Since the first published reports in 1986 on the use of microwave irradiation to "accelerate" organic chemical transformations, there has been considerable speculation and discussion of this effect. Much of the debate has centered around the question whether the observed effects can in all instances be rationalized by purely thermal/kinetic phenomena (thermal microwave effects) arising from the rapid heating and high bulk reaction temperatures attained with microwave dielectric heating, or whether some effects are connected to so-called specific or nonthermal microwave effects. Unfortunately, the definitions of what constitutes a specific or nonthermal microwave effect are somewhat vague and different scientific communities may in fact have different definitions. Most scientists today will agree that the energy of the microwave photon is far too low to directly cleave molecular bonds, and that therefore microwaves cannot "induce" molecules to undergo chemical reactions upon direct absorption of electromagnetic energy, as opposed to ultraviolet and visible radiation (photochemistry). However, in the organic chemistry community claims of the existence of nonthermal microwave effects persist. These effects have been postulated to result from a direct, often stabilizing interaction of the electromagnetic field with specific molecules, intermediates, or even transition states in the reaction medium, which is not connected to a macroscopic change in reaction temperature. It has been argued, for example, that the presence of an electric field affects the orientation of dipolar molecules or intermediates and hence changes the prc-exponential factor A or the activation energy (entropy term) in the Arrhenius equation for certain types of reactions. Furthermore, a similar effect has been proposed for polar reaction mechanisms, where the polarity increases going from the ground state to the transition state, resulting in an enhancement of reactivity by a decrease of the activation energy. Specific microwave effects are caused by the uniqueness of the microwave dielectric heating mechanisms and include, for example, 1) the superheating effect of solvents at atmospheric pressure, 2) the selective heating of, for example, strongly microwave-absorbing heterogeneous catalysts or reagents in a less polar reaction medium (and effects resulting from the differential/selective heating of bi- or multiphasic liquid/ liquid systems), 3) the formation of "molecular radiators" by direct coupling of microwave energy to specific reagents in homogeneous solution (microscopic hotspots), and 4) the elimination of wall effects caused by inverted temperature gradients.
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