Solvent-cage effects

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... 

Let us estimate a typical value for A. Choosing ta = rt = 5 A, p. = 2 x 10 g,T = 300 K, we find A 4 x 10 M s". This is for the gas phase. In solution the situation is somewhat different because of the solvent cage effect described in Section 4.1. During each bimolecular encounter within a solvent cage, several collisions may occur. This results in a predicted A value for liquid solutions somewhat larger than that for gases. ... 

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,... 

We have also investigated the kinetics of free radical initiation using azobisisobutyronitrile (AIBN) as the initiator [24]. Using high pressure ultraviolet spectroscopy, it was shown that AIBN decomposes slower in C02 than in a traditional hydrocarbon liquid solvent such as benzene, but with much greater efficiency due to the decreased solvent cage effect in the low viscosity supercritical medium. The conclusion of this work was that C02 is inert to free radicals and therefore represents an excellent solvent for conducting free radical polymerizations. 

Clear-cut evidence on the intermolecular nature of the excitation step and the need for in-cage radical annihilation has been gained from a study on the solvent-cage effect in the peroxyoxalate reaction. The viscosity effect on the singlet quantum yields was verified using the binary solvent system toluene-diphenyhnethane. A rise in solvent viscosity from 0.50 cP (toluene) to 2.67 cP (diphenylmethane) leads to an up to tenfold increase in the quantum yields, demonstrating the intermolecular nature of the excitation step. 

I do believe in the solvent cage effect, and that it takes a while for species to diffuse in and out so the analog of the gas phase collision is the much less frequent encounter. ... 

The quantum yield for the primary photochemical process differs from that of the end product when secondary reactions occur. Transient species produced as intermediates can only be studied by special techniques such as flash photolysis, rotating sector devices, use of scavengers, etc. Suitable spectroscopic techniques can be utilized for their observations (UV, IR, NMR, ESR, etc.). A low quantum yield for reaction in solutions may sometimes be caused by recombination of the products due to solvent cage effect. 

It may be assumed that in absence of oxygen, a large fraction of. CCi3 radicals will recombine due to solvent cage effect to give the addition product trichloro-dihydroanthracene I. This compound is expected to be unstable and eliminates HC1 in a number of ways. 

Figure 5 Illustration of solvent cage effect in delaying excited state thermal equilibration (reproduced with permission from J. Chem. Educ., 1983, 60, 797)...

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],... 

The initiation step could also be positively affected by the above-mentioned transport properties, as the efficiency factor f assumes higher values with respect to conventional liquid solvents due to the diminished solvent cage effect One further advantage is constituted by the tunability of the compressibility-dependent properties such as density, dielectric constant, heat capacity, and viscosity, all of which offer additional possibilities to modify the performances of the polymerization process. This aspect could be particularly relevant in the case of copolymerization reactions, where the reactivity ratios of the two monomers, and ultimately the final composition of the copolymer, could be controlled by modifying the pressure of the reaction system. 

Photolysis of iodine. Unlike DBK, molecular iodine photodissociates via a one bond scission. Also the wavelength of the laser pulse used by Otto et al. (19) results in a relatively mild, low energy separation of the photogenerated radicals. Therefore 12 photodissociation should be a more sensitive probe for the characterization of solvent cage effects. 

Braden DA, Parrack EE, Tyler DR. Solvent cage effects. I. Effect of radical mass and size on radical cage pair recombination efficiency. II. Is geminate recombination of polar radicals sensitive to solvent polarity Coord Chem Rev 2001 211 279-94. 

The rebinding dynamics to microperoxidase are nonexponential due to a solvent cage effect. To deduce the time constant for rebinding CO from the solvent cage, the recombination kinetics were modeled according to... 

As a result of the solvent-cage effect, any slow reaction occurring between two or more species in solution may be divided into two steps. The first step involves a diffusional encounter and the second step is a rearrangement of the encounter complex in the first step to yield a product [1,2]. If the overall rate is more than an order of magnitude slower than the diffusion-controlled limit, the first step may be treated as a pre-equilibrium process. The steps involved may be written as... 

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