Non-thermal effects

In general, the reasons for rate-enhancements in microwave-assisted transformations in comparison to conventional heating are not always fully understood. Some authors have postulated a specific non-thermal microwave effect for those effects that could not be rationalised as a simple consequence of superheated solvents and higher reaction temperatures. Stadler and Kappe therefore carried out a kinetic comparison of the thermal coupling of benzoic acid to chloro-Wang resin at 80° C, with the microwave-assisted coupling at the identical temperature of 80°C and otherwise identical reaction parameters. However, the reaction rates for the two runs were quite similar and the small observed differences could not be attributed to non-thermal effects. In order to confirm this hypothesis, the authors also carried out coupling experiments with... 

Porcelli, M., Cacciapuoti, G., Fusco, S., et al. Non-thermal effects of microwaves on proteins thermophilic enzymes as model system. FEBS Letters 1997 402, 102-6. 

Barnes, F. S. Hu, C. L. Model for some non-thermal effects of radio and microwave fields on biological membranes. IEEE Trans. Microwave Theory Tech., 1977, MTT-25, 742-746. 

Kwan, K. M., and Benatar, A., Investigation of Non-Thermal Effects Produced by Ultrasonic Heating on Curing of Two-Part Epoxy Adhesive, Society of Plastic Engineers Annual Technical... 

Tests involving new, more sophisticated measurement tools have provided new interpretations and equations for the cavitation phenomenon [14,15]. The thermal and non-thermal effects of non-inertial cavitation, and the chemical and mechanical effects of Inertial cavitation in relation to their impact on ultrasound safety have recently been Investigated [16]. 

The eco-friendly advantages of these reactions may be found in instances where catalytic amounts of reagents or supported agents are used since they provide reduction or elimination of solvents or utilize water as reaction media, thus preventing pollution at source . Although not delineated completely, the reaction rate enhancements achieved in these methods may be ascribable to non-thermal effects. The rationalization of microwave effects and mechanistic considerations possibly involving the intermediacy of polar transition states have been discussed.The use of synthesis processes that employ MW/ultrasonic irradiation to shorten the reaction time and eliminate or minimize side product formation is already finding acceptance in pharmaceutical industry (combinatorial chemistry) and polymer syntheses... 

Several authors also postulate the occurrence of a non-thermal effect that originates from the polarizing electromagnetic field. In this vay the most polar transition state, i.e. the harder transition state, vill be favored under the action of micro-wave irradiation (Chapter 4). These results have been supported by computational calculations. The experimental and theoretical results described here now need to be supported with further examples. If these results are confirmed, however, such systems could be used as a predictive tool to show which reactions can be improved or have modified selectivity under the action of microwave irradiation and even to predict the result of the reaction. 

Long chain 2-alkyl-l-(2-hydroxyethyl)-2-rmidazolmes (at 150 °C) and their amide precursors at 120 °C were readily obtained by condensation of aminomethy-lethanolamine with fatty acids using CaO as inorganic support [170]. A non-thermal effect is implicated. A one-pot selective synthesis of ds- and trans-2,4,5-triarylimidazolines from aromatic aldehydes was achieved under irradiation conditions via methadiamines and selected bases [171]. 

However, direct evidence for these suggestions is currently not available, and the circumstantial evidence is neither overwhelming nor definitive. The recent development of in situ, real time monitoring of MW-accelerated reactions by Raman spectroscopy will perhaps provide answers to the important questions. Loupy has proposed that the non-thermal effects depend on the reaction medium, that is, solvent-free or polar or nonpolar solvent, the polarity of the transition state, and the position of the transition state along the reaction coordinate. He has suggested that specific MW effects are more apparent in apolar solvents and those reactions which have late transition states. 

Results have appeared recently that quantify directly the non-thermal effects of photons and low-energy ions on siuface diffusioa Such work has important applications in thin film deposition. 

Non-Thermal Effects and Laser Fusion Applications," UCRL-74938, Lawrence Livermore Laboratory (1972). 

The non-thermal effects of illumination upon surface diffusion at high temperatures were measured. It was found that the activation energies and pre-exponential factors for the diffusion of Ge changed upon illumination with photons having energies that were greater than the substrate band-gap ... 

Here is the mobility tensor satisfying f = X- Again, this diffusion coefficient contains two contributions, one depending on the temperature T of the flmd and another one independent from it and proportional to the velocity gradient of the imposed external flow. Therefore, the first contribution is related to the usual thermal Brownian motion whereas the last one is related to non-thermal effects. Notice also that the Smoluchowski equation (26) also contains the usual convective term V (pvo). 

Provided that thermodynamic properties such as internal energy and Gibbs free energy of materials with permanent dipole moments undergo significant changes under the influence of microwave fields, shifts in the reaction equilibrium and in kinetics, as compared to thermal fields at the same temperature, can be foreseen [11]. At present, it seems that final conclusions about non-thermal effects can be arrived at only after much more carefully performed experiments have been carried out. However, it is clear that sub-THz radiations - and especially microwaves - can serve as a tool for rapid and efficient heating, with no further influence on most reactions [10]. 

Perreux, L and Loupy, A. (2006) Non-thermal Effects of Microwaves in Organic Synthesis, in Microwaves in Organic Synthesis (ed. A. Loupy), 2nd edn, Wiley-VCH, Weinheim, Ch. 3. 

Obermayer D, Gutmann B, Kappe CO. Microwave chemistry in silicon carbide reaction vials separating thermal from non-thermal effects. Angew. Chem. Int. Ed. 2009 48 8321-8324. 

However, Frohlich proposed the concept of coherent excitation. According to this concept, a pulsing electric field may force the dipolar components of the cell membrane, which are otherwise vibrating randomly, into coherent vibration . Such a coherent state should become a trigger of a critical step in the cell cycle. In such a case, a low energy stimulus causes a drastic phenomena. Thus, it is called a non-thermal effect. 

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