Characteristics dielectric constant

Solvent effects can significantly influence the function and reactivity of organic molecules.1 Because of the complexity and size of the molecular system, it presents a great challenge in theoretical chemistry to accurately calculate the rates for complex reactions in solution. Although continuum solvation models that treat the solvent as a structureless medium with a characteristic dielectric constant have been successfully used for studying solvent effects,2,3 these methods do not provide detailed information on specific intermolecular interactions. An alternative approach is to use statistical mechanical Monte Carlo and molecular dynamics simulation to model solute-solvent interactions explicitly.4 8 In this article, we review a combined quantum mechanical and molecular mechanical (QM/MM) method that couples molecular orbital and valence bond theories, called the MOVB method, to determine the free energy reaction profiles, or potentials of mean force (PMF), for chemical reactions in solution. We apply the combined QM-MOVB/MM method to... 

Various organic solvents were used as reactionary medium at nonequilibrium polycondensation in solution realization [96]. The solvent type influence on the synthesis reaction main characteristics (conversion degree Q and molecular weight MM) is well known and is explained usually by solvent various characteristics (dielectric constant, solubility parameter, heat of dissolution and so on) [96]. However, up to now the indicated effects general theoretical explanation is not obtained. Besides, at the solvent type influence analysis its correlation with polycondensation process quantitative characteristics (the same Q and MM) is usually considered, but any changes of polymer structure or reaction mechanism are not assumed, although the possibility of side reactions is noted repeatedly [96]. The authors [71, 127] studied the solvent influence on the enumerated above characteristics on die example of the rules of chloranhydride of terephthalic acid and phenolfthaleine low-temperature polycondensation (polyarylate F-2), performed in 8 different solvents [128]. 

Dielectric constant A dimensionless constant that expresses the screening effect of an intervening medium on the interaction between two charged particles. Every medium (such as a water solution or an intervening portion of an organic molecule) has a characteristic dielectric constant. 

State Characteristics Dielectric constant (f — 1), gas H2 gas Light gas +264xl0- He gas Noble gas +68xl0 s Units Remarks At 273K... 

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... 

Solvents exert their influence on organic reactions through a complicated mixture of all possible types of noncovalent interactions. Chemists have tried to unravel this entanglement and, ideally, want to assess the relative importance of all interactions separately. In a typical approach, a property of a reaction (e.g. its rate or selectivity) is measured in a laige number of different solvents. All these solvents have unique characteristics, quantified by their physical properties (i.e. refractive index, dielectric constant) or empirical parameters (e.g. ET(30)-value, AN). Linear correlations between a reaction property and one or more of these solvent properties (Linear Free Energy Relationships - LFER) reveal which noncovalent interactions are of major importance. The major drawback of this approach lies in the fact that the solvent parameters are often not independent. Alternatively, theoretical models and computer simulations can provide valuable information. Both methods have been applied successfully in studies of the solvent effects on Diels-Alder reactions. 

PPQs possess a stepladder stmcture that combines good thermal stabiUty, electrical insulation, and chemical resistance with good processing characteristics (81). These properties allow unique appHcations in the aerospace and electronics industries (82,83). PPQ can be made conductive by the use of an electrochemical oxidation method (84). The conductivities of these films vary from 10 to 10 S/cm depending on the dopant anions, thus finding appHcations in electronics industry. Similarly, some thermally stable PQs with low dielectric constants have been produced for microelectronic appHcations (85). Thin films of PQs have been used in nonlinear optical appHcations (86,87). 

Two parallel plates of conducting material separated by an insulation material, called the dielectric, constitutes an electrical condenser. The two plates may be electrically charged by connecting them to a source of direct current potential. The amount of electrical energy that can be stored in this manner is called the capacitance of the condenser, and is a function of the voltage, area of the plates, thickness of the dielectric, and the characteristic property of the dielectric material called dielectric constant. 

