Dielectric tuning

A plot of ee and e as a function of pressure (Figure 4.7-7(a)) shows the reverse relationship between the polarity of the medium and die enrichment of the (/ ,5)-enantiomer. The increase in asymmetric induction with decreasing pressure is fully in line with its increase with decreasing e in liquid solvents, as demonstrated for the enantiomeric ratio in Figure 4.7-7(b). This new result demonstrates the possibility of dielectric tuning of catalytic reactions in SCFs. 

Now, we should ask ourselves about the properties of water in this continuum of behavior mapped with temperature and pressure coordinates. First, let us look at temperature influence. The viscosity of the liquid water and its dielectric constant both drop when the temperature is raised (19). The balance between hydrogen bonding and other interactions changes. The diffusion rates increase with temperature. These dependencies on temperature provide uS with an opportunity to tune the solvation properties of the liquid and change the relative solubilities of dissolved solutes without invoking a chemical composition change on the water. 

An additional benefit of this technology is that the choice of solvent for a given reaction is not governed by the boiling point (as in a conventional reflux set-up) but rather by the dielectric properties of the reaction medium, which can be easily tuned, e.g., by the addition of highly polar materials such as ionic liquids. 

One approach to the production of high-performance dielectrics relies on the use of mixed-metal, multiple-component oxides. These oxides provide convenient means for controlling the dielectric-constant breakdown-field product through incorporation of components that specifically contribute to performance via dielectric constant or breakdown. At the same time, the mixed materials can inhibit crystallization, resulting in deposition of amorphous films with extremely flat surfaces. Common candidates, base oxides for tuning these properties, are listed in Table 4.1. 

Many solvent properties are related to density and vary with pressure in a SCF. These include the dielectric constant (er), the Hildebrand parameter (S) and n [5], The amount a parameter varies with pressure is different for each substance. So, for example, for scC02, which is very nonpolar, there is very little variation in the dielectric constant with pressure. However, the dielectric constants of both water and fluoroform vary considerably with pressure (Figure 6.3). This variation leads to the concept of tunable solvent parameters. If a property shows a strong pressure dependence, then it is possible to tune the parameter to that required for a particular process simply by altering the pressure [6], This may be useful in selectively extracting natural products or even in varying the chemical potential of reactants and catalysts in a reaction to alter the rate or product distributions of the reaction. 

Recently, much progress has been made in fine-tuning the properties of fluorinated polyimides by copolymerization. Serious property conflicts, such as the countertrends of the desirable decrease in dielectric constant with fluorine incorporation and the undesirable increase in CTE have been significantly resolved by judicious choice of comonomers and their ratios. 

The dielectric constant of a substance may be measured by determining the ratio of the capacity of a condenser filled with the substance and the capacity of the empty condenser. The electrical apparatus involves the condenser whose capacity is to be determined in parallel with a calibrated variable condenser, in a tuned resonant circuit the determination J3 made by adjusting the variable condenser to keep the resonance frequency constant, and this requires that the sum of the capacities of the two condensers be constant.1... 

Several workers have employed monomodal cavities for microwave chemistry on the sub-gram scale. In some cases in which monomodal cavities have been used7, special benefits of so-called focussed microwaves have been claimed. As mentioned earlier, the dielectric properties of a sample can alter substantially with temperature and/or with changing chemical composition. Hence, regardless of whether multi-modal or unimodal cavities are employed, frequent tuning may be necessary if heating efficiency is to be retained. This aspect has often been overlooked by proponents of focussed microwaves. The nett result is that transfer of microwave conditions between monomodal to multi-modal cavities is usually facile. With the MBR (which had a tunable multimodal cavity), Cablewski et al. performed five reactions that had been conducted earlier on the gram scale or below with focussed microwaves (T. Cablewski, B. Heilman, P. Pilotti, J. Thorn, and C.R. Strauss, personal communication see also Ref. 117 for conference poster). These were scaled-up between 40- and 60-fold and reaction conditions... 

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