Dipolar polarization mechanism

Microwave radiation, as all radiation of an electromagnetic nature, consists of two components, i.e. magnetic and electric field components (Fig. 1.3). The electric field component is responsible for dielectric heating mechanism since it can cause molecular motion either by migration of ionic species (conduction mechanism) or rotation of dipolar species (dipolar polarization mechanism). In a microwave field, the electric field component oscillates very quickly (at 2.45 GHz the field oscillates 4.9 x 109 times per second), and the strong agitation, provided by cyclic reorientation of molecules, can result in an... 

The dipolar polarization mechanism is present in substances containing molecules with permanent dipole moments. In these compounds, the bulk material is neutral, since normally the orientation of the polar molecules constituting the material is randomly distributed due to the action of thermal energy. In this situation, under the influence of the electric field, the dipoles will be partially oriented in the direction of the electric field. The dipole moment can be written as... 

Aromatic plants are usually constituted from cellulose, essential oil, and water. If these three compounds are heated by microwaves at a fixed radiation power and for a set time, the heating rate will be the highest for water, followed by essential oil and cellulose, respectively. One of the interactions of the microwave energy with the matrix is called the dipolar polarization mechanism. A substance can generate heat when irradiated with microwaves if it has a dipole moment, for example that of the water molecule. A dipole is sensitive to external electric fields and will attempt to align itself with the field by rotation. 

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

Microwave effects result from material-wave interactions and, because of the dipolar polarization phenomenon, the greater the polarity of a molecule (such as the solvent) the more pronounced the microwave effect when the rise in temperature [43] is considered. In terms of reactivity and kinetics the specific effect has therefore to be considered according to the reaction mechanism and, particularly, with regard to how the... 

Amino-l,2,4-thiadiazoles 191 are obtained when ether is used (249), while 5-alkylthio-1,2,3-triazoles 192 result when the reaction is carried out in THF (250). Reaction of 3 with carbon disulfide leads to 5-alkylthio-l,2,3-thiadiazoles 193 (251). While 3 can act as a synthetic equivalent of the RC—N—N synthon (R = H, SiMea) in all these reactions, it should be emphasized that it does not react by a concerted 1,3-dipolar cycloaddition but rather by a stepwise polar mechanism. The highly nucleophilic character of 3 can account for why diazomethane and... 

Dielectric relaxation study is a powerful technique for obtaining molecular dipolar relaxation as a function of temperature and frequency. By studying the relaxation spectra, the intermolecular cooperative motion and hindered dipolar rotation can be deduced. Due to the presence of an electric field, the composites undergo ionic, interfacial, and dipole polarization, and this polarization mechanism largely depends on the time scales and length scales. As a result, this technique allowed us to shed light on the dynamics of the macromolecular chains of the rubber matrix. The temperature as well as the frequency window can also be varied over a wide... 

Let us assume now, for example, that a step-like constant electric field of magnitude E0 is applied within a dielectric at any time t0, and remains constant for t>t0 (see Figure 1.27). Then, Pao = Pe + P, is almost instantaneously established. Thereafter, the acting relaxation processes (i.e., dipolar and/or charge-hopping and/or space charge polarization mechanisms) provoke that the polarization is not instantaneously established. 

In practice, the Cole-Cole diagrams observed differ from those of Fig. 13 in two ways. The first is due to electrode polarization, discussed in Section 3.2.1 the second is due to the fact that in real materials, the dipolar hindering mechanisms are... 

Stereospecificity, the property that the stereochemistry of the starting materials determines the stereochemistry of the product, is one of the hallmarks of pericyclic reactions. It is possible to draw two-step nonconcerted, polar or free-radical mechanisms for many pericyclic reactions, but these two-step mechanisms fail to account for the stereospecificity of the reactions. For example, a two-step polar mechanism can be drawn for the Diels-Alder reaction between 2-methoxybutadiene (a nucleophile) and ethyl cA-crotonatc (an electrophile). This mechanism proceeds through a dipolar intermediate in which one new cr bond has formed. In this intermediate, there is free rotation about the two C atoms of the dienophile, so the cis stereochemical relationship between the Me and CC Et groups is expected to be lost in the product. In fact, though, the product is exclusively cis. This finding does not completely rule out a polar mechanism— it is possible that the intermediate exists but that ring closure occurs more quickly than rotation about the cr bond—but it does limit the lifetime of the dipolar intermediate to such an extent that one can say practically that it does not exist. 

Temperature will influence only the polarization mechanisms that depend on long-range ionic displacement such as dipolar polarization. Ionic polarization is not strongly affected by temperature since long-range mobility of the ions is not required for it to be operative." ... 

The application of an electric field E across a linear dielectric material results in polarization P or the separation of positive and negative charges. The relative dielectric constant k is a measure of the capacity of a solid to store charge relative to vacuum and is related to the extent to which the charges in a solid polarize. Atomically there are four main polarization mechanisms electronic, ionic, dipolar, and space charge. 

If one wishes to extract stereochemical information about substrates in solution from the data obtained in the NMR experiments, two physical-mathematical models are available. For shifts induced by a dipolar (pseudocontact) mechanism, the McConnell-Robertson equation (Equation 1) (28, 29)—where r, i, and are the spherical polar coordinates of the ith resonating nucleus in the coordinate system of the principal magnetic axes—relates the direction and magnitude of the shift to the geometry of the substrate-chelate complex. If the substrate-chelate... 

MW-enhanced chemistry is based on the efficiency of interactions of molecules with waves by microwave dielectric heating effects. This phenomenon depends on the ability of materials to absorb MW radiation and convert it into heat. The electric component of the electromagnetic field has been shown to be the most important [22-24]. It results in two main mechanisms - dipolar polarization and ionic conduction. Irradiation of polar molecules at MW frequencies results in orientation of the dipoles or ions in the applied electric field (Scheme 4.1) [25]. 

A note In some texts you will find that the polarization mechanism occurring in BaTiOs is described as dipolar and in others as ionic. We prefer the former because BaTiOs contains permanent dipoles (a condition of dipolar polarization) that are being oriented in an electric field. Although the permanent dipoles in BaTiOs are the result of ion displacements, the term ionic polarization refers to the movement of any ions in an electric field (whether the material has a permanent dipole or not). 

It is, of course, recognized that there are polarization mechanisms with a fundamentally molecular basis in addition to the electronic and the dipolar orientation types referred to above. Indeed, the two other principal mechanisms, atomic and nomadic polarization, provide... 

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