Microwave absorbers

The dielectric properties of most foods, at least near 2450 MH2, parallel those of water, the principal lossy constituent of food (Fig. 1). The dielectric properties of free water are well known (30), and presumably serve as the basis for absorption in most foods as the dipole of the water molecule interacts with the microwave electric field. By comparison, ice and water of crystaUi2ation absorb very Httie microwave energy. Adsorbed water, however, can retain its Hquid character below 0°C and absorb microwaves (126). 

Because a chemical bond is only about 10 10 m long, special techniques have to be used to measure its length. There are two principal techniques one for solids and the other for gases. The technique used for solids, x-ray diffraction, is described in Major Technique 3, billowing Chapter 5. Microwave spectroscopy, discussed here, is used to determine bond lengths in gas-phase molecules. This branch of spectroscopy makes use of the ability of rotating molecules to absorb microwave radiation, which has a wavelength close to 1 cm. 

Fibers of the conducting polymer polypvrrole are woven into radar camouflage cloth. Because it absorbs microwaves, rather than reflecting them back to their source, the cloth appears to be a patch of empty space on radar. 

In order to perform extraction of additives or dissolution of polymers, solvents that absorb microwave energy are necessary. This is more important than direct absorption of microwave energy by the polymer or additives. When microwave extraction of additives... 

MAP makes use of physical phenomena that are fundamentally different compared to those applied in current sample preparation techniques. Previously, application of microwave energy as a heat source, as opposed to a resistive source of heating, was based upon the ability to heat selectively an extractant over a matrix. The fundamental principle behind MAP is just the opposite. It is based upon the fact that different chemical substances absorb microwave energy... 

In effect the sample will no longer absorb microwave power when the levels become equally populated. Such a phenomenon is known as saturation. 

Most of the parallel reactions described in Schemes 4.23 and 4.24 were performed as dry-media reactions, in the absence of any solvent. In many cases, the starting materials and/or reagents were supported on an inorganic solid support, such as silica gel, alumina, or clay, that absorbs microwave energy or acts as a catalyst for the reaction (see also Section 4.1). In this context, an interesting method for the optimization of silica-supported reactions has been described [83], The reagents were co-spotted neat or in solution onto a thin-layer chromatographic (TLC) plate. 

Changes to the physical properties of a compound or material can have a dramatic influence on the susceptibility to microwave radiation. For example, ice has dielectric properties (e, 3.2 tan 8, 0.0009 e", 0.0029) that differ significantly from those of liquid water at 25 °C (s, 78 tan <5, 0.16 e", 12.48) [31], rendering it essentially microwave-transparent. Although liquid water absorbs microwave energy efficiently, the dielectric constant decreases with increasing temperature and supercritical water (Tc 374 °C) is also microwave-transparent. 

For liquid products (solvents), only polar molecules selectively absorb microwaves, because nonpolar molecules are inert to microwave dielectric loss. In this context of efficient microwave absorption it has also been shown that boiling points can be higher when solvents are subjected to microwave irradiation rather than conventional heating. This effect, called the superheating effect [13, 14] has been attributed to retardation of nucleation during microwave heating (Tab. 3.1). 

It must be stressed that a liquid component can be substituted with an efficient absorber of microwave irradiation together with a low-melting component. The use of most typical PTC solvents (nonpolar aromatic or aliphatic hydrocarbons, or highly chlorinated hydrocarbons) is most interesting for microwave activation, because such solvents are transparent or absorb microwaves only weakly. They can, therefore, enable specific absorption of microwave irradiation by the reagents, and the results or product distributions might be different under microwave and conventional conditions [7]. 

For instance, cycloadditions of [60]fullerene (4) under the action of microwave irradiation usually require the use of this technique, because reactions are performed on a very small scale and C60. in common with many dienophiles, does not absorb microwaves efficiently [19]. 

In MORE chemistry the solvent of choice absorbs microwaves in an energy-efficient manner and is, therefore, heated rapidly under microwave irradiation. The solvent should also have a boiling point at least 20-30 degrees higher than the desired reaction temperature [17]. 

Alumina spheres polluted by carbon residues have been also reactivated by use of microwaves [33]. Their regeneration has been performed in a stream of air and in the presence of silicon carbide as an auxiliary microwave absorber. Microwave heat treatment led to full recovery of the catalyst in times varying from a half to a quarter of the conventional treatments. Regeneration of a commercial Ni catalyst (Ni/Al203) deactivated, presumably, by coke formation, by means of a flow of hydrogen or oxygen and water vapor under the action of microwave irradiation was, however, unsuccessful [34]. 

Just as the absorption of UV or visible light causes electrons to excite between different electronic quantum states, so absorption of infrared photons causes excitation between allowed vibrational states, and absorbing microwave radiation causes excitation between allowed rotational states in the absorbing molecule. As a crude physical representation, these quantum states correspond to different angular velocities of rotation, so absorption of two photons of microwave radiation by a molecule results in a rotation that is twice as rapid as following absorption of one photon. 

To learn that the most common problems experienced with in situ EPR spectroelectrochemistry work result from employing a solvent that itself absorbs microwave radiation, and from using polymers which contain radical species. 

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. 

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