Effect, dielectric

Another special case of pyroelectricity is ferrodastidty. In these materials, the direction of the spontaneous polarization can be changed by applying a mechanical stress. In some crystals such as gadolinium molybdate [Gd2(Mo04)3 GMO] the ferroelastic effect is coupled with a ferroelectric effect (Bohm and Kiirten, 1973). 

Piezoelectrics are materials in which a mechanical stress results in a shift of charge carriers, and hence a polarization (see Section 8.5.2). The basic requirement of piezoelectricity is the existence of a crystal structure lacking a center of symmetry. Conversely, this asymmetry causes a strain which is linearly proportional to the electric field strength, an effect termed the reverse piezodectric effed. 

In a symmetric crystal, the strain is proportional to the square of the electric field strength, known as the electrostrictive strain effect. 

Ferroelectric and piezoelectric ceramics, in particular, play an ever-increasing role as materials for electrical and electronic applications that include multilayer capacitors (MLCs), bypass capacitors, dielectric resonators for frequency stabilization of microwave circuits, low-noise oscillators and low-insertion loss bandpass filters for microwave communication components, dielectric waveguide resonators, piezoelectric transducers and sensors, piezomechanical actuators and motors, 

PTC thermistors, and a large variety of novel emerging utilizations. Some of these applications are described later in the chapter (see Section 8.7). 

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N. G. McCmm, B. E. Read, and G. WiUiams, Anelastic and Dielectric Effects in Polymer Solids,]ohxs Wiley Sons, Inc., New York, 1967. 

The treatment of electrostatics and dielectric effects in molecular mechanics calculations necessary for redox property calculations can be divided into two issues electronic polarization contributions to the dielectric response and reorientational polarization contributions to the dielectric response. Without reorientation, the electronic polarization contribution to e is 2 for the types of atoms found in biological systems. The reorientational contribution is due to the reorientation of polar groups by charges. In the protein, the reorientation is restricted by the bonding between the polar groups, whereas in water the reorientation is enhanced owing to cooperative effects of the freely rotating solvent molecules. 

AK Churg, A Warshel. Control of the redox potential of cytochrome c and microscopic dielectric effects m proteins. Biochemistry 25 1675, 1986. 

MCCRUM, N. G., READ. B. E. and WILLIAMS, G., Anelostic and Dielectric Effects in Polymeric Solids, Wiley, New York, London (1967)... 

McCrum, N.G., Read, B.E. and Williams, G. (1967) Anelastic and Dielectric Effects in Polymeric Solids (Wiley, London and New York) (Reprinted in 1991 by Dover). 

Another different 7t -scale which indicates solvent dipolarity/polarizability and which is a measure of the ability of the solvent to stabilize a charge or a dipole by virtue of the dielectric effect, has been proposed for numerous Lewis bases including sulphoxides89. 

ELECTROSTATIC FREE ENERGIES AND DIELECTRIC EFFECTS IN MACROMOLECULES... 

FIGURE 6.6. The type of model compounds that were used to estimate the electrostatic stabilization in lysozyme (the only hydrogen atom shown, is the one bonded to the oxygen). Such molecules do not show a large rate acceleration due to electrostatic stabilization of the positively charged carbonium transition state. However, the reaction occurs in solution and not in a protein-active site, and the dielectric effect is expected to be very different in the two cases. 

Electrostatic Free-Energies and Dielectric Effects in Macromolecules, 122... 

Taft et al. [28] proposed the solvatochromic parameters, tt, a, and p, which describe three solvent s abilities, respectively, to stabilize a charge or a dipole by virtue of its dielectric effect, to donate a proton (or accept an electron pair), and to accept a proton (or donate an electron pair). It was shown that the AN for nonprotonic solvents correlates... 

The charge-state section highlighted the value of Bjerrum plots, with applications to 6- and a 30-pKa molecules. Water-miscible cosolvents were used to identify acids and bases by the slope in the apparent pKa/wt% cosolvent plots. It was suggested that extrapolation of the apparent constants to 100% methanol could indicate the pKa values of amphiphilic molecules embedded in phospholipid bilayers, a way to estimate pAi m using the dielectric effect. 

The dielectric effect on the interactions among inner region atoms is represented... 

