Dielectric constant medium effects from change

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]). 

Attempts to verify the linear relationship between — log K and 1/D by means of a series of dioxane-water mixtures have brought to light considerable discrepancies. The addition of dioxane to water results in a much greater decrease in the dissociation constant than would be expected from the change in the dielectric constant of the medium. Since the organic acids studied are more soluble in dioxane than in water, it is probable that molecules of the former solvent are preferentially oriented about the acid anion the effective dielectric constant would then be less than in the bulk of the solution. It is thus p>0B8ible to... 

Moreover, other effects are as important as the ligands. The dielectric properties of the protein matrix are very different from those of water. It has often been argued that it behaves as a medium with a low dielectric constant (around 4 compared to 80 in water) [47,123,124]. Figure 11 shows that this gives rise to a very prominent change in the reduction potential of a blue-copper site [45]. It increases by 0.8-1 V as the site is moved from water solution to the centre of a protein with a radius of 1.5 nm (like plastocyanin) or 3.0 nm (like an azurin tet-ramer). It can also be seen that it is not necessary to move the site to the centre of the protein to get a full effect. Already at the surface of the protein, 80% of the maximum effect is seen, and when the site is 0.5 nm from the surface (as is typi-... 

In this section, we shall distill from the examples presented a number of generalizations which, hopefully, may be of use to the experimentalist in understanding irreversible processes in confined systems. Considered sequentially will be the role of system size, dimensionality, shape, and internal connectivity, all facets of the underlying topology of the system. Then, the role of intermolecular potentials and the effects of changes in the multipolarity of the potential, the temperature and the dielectric constant of the ambient medium will be reviewed. 

Medium effects for transfer of acid-base equilibria from water to a dipolar aprotic solvent may be expected to differ from those from water to methanol. The change in dielectric constant is about the same, but other solute-solvent interactions are very different. For a cation acid... 

The equation for the total van der Waals interaction between two atoms or molecules [Eq. (4.40)] includes a factor for corrections due to changes in the dielectric characteristics of an intervening medium other than vacuum. That aspect of the theory can be of great importance both quantitatively and qualitatively and has significant ramifications in practical systems. A full discussion of the theoretical aspects of the effects of medium on van der Waals interactions is beyond the scope of this book, but the reader is referred to the work by Israelachvili for further enlightenment. From a practical standpoint, however, several important points arise from an analysis of the dispersion force equation for media of differing dielectric constants. The relevant points include the following ... 

---