Dielectric constant/effect/function high- dielectrics

With the addition of a pseudopotential interaction between electrons and metal ions, the density-functional approach has been used82 to calculate the effect of the solvent of the electrolyte phase on the potential difference across the surface of a liquid metal. The solvent is modeled as a repulsive barrier or as a region of dielectric constant greater than unity or both. Assuming no specific adsorption, the metal is supposed to be in contact with a monolayer of water, modeled as a region of 3-A thickness (diameter of a water molecule) in which the dielectric constant is 6 (high-frequency value, appropriate for nonorientable dipoles). Beyond this monolayer, the dielectric constant is assumed to take on the bulk liquid value of 78, although the calculations showed that the dielectric constant outside of the monolayer had only a small effect on the electronic profile. 

Useful solvents must themselves resist oxidation or reduction, should dissolve suitable ionic solutes and nonelectrolytes, and in addition should be inexpensive and obtainable in high purity. Kratochvil indicated that the most potentially useful solvents are those that have a dielectric constant greater than about 25 and have Lewis-base properties. Some solvents meeting these criteria are acetonitrile, dimethyl-sulfoxide, dimethylformamide, dimethylacetamide, propylene carbonate, ethylene carbonate, formamide, sulfolane, and y-butyrolactone. Solvents of the Lewis-base type show specific solvation effects with many metal cations (Lewis acids). Thus acetonitrile functions as a Lewis base toward the silver ion. At the same time it reacts but little with the hydrogen ion. 

As a rule, the presence of a high dielectric constant medium leads to the rise of the dipole moment of the compound. However, the relative increase of the dipole moment varies substantially from one structure to another. For instance, the calculated dipole moment of 2(H)-3-pyrazolone (7) increases by 72% when transferred from the medium with e = 1 to the medium with s = 80, whereas the corresponding change in the dipole moment of pyrazole (3) is only 15%. Therefore, it is questionable to assume constant dipole moment values for a series of congeneric compounds in different dielectric media and derive correlations of the chemical reactivity or physical properties of such series of compounds with some fixed function of the dielectric constant of the solvent (a common approach in the linear free energy relationship studies of solvent effects, cf. [65-67]). 

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