Dielectric Film Deposition. Dielectric films are found in all VLSI circuits to provide insulation between conducting layers, as diffusion and ion implantation (qv) masks, for diffusion from doped oxides, to cap doped films to prevent outdiffusion, and for passivating devices as a measure of protection against external contamination, moisture, and scratches. Properties that define the nature and function of dielectric films are the dielectric constant, the process temperature, and specific fabrication characteristics such as step coverage, gap-filling capabihties, density stress, contamination, thickness uniformity, deposition rate, and moisture resistance (2). Several processes are used to deposit dielectric films including atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or plasma-enhanced CVD (PECVD) (see Plasma technology). 

Electrical Properties. AH polyolefins have low dielectric constants and can be used as insulators in particular, PMP has the lowest dielectric constant among all synthetic resins. As a result, PMP has excellent dielectric properties and alow dielectric loss factor, surpassing those of other polyolefin resins and polytetrafluoroethylene (Teflon). These properties remain nearly constant over a wide temperature range. The dielectric characteristics of poly(vinylcyclohexane) are especially attractive its dielectric loss remains constant between —180 and 160°C, which makes it a prospective high frequency dielectric material of high thermal stabiUty. 

The dielectric constants of amino acid solutions are very high. Thek ionic dipolar structures confer special vibrational spectra (Raman, k), as well as characteristic properties (specific volumes, specific heats, electrostriction) (34). 

Electronic and Electrical Applications. Sulfolane has been tested quite extensively as the solvent in batteries (qv), particularly for lithium batteries. This is because of its high dielectric constant, low volatUity, exceUent solubilizing characteristics, and aprotic nature. These batteries usuaUy consist of anode, cathode polymeric material, aprotic solvent (sulfolane), and ionizable salt (145—156). Sulfolane has also been patented for use in a wide variety of other electronic and electrical appHcations, eg, as a coil-insulating component, solvent in electronic display devices, as capacitor impregnants, and as a solvent in electroplating baths (157—161). 

Barium carbonate also reacts with titania to form barium titanate [12047-27-7] BaTiO, a ferroelectric material with a very high dielectric constant (see Ferroelectrics). Barium titanate is best manufactured as a single-phase composition by a soHd-state sintering technique. The asymmetrical perovskite stmcture of the titanate develops a potential difference when compressed in specific crystallographic directions, and vice versa. This material is most widely used for its strong piezoelectric characteristics in transducers for ultrasonic technical appHcations such as the emulsification of Hquids, mixing of powders and paints, and homogenization of milk, or in sonar devices (see Piezoelectrics Ultrasonics). 

The observed dielectric constant M and the dielectric loss factor k = k tan S are defined by the charge displacement characteristics of the ceramic ie, the movement of charged species within the material in response to the appHed electric field. Discussion of polarization mechanisms is available (1). 

The above are of course only some of the most common characteristics. Individual materials may have special properties such as photoconductivity, very low coefficient of friction to steel, high dielectric constant, high ultraviolet light transmission and so on. 

At low frequencies when power losses are low these values are also low but they increase when such frequencies are reached that the dipoles cannot keep in phase. After passing through a peak at some characteristic frequency they fall in value as the frequency further increases. This is because at such high frequencies there is no time for substantial dipole movement and so the power losses are reduced. Because of the dependence of the dipole movement on the internal viscosity, the power factor like the dielectric constant, is strongly dependent on temperature. 

So far, all of the calculations we ve done have been in the gas phase. While gas phase predictions are appropriate for many purposes, they are inadequate for describing the characteristics of many molecules in solution. Indeed, the properties of molecules and transition states can differ considerably between the gas phase and solution. For example, electrostatic effects are often much less important for species placed in a solvent with a high dielectric constant than they are in the gas phase. 

Deuteron Transfers in D20. As shown in Table 1 the characteristic temperature d for the dielectric constant of D20 is almost the same as for H20. The values of the dielectric constant of D20 are given with... 

Figure 9-26. Expcrimenial and calculated (solid lines) UV characteristics of ITO/PPV/Au hole-only devices of various thicknesses, L. The current flow in all devices is described by SLC with a hole mobility //, = 1(r cm2 V"1 s 1 and with a dielectric constant of 3. The inset shows the chemical structure of the PPV investigated in that study (R =CHi, R2=CiUH2i). Reproduced from Ref. 185).

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