Microwaves dipolar polarization

Due to previous organization of the polar system under microwaves (dipolar polarization), activation parameters, and essentially AS , can be modified. This was experimentally proved by Lewis et al. [108] during imidization of polyamic acid either by conventional heating or under MW activation [Eq. (58)]. 

A decrease in the activation energy AG is certainly a major effect. Because of the contribution of enthalpy and entropy to the value of AG (= AH -TAS ), it might be predicted that the magnitude of the -TAS term would increase in a microwave-induced reaction, because organization is greater than with classical heating, as a consequence of dipolar polarization. 

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... 

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... 

MAOS is mainly based on the efficient heating of materials by the microwave dielectric heating effect [15] mediated by dipolar polarization and ionic conduction. When irradiated at microwave frequencies, the dipoles (e.g., the polar solvent... 

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]. 

Although there is some speculation that microwaves can reduce activation energy by dipolar polarization, this has yet to be proven. Microwave energy will affect the temperature of the system, however. In the Arrhenius equation, T measures the average bulk temperature of all components of the system. It is known that for a given temperature the molecules in the system are at a range of temperatures as shown in the Boltzmann equation, F( ) = Because not all compo-... 

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. 

Rolla et ah, used microwave dielectric measurements to monitor the polymerization process of mono functional n-butyl acrylate as well as 50/50 w/w blends with a difunctional hexane-diol diacrylate that gave highly cross-linked networks. In these real time cure experiments the decreasing acrylate monomer concentration was studied via a linear correlation with the dielectric loss index at microwave frequencies. This correlation is a result of the largely different time scales for dipolar polarization in the monomer on one hand and in the polymerized reaction product on the other hand. 

The dielectric permittivity as a function of frequency may show resonance behavior in the case of gas molecules as studied in microwave spectroscopy (25) or more likely relaxation phenomena in soUds associated with the dissipative processes of polarization of molecules, be they nonpolar, dipolar, etc. There are exceptional circumstances of ferromagnetic resonance, electron magnetic resonance, or nmr. In most microwave treatments, the power dissipation or absorption process is described phenomenologically by equation 5, whatever the detailed molecular processes. 

Entropic contributions to the acceleration of first-order reactions by microwaves should be negligible (AS = = 0). When ionization (SN1 or E,) or intramolecular addition (cyclizations) processes are involved a microwave effect could be viewed as resulting from a polarity increase from GS to TS, because of the development of dipolar intermediates (Scheme 3.5). 

These reactions are among the most propitious for revealing specific microwave effects, because the polarity is evidently increased during the course of the reaction from a neutral ground state to a dipolar transition state. 

This observation is consistent with the assumptions of the authors predicting microwave effects when the polarity is enhanced in a dipolar TS. The kinetic rate-determining step consists of an intramolecular attack of the nitrogen lone pair on the carbon atom of the carbonyl moiety (Scheme 3.15). 

In microwave-assisted synthesis, a homogeneous mixture is preferred to obtain a uniform heating pattern. For this reason, silica gel is used for solvent-free (open-vessel) reactions or, in sealed containers, dipolar solvents of the DMSO type. Welton (1999), in a review, recommends ionic liquids as novel alternatives to the dipolar solvents. Ionic liquids are environmentally friendly and recyclable. They have excellent dielectric properties and absorb microwave irradiation in a very effective manner. They exhibit a very low vapor pressure that is not seriously enhanced during microwave heating. This makes the process not so dangerous as compared to conventional dipolar solvents. The polar participants of organic ion-radical reactions are perfectly soluble in polar ionic liquids. 

The second approach is the use of the dynamic nuclear polarization (DNP) detection principle. Dorn and co-workers have pioneered the application of this technique [9,10], Whereas the NOE enhancement of 13C nuclei in the conventional 13C H recording is dependent upon the 7h/7c ratio (NOE = Th/ Tc = 2 1), the DNP enhancement relates to the ye/yuc ratio (2640 1). In an electron-nucleus spinsystem, the electron-electron transitions are saturated by microwave irradiation and magnetization transfer from electron to nucleus (Overhauser effect) occurs via a scalar and/or dipolar mechanism. The DNP enhancement, A, is described by the following equation ... 

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