Transfer of microwave energy

The transfer of microwave energy to a molecule is extremely fast and results from the direct interaction of a molecule with the high-frequency, oscillating electric component of the microwave field. Because the absorption of microwave energy depends on the polarity of the molecule, the reactant molecules in an... 

These effects have been proposed in order to explain unusual observations in microwave chemistry. As the name suggests, these effects are supposed not to require the transfer of microwave energy into thermal energy. Instead, the microwave energy itself directly couples to energy modes within the molecules or lattice. Non-thermal effects in liqtrids are almost certainly nonexistent (De la Hoz et al., 2005 Kappe,... 

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

Precautions should be taken, especially in a scale-up approach, when dealing with exothermic reactions in the microwave field. Due to the rapid energy transfer of microwaves, any uncontrolled exothermic reaction is potentially hazardous (thermal runaway). Temperature increase and pressure rise may occur too rapidly for the instrument s safety measures and cause vessel rupture. 

Abramovitch, R.A., Abramovitch, D.A., Iyanar, K. and Tamareselvy, K., Transfer hydrogenation, atom economy activation, palladium, chemistry Application of microwave-energy to organic-synthesis - improved technology Tetrahedron Lett., 1991, 32, 5251-5254. 

One last factor to consider is the saturation behavior of the carbon radical - its response to an applied microwave field. At low microwave powers the energy which a carbon radical absorbs from the microwave field is dissipated to its environment in a relatively short period of time in a process called spin-lattice relaxation. At sufficiently high microwave powers this relaxation process is no longer able to transfer the microwave energy absorbed by the spins to the environment rapidly enough for the spins to remain in thermodynamic equilibrium. This... 

Another notable feature of microwave energy transfer over conductive energy transfer is that the applied energy is available with an instant on/ofi control. As detailed above, microwave energy enables the reaction to proceed in a more controlled manner in a decreased time period. Controlling the kinetics of the reaction becomes easy when the control of the applied energy becomes more direct and precise. 

The use of Chloramine-T or Bromamine-T as the nitrogen source has been useful as the transfer reagent in the aziridination reaction. Bedekar and co-workers have found that simple cyclic olefins such as norbornene, cyclohexene, or cyclooctene produce low yields of aziridinated products. However, the same reaction run in a simple microwave oven increases the chemical yield dramatically <2001JOC30>. The reaction shown in Equation (13) was run without the assistance of microwave energy and provided approximately half of the chemical yield at 38%. 

The ionic conduction effect arises because ionic species in the presence of an electric field wiU migrate in one direction or the other. Energy is transferred firom the electric field, causing ionic interactions that speed up the heating of a solution. Ionic absorbers become stronger absorbers of microwave energy as they are heated since ionic conductance increases with temperature. Deionized water heats slowly, but if salt is added, it heats rapidly. Acids, of course, are good conductors and heat rapidly. 

This is what is used in a mierowave oven. Food (sueh as ehieken) on a ceramic plate is irradiated by microwaves. This causes rotational excitation of the water molecules that are always present in food. The rotating water molecules cause a transfer of kinetic energy to protein, similar to what would happen in traditional cooking. After removing the food from the microwave, the chicken is hot, but the plate is cool (as there is nothing to rotate in the material that makes it up). 

---