McCrum N. G., Read B. E., and Williams G., Anelastlc and Dielectric Effects in Polymer Solids, John Wiley and Sons (1967). 

In addition to the described above methods, there are computational QM-MM (quantum mechanics-classic mechanics) methods in progress of development. They allow prediction and understanding of solvatochromism and fluorescence characteristics of dyes that are situated in various molecular structures changing electrical properties on nanoscale. Their electronic transitions and according microscopic structures are calculated using QM coupled to the point charges with Coulombic potentials. It is very important that in typical QM-MM simulations, no dielectric constant is involved Orientational dielectric effects come naturally from reorientation and translation of the elements of the system on the pathway of attaining the equilibrium. Dynamics of such complex systems as proteins embedded in natural environment may be revealed with femtosecond time resolution. In more detail, this topic is analyzed in this volume [76]. 

In typical QM-MM simulations, no dielectric constant is included. Orientational dielectric effects come naturally from reorienting and translation of the elements of the system, providing the system comes to equilibrium. What is left out of the model is electronic polarization of molecules, which makes a minor contribution. 

Rinaldi, D. and J.-L. Rivail. 1973. Molecular Polarizability and Dielectric Effect of Medium in the Liquid Phase. Theoretical Study of the Water Molecule and Its Dimer. Theoret. Chim. Acta 32, 57 (in French). 

Considering this chain of electrical resistors, we should take into account the influence of the binder (dielectric) effecting on all the above contact resistances in the circuit and increasing their value (Rb). Evidently, the main influence on the ESR of electrode is this circuit because of a big number of contact resistances distributed in the bulk of the electrode. 

Experimental and theoretical results are presented for four nonlinear electrooptic and dielectric effects, as they pertain to flexible polymers. They are the Kerr effect, electric field induced light scattering, dielectric saturation and electric field induced second harmonic generation. We show the relationship between the dipole moment, polarizability, hyperpolarizability, the conformation of the polymer and these electrooptic and dielectric effects. We find that these effects are very sensitive to the details of polymer structure such as the rotational isomeric states, tacticity, and in the case of a copolymer, the comonomer composition. 

When a strong static electric field is applied across a medium, its dielectric and optical properties become anisotropic. When a low frequency analyzing electric field is used to probe the anisotropy, it is called the nonlinear dielectric effect (NLDE) or dielectric saturation (17). It is the low frequency analogue of the Kerr effect. The interactions which cause the NLDE are similar to those of EFLS. For a single flexible polar molecule, the external field will influence the molecule in two ways firstly, it will interact with the total dipole moment and orient it, secondly, it will perturb the equilibrium conformation of the molecule to favor the conformations with the larger dipole moment. Thus, the orientation by the field will cause a decrease while the polarization of the molecule will cause an... 

Figure 2. Nonlinear dielectric effect constant ( 10 scm7SC 2moF ) of Br(CH2)n, Br at 25 °C. Points represent the experimental results of Chelkowski (19, 20). The solid and dashed lines are calculated with 4>s, = 120 ° and +80 °, respectively, for thefirst and last C-C bonds. (Reproduced with permission from Ref 6. Copyright 1981, American...

Since the absorption process is 7r/2 out of phase with the electromagnetic field its effect on the index of refraction will also lag in the same way behind the simple dielectric effect. The total index of refraction is therefore formulated as 77 = n + ik, where k is the absorptive contribution. 

Application of CBS extrapolations to the A5-ketosteroid isomerase-catalyzed conversion of A5-androstene-3,17-dione to the A4 isomer (Fig. 4.10) provides a test case for extensions to enzyme kinetics. This task requires integration of CBS extrapolations into multilayer ONIOM calculations [56, 57] of the steroid and the active site combined with a polarizable continuum model (PCM) treatment of bulk dielectric effects [58-60], The goal is to reliably predict absolute rates of enzyme-catalyzed reactions within an order of magnitude, in order to verify or disprove a proposed mechanism. 

Ionization of a covalent compound may be defined as the process leading to the formation of solvated ions independent of their presence as associated ions or as free entities (Fig. 6). In a medium of low dielectric constant the formation of associated ions is favored. It is therefore conceivable to consider the overall process of ionization as consisting of two steps, i.e., the formation of associated ions due to cation-coordination and anion-solvation and the dissociation of the associated ions in solution as a dielectric effect. 

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