Dielectric properties of solvents

According to the mode of parameter correlation, Ej(30) was introduced in the group of parameters that describe the acidity of solvent and partially its polarity, and n in the group of parameters that present the dielectric properties of solvents. Quantitative relations between different parameters of polarity, such as correlation between the n scale or and the dielectric constant and refractive index of... 

The observations of the effect of hydrogen bond formation on the electronic spectra of organic molecules represent experimental evidence for the polarization, i.e. the electron-shifts in the ground state, arising from true hydrogen bond formation. The difficulty to separate this effect from that due to the normal dielectric properties of solvents can be overcome in the case of phenolic substances by a comparison with the corresponding methyl ethers. 

Horiuchi T, Takahashi M, Kikuchi J, Yokoyama S, Maeda H (2005) Effect of dielectric properties of solvents on the quality factor for a beyond 900 MHz cryogenic probe model. J Magn Reson 174 34 2... 

As seen in the preceding sections, for a quantitative description of the electrostatic part of the solvent effect, or even for its qualitative evaluation, it is necessary to know the dielectric constant (relative permittivity) of the solvent (or possibly that of the solution). Accordingly, even in the early stages of research, the study of the dielectric properties of solvents and solutions served as the basis of attempts at elucidating the solvent effect. 

Oh et al investigated the influence of the dielectric properties of solvents used in the hydrothermal reaction on the morphology and particle size of ceria nanocrystals. Cerium nitrate and KOH were used as starting materials, and methanol, ethanol, ethylene glycol, and 1,4-butylene glycol were chosen as different dielectric solvents. They found that the size of the final products increased along with an increase of the dielectric constant of the solvent and reaction temperature, and a decrease in the pff values of the reaction medium. 

Electron transfer reaction rates can depend strongly on tire polarity or dielectric properties of tire solvent. This is because (a) a polar solvent serves to stabilize botli tire initial and final states, tluis altering tire driving force of tire ET reaction, and (b) in a reaction coordinate system where the distance between reactants and products (DA and... 

The Structure of Ice and Water. It has not yet been necessary to consider in detail the properties of particular solvents. In Table 1 we gave a list of values for the dielectric constants of various solvents but apart from this we have not yet paid attention to the observed properties of solvents or of the ions which are commonly dissolved in them. Before continuing the discussion which was in progress in Sec. 23, it will be useful to review in some detail the state of our knowledge of the liquids which are used as solvents, and of the species of ions which are most often studied in solution. Although non-aqueous solutions are of great interest for the sake of comparison, nevertheless aqueous solutions are still of paramount importance, and we shall pay most of our attention to H20 and D20 and to ions dissolved in these liquids. 

In order to obtain better separations it is very important to know the bulk physical properties of solvents (viscosity, refractive index, dielectric constant, dipole moment. 

An interesting hypothesis may be put forward. The interfacial pA lcm (Fig. 5.1) that a solute exhibits depends on the dielectric environment of its location in the bilayer. Simple isotropic water-miscible solvents may be used to approximate p mem pure methanol (e 32), may do well for the bilayer zone containing the phosphate groups pure 1,4-dioxane (e 2) may mimic some of the dielectric properties of the hydrocarbon region. It appears that psKa values of several weak bases, when extrapolated to 100% cosolvent, do approximate pvalues [119,162,172]. Fernandez and Fromherz made favorable comparisons using dioxane [448]. This idea is of considerable practical use, and has been largely neglected in the literature. 

Instead of the quantity given by Eq. (15), the quantity given by Eq. (10) was treated as the activation energy of the process in the earlier papers on the quantum mechanical theory of electron transfer reactions. This difference between the results of the quantum mechanical theory of radiationless transitions and those obtained by the methods of nonequilibrium thermodynamics has also been noted in Ref. 9. The results of the quantum mechanical theory were obtained in the harmonic oscillator model, and Eqs. (9) and (10) are valid only if the vibrations of the oscillators are classical and their frequencies are unchanged in the course of the electron transition (i.e., (o k = w[). It might seem that, in this case, the energy of the transition and the free energy of the transition are equal to each other. However, we have to remember that for the solvent, the oscillators are the effective ones and the parameters of the system Hamiltonian related to the dielectric properties of the medium depend on the temperature. Therefore, the problem of the relationship between the results obtained by the two methods mentioned above deserves to be discussed. 

The heating characteristics of a particular material (for example, a solvent) under microwave irradiation conditions are dependent on the dielectric properties of the material. The ability of a specific substance to convert electromagnetic energy into heat at a given frequency and temperature is determined by the so-called loss tangent, tan <5. The loss factor is expressed as the quotient tan (3 = "/ , where e" is the dielectric loss, indicative of the efficiency with which electromagnetic radiation is... 

A selective heating in liquid/liquid systems was exploited by Strauss and coworkers in a Hofmann elimination reaction using a two-phase water/chloroform system (Fig. 2.10) [32]. The temperatures of the aqueous and organic phases under micro-wave irradiation were 110 and 55 °C, respectively, due to the different dielectric properties of the solvents (Table 2.3). This temperature differential prevented decomposition of the final product. Comparable conditions would be difficult to obtain using traditional heating methods. A similar effect has been observed by Hallberg and coworkers in the preparation of /3,/3-diarylated aldehydes by hydrolysis of enol ethers in a two-phase toluene/aqueous hydrochloric acid system [33],... 

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. 

It has been pointed out, that the dielectric constant is an important quantity in determining the extent of dissociation of associated ions. The dissociation constants Aj)iss of quaternary ammonium salts may be considered as a qualitative indication of the dielectric properties of a solvent. For example, in a medium of e 10 A diss 10 4 and in a medium of e 35 Aoiss 10 2 ... 

The dielectric properties of the solvent have also an influence on the ionization constant of an incompletely ionized substrate. By the process of ion dissociation the concentration of associated ions is decreased this results because the latter are in equilibrium with non-ionized species and the ionization equilibrium will be restored by the formation of additional associated ions. 

The physical properties of solvents greatly influence the choice of solvent for a particular application. The solvent should be liquid under the temperature and pressure conditions at which it is employed. Its thermodynamic properties, such as the density and vapor pressure, temperature and pressure coefficients, as well as the heat capacity and surface tension, and transport properties, such as viscosity, diffusion coefficient, and thermal conductivity, also need to be considered. Electrical, optical, and magnetic properties, such as the dipole moment, dielectric constant, refractive index, magnetic susceptibility, and electrical conductance are relevant, too. Furthermore, molecular... 

Catalyst stability in this system is substantially influenced by the characteristics of solvents and promoters. Indeed, the properties of solvents and promoters which improve the catalytic activity for ethylene glycol production (increased dielectric constant, greater cation complexing ability, or... 

In the homogeneous mechanism, the reaction is assumed to start by protonation of one of the reactants, either ester (mechanisms denoted as Aac1 and Aac2 [397,398]) or, less frequently, alcohol (mechanism Aal1). It seems likely that protonation of reactants is an important step in esterification catalysed by ion exchangers, too. This follows from all that has been said above about the effect of the acidic properties of ion exchangers on their catalytic activity and is further supported by the effect of the dielectric constant of solvents (Fig. 18), which indicates that the reaction mechanism involves a positive ion and a dipolar molecule [454]. 

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