Cage effects supercritical fluids

The chlorine atom cage effect was used as a highly sensitive probe for studying the effect of viscosity and the possible role of solvent clusters on cage lifetimes and reactivity for reactions carried out in supercritical fluid solvents. The results of these experiments provide no indication of an enhanced cage effect near the critical point in SC-CO2 solvent. The magnitude of the cage effect observed in SC-CO2 at all pressures examined is well within what is anticipated on the basis of extrapolations from conventional solvents (Fletcher et al., 1998). 

Much of the research discussed in this chapter has demonstrated that sc C02 is a viable alternative solvent for free-radical reactions. Moreover, several of these studies demonstrate that the unique properties of a supercritical fluid, specifically the ability to change important solvent properties such as viscosity by varying pressure (and temperature), can be exploited to manipulate reaction yield and selectivity. Solvent viscosity is particularly important for reactions that are diffusion-controlled or those for which cage-effects are important. 

There is one more unique feature of supercritical fluid solvents that will be a recurring theme in this chapter. Several studies have demonstrated that near the critical point, the density of the solvent about a solute is enhanced relative to the bulk density (solvent/solute clustering). As such, the mobility of the solute may be impeded to an extent greater than expected on the basis of the bulk viscosity. This phenomenon may also affect reactivity for reactions that are diffusion-controlled or for which cage effects are important, particularly near the critical point (vide infra). 

Fletcher, B. Suleman, N. K. Tanko, J. M. Free Radical Chlorination of Alkanes in Supercritical Carbon Dioxide The Chlorine Atom Cage Effect as a Probe for Enhanced Cage Effects in Supercritical Fluid Solvents. J. Am. Chem. Soc. 1998, 120, 11839-11844. 

Cage effects in supercritical fluids have attracted a great deal of attention lately because there is some indication diat near the critical point, the cage effect may be... 

Finally, the PFR of 1-naphthyl acetate has been performed in supercritical carbon dioxide the obtained resnlts point to an enhanced solvent cage effect on the product ratios at lower supercritical fluid bulk densities, which is consistent with the existence of supercritical solvent-solute clusters with a lifetime similar to or longer than that of the primary singlet radical pair [143]. 

B Fletcher, NK Suleman, JM Tanko. Free radical chlorination of alkanes in supercritical carbon dioxide the chlorine atom cage effect as a probe for enhanced cage effects in supercritical fluid solvents. J Am Chem Soc 120 11839, 1998. 

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