvent [12]. For polymerization reactions, in particular, the solvency of C02 as a medium and the plasticization effects of C02 on the resulting polymeric products represent the properties of central importance. These significant properties of C02 are explored in detail below. When all of these factors are combined with the fact that C02 may obviate the use of much more expensive and hazardous solvents,... 

Yang H, Snee PT, Kotz KT, Payne CK, Harris CB. Femtosecond infrared study of the dynamics of solvation and solvent caging. J Am Chem Soc 2001 123 4204-10. 

Kinetic studies of free-radical initiation in CO2 were conducted using AIBN as the initiator [16, 29]. The study was undertaken using high-pressure UV spectroscopy. The results showed decomposition to be slower in CO2 relative to that measured in benzene owing to the reduced dielectric strength of carbon dioxide. However, it was also determined that the initiator efficiency is much higher in CO2 because of reduced viscosity and solvent cage effects. Initiator... 

For all cases, the. competing homolysls yields oxy radical pairs capable of self-dlsproportlonatlon, a complicating feature for both product analyses and solvent-cage considerations. 

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 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... 

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 ... 

When the decomposition is carried out in an inert solvent, methyl acetate and ethane are formed, whereas in the gas-phase decomposition methyl acetate is completely absent and ethane is produced in much smaller quantity, It was suggested that the dimers in solution represent the recombination of methyl, and the combination of methyl and acetoxy radicals, within the solvent cage. ... 

Evidence that the actual methylation of the anion can be divided into SnI, Eq. (3), and Sx2 types, Eq, (4), is provided by a whole series of investigations. " The terms S l and 8 2 must be taken to mean reactions with, respectively less or greater nucleophilic participation of the anion in the transition state. The importance of oriented ion pairs" in the solvents of low polarity frequently used in reactions involving diazomethanc, e.g., the ions formed by a diazoalkane and benzoic acid in ether, should be emphasized. The expression oriented ion pair means that, because of insufficient solvation, the ions are not individually solvated but exist as ion pairs within a solvent cage. The orientation within the ion pair is defined electrostatically, and this orientation fixes the path for the productdetermining step. Several indications (cf, foo otes 22-24) in the literature indicate the occurrence of carbonium ions and oriented ion pairs in Broensted-type equilibria of the type of Eq. (2). 

The Lewis acid complex 4 can cleave into an ion-pair that is held together by the solvent cage, and that consists of an acylium ion and a Lewis acid-bound phenolate. A fr-complex 6 is then formed, which further reacts via electrophilic aromatic substitution in the ortho- or para-position ... 

The high rate of decarboxylation of aliphatic acyloxy radicals is also the prime reason behind low initiator efficiencies (see 3.3.2.1.3). Decarboxylation occurs within the solvent cage and recombination gives alkane or ester byproducts. Cage return for LPO is 18-35% at 80 °C in -octane as compared to only 4% for BPO under similar conditions.144... 

The mean distance between two solutes at 0.02 M concentration is roughly the same as in the gas phase at one atmosphere pressure.1 Depending on the sizes of solute and solvent, about 4-12 solvent molecules make a cage. A solute molecule enclosed in... 

Rate constants that are near the diffusion-controlled limit may need to have a correction applied, if they are to be compared with others that are slower. To see this, consider a two-step scheme. In the first, diffusion together and apart occur the second step is the unimolecular reaction within the solvent cage. We represent this as... 

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