Dielectric absorption

Dielectric Behavior of Adsorbed Water. Determination of the dielectric absorption of adsorbed water can yield conclusions similar to those from proton NMR studies and there is a considerable, although older literature on the subject. Figure XVI-7 illustrates how the dielectric constant for adsorbed water varies with the frequency used as well as with the degree of surface coverage. A characteristic relaxation time r can be estimated... 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

From the point of view of biological relevance ideally EPR spectra should be taken from aqueous solution samples at physiological temperatures. Not-so-ideal reality brings along two major practical problems paramagnetic relaxation and dielectric absorption. 

Measurements of dipole moments, Kerr constants, and dielectric absorption have been employed (81RCR336) widely to obtain information on the conformational equilibrium in acyl heterocycles. Details on conformer structures and populations depend on the choice of additive scheme, group moments, or polarizability tensor in the case of Kerr constants. Several early conclusions, especially for furan- and thiophene-2-carboxaldehyde, appeared contradictory, owing to the choice of these quantities. A more precise definition of polarizability tensors for several heterocycles and a choice of group moments and additive schemes tested on a large amount of available experimental results and supported by accurate theoretical calculations have led to more confidence in the use of experimental dipole moments and Kerr constants in conformational analysis. A limitation of the method is that the... 

Dielectric absorption on furan-2-carboxaldehyde has been measured (78JCS(F2)727 81ZPC147). Even in a polymer matrix, the energy barrier obtained for internal rotation is close to that determined with other experimental techniques, suggesting a low influence of the surrounding medium on the torsional process. 

One of the most direct methods of examining reorientational motion of molecules is by far infrared absorption spectroscopy or dielectric absorption. In the absence of vibrational relaxation, the relaxation times obtained by IR and dielectric methods are equivalent. In both these techniques we obtain the correlation function, [Pg.209]

Dielectric Absorption of Heterocyclic Molecules , S. Walker, in Physical Methods in Heterocyclic Chemistry , ed. A. R. Katritzky, Academic Press, New York, 1971, vol. 

Hydrated Zeolites. Figure 3 gives a typical plot of the conductivity vs. the reciprocal temperature for hydrated NaF86.5. The other samples behave qualitatively in the same way. Conduction and dielectric absorption are superposed. The position of the maximum of dielectric absorption is frequency dependent it shifts to higher temperatures with increasing frequency. In some favorable cases a second conduction phenomenon is observed on the low temperature side of the relaxation phenomenon (Figure 3). Because of a lack of reproducibility we cannot interpret it. 

The presence of a dipole moment and consequently the existence of the dielectric absorption phenomenon permits a study of the weak interactions that occur between solute... 

The time and wavelength resolved fluorescence dynamics of bianthryl has been investigated by several groups [30, 82, 132, 133, 115, 116]. In addition, this molecule has been studied by picosecond absorption spectroscopy [115], electric field induced fluorescence anisotropy measurements [117] and optically induced dielectric absorption (microwave) measurements [118, 119]. The results are generally in accord with the theoretical model presented in Sections III.A and III.B. One of the challenges of studying the photodynamics of BA is that the LE and CT interconversion is so rapid (i.e., on the time scale of solvation) that it is necessary to employ ultraviolet subpicosecond and even femtosecond fluorescence spectroscopy which has only recently become available [30, 82, 132, 133]. 

Solids not containing ions, such as ice and plastics, cannot strictly be thought of as polar. These molecules are locked into place by their structure and are unable to move easily. They thus are unable to participate readily in dielectric absorption and consequently have low values of dielectric constant and loss factor (see Table 1). 

Effect of the Spacer Group on Dynamic Mechanical and Dielectric Absorptions... 

Microwave dielectric loss measurements in the region 1-9 GHz have been reported for beryllocene in different solvents (cyclohexane, decalin, benzene, and 1,4-dioxane) (279). The dipolar nature of beryllocene was confirmed. The dielectric absorption was interpreted to suggest rocking movements of the cyclopentadienyl groups synchronous with oscillation of the beryllium atom between two equivalent positions in an rj5, a structure, as indicated in CII. 

FIG. 11.6 Dispersion curves of the dielectric constant (s ) and dielectric absorption (s") in the regions of electrical, infrared and optical frequencies. 

The ions interpolated in tunnel structures, listed in Table I, are situated in continuous cavities enclosed by the fixed framework of the host (39). Although there is no diffraction evidence that the guest ions are ordered, dielectric absorption in the phase Baa,Ti8 a.Mga.016 favors a model with sequential ordering of barium ions and vacant positions in any one tunnel, but with no relationship to the sequence in any other tunnel (22). This seems to be true in some, at least, of the other nonstoichiometric phases falling into this group, and the application of newer techniques can be expected to amplify the rather limited knowledge of these substances. 

These are, with decreasing resolution on the time scale, the methods of infrared and Raman spectroscopy, dielectrical absorption and NMR spectroscopy, and inelastic neutron scattering. Thermodynamic properties depend on the long lifetime average structures, which are observed by X-ray and neutron scattering. For each of these methods, the descriptor structure has a different meaning. 

Relaxation times and dispersion amplitudes" change when ions are added. If ion pairs are formed, a new relaxation region appears on the solvent relaxation spectrum on the low-frequency side. Figure 4.105 shows the dielectric absorption spectrum of LiBr in acetonitrile, and how a maximum is developed in the low-frequency region as the concentration of solute increases and ion pairs are formed. Association constants can be determined from these data and contribute to the identification of the ion pan-present. 

Fig. 4.105. Dielectric absorption spectrum (imaginary part of the complex permittivity, e") of LiBr solutions in acetonitrile at 25 °C. 1, Pure solvent 2, 0.107 M 3,0.194 M 4,0.303 M 5,0.479 M 6, 0.657 M. S and IP indicate the frequency regions of the relaxation processes of solvent and solute. For the sake of clarity, experimental data ( ) are added only for curves 1, 4, and 6 (J. Barthel, H. Hetzenauer, and R. Buchner, Ber. Bunsenges. Phys. Chem. 96 988, 1992).

Fessenden RW, Carton PM, Shimamori H, Scaiano JC. (1982) Measurement of the dipole moments of excited states and photochemical transients by microwave dielectric absorption. JPhys Chem 86 3803-3811. 

---