Nonthermal specific

We shall, therefore, be concerned with relating electronic structure of solids with chemical instability in the most general theoretical analyses, with energy transport by electronic mechanisms during detonation, with the effect of occupancy of electronic states (Fermi level) on initiation, and with the possibility of initiation by nonthermal, specifically electronic, mechanisms in both single-crystal and amorphous explosives. 

With the same type of molecule, nonthermal specific microwave effects were apparent in the Wilkinson complex-catalyzed orthoalkylation of ketimines [103], which occurred via initial transimrnation (Eq. 18) ... 

The reactions have also been performed under solvent-free conditions with considerable success and almost quantitative yields of the products [35a]. For example, the last entry in Table 7.2 was improved to 100% when irradiation was performed for 3 min. at 50 °C. Under solvent-free conditions, the existence of nonthermal specific microwave effects has been proposed on the basis of comparison with control reactions performed using conventional heating. 

The essential questions raised by the assumption of athermal or specific effects of microwaves are, then, the change of these characteristic terms (free energy of reaction and of activation) of the reaction studied. Hence, in relation to previous conclusions, five criteria or arguments (in a mathematical sense) relating to the occurrence of microwave athermal effects have been formulated by the author [25], More details can be found in comprehensive papers which analyze and quantify the likelihood of nonthermal effects of microwaves. This paper provides guidelines which clearly define the character of nonthermal effects. 

Significant rate accelerations and higher loadings are observed when the micro-wave-assisted and conventional thermal procedures are compared. Reactions times are reduced from 12-48 h with conventional heating at 80 °C to 5-15 min with microwave flash heating in NMP at temperatures up to 200 °C. Finally, kinetic comparison studies have shown that the observed rate enhancements can be attributed to the rapid direct heating of the solvent (NMP) rather than to a specific nonthermal microwave effect [17]. 

More interesting fundamentally is the very strong specific nonthermal effect of microwaves, as evidenced by comparison with classical heating. This effect grows as ester reactivity falls. 

Several reasons have been proposed to account for the effect of microwave heating on chemical reactions and catalytic systems. The results summarized in 1 to 7, above, show that under specific conditions microwave irradiation favorably affects reaction rates of both the liquid- and gas-phase processes. This phenomenon has been explained in terms of microwave effects, i. e. effects which cannot be achieved by conventional heating. These include superheating, selective heating, and formation of hot spots (and possibly nonthermal effects). 

The energy necessary for the ionization and evaporation of the analyte can be delivered either by use of thermal energy or by means of nonthermal energy sources. A number of techniques following different physical principles have been developed for the ion formation in MS, each having specific advantages for different classes of analytes. 

Maoz, R., Cohen, H. and Sagiv, J., Specific nonthermal chemical structural transformation induced by microwaves in a single amphiphilic bilayer self-assembled on silicon, Langmuir, 1998,14, 5988. 

Microwave-assisted synthesis in general is likely to have a tremendous impact in the medicinal/combinatorial chemistry communities because it shortens reaction times, improves final yields and purities, and can carry out reactions that previously were thought impossible to do. It should be stressed that in general the rate enhancements seen in microwave-assisted synthesis can be attributed to the very rapid heating of the reaction mixture (flash heating) and the high temperatures that can be reached, rather than to any other specific or nonthermal microwave effect.40... 

Electric arc processes have been given a new lease on fife in the guise of plasma reactors, especially those involving cold, or nonthermal plasmas, with electron temperatures of I (E-IO5 K and gas temperatures of 102-103 K. Plasmas of this kind can be used to activate and functionalize inert molecules, but usually with only poor selectivities and low energy yields ( 0.01 mol/kWh ). The use of catalytic additives may offer some potential for improvement, but reactive plasma processes will probably remain restricted to a few specific applications. 

Research on the bioeffects of nerve and muscle exposure to electromagnetic fields has resulted in a literature filled with contradicting phenomenology. Thus, a number of researchers have published evidence which supports (21,22,23,24) or does not support (25,26,27,28) the hypothesis that AC fields can affect excitable cells by nonthermal mechanisms. With few exceptions, little attention has been given to possible underlying mechanisms of interaction. It is hoped that this paper will help to provide a rational basis for design of experiments based on specific hypotheses which can be tested in the laboratory. 

Microwave dielectric heating was initially categorized by thermal effects and nonthermal effects. Thermal effects are those which are caused by the different temperature regime which can be created due to microwave dielectric heating. Nonthermal effects are effects, which are caused by effects specifically inherent to... 

Differences between chemistry observed with microwave and conventional heating can often be attributed to the different transfer rate or spatial distribution of heat. Once appropriate temperatures are known in various parts of the system, conventional laws of thermodynamics or kinetics commonly apply. Such cases may be called microwave specific effects. However, there are also cases where it is proposed that an additional effect operates, perhaps through the action of the electric field 23 such effects are commonly called nonthermal or athermal microwave effects. Although their existence is controversial in fluid phases,6,24-26 there is a strong body of evidence supporting such effects in solid phases.27-29... 

Because of the enormous photon flux available at a synchrotron source, both thermal and nonthermal effects of X-radiation must be considered as potential problems when making TRXRD measurements. Nonthermal radiation damage has been shown to be significant in the case of hydrated phosphatidylcholine [15]. However, the degree of damage is lipid species specific at the least and each sample must be evaluated on an... 

The observed rate accelerations and sometimes altered product distributions compared to classical oil-bath experiments have led to speculation on the existence of specific or nonthermal microwave effects. " Historically, such effects were claimed when the outcome of a synthesis performed under microwave conditions was different from that of the conventionally heated counterpart. When reviewing the present literature, it appears that most scientists now agree that in the majority of cases the reason for the observed rate enhancements is a purely thermal/kinetic effect. Even though for the industrial chemist this discussion seems largely irrelevant, the debate on microwave effects is undoubtedly going to continue for many years in the academic world. Today, microwave chemistry is as reliable as the vast arsenal of synthetic methods that preceded it. Microwave heating not only reduces reaction times significantly, but is also known to reduce side reactions, increase yields, and improve reproducibility. 

It is well known that a wide variety of organic reactions are accelerated substantially by microwave irradiation in sealed tubes. These rate enhancements can be attributed to superheating of the solvent, because of the increased pressure generated when the reactions are performed in the a.m. manner. Furthermore several reports have described increased reaction rates for reactions conducted under the action of microwave irradiation at atmospheric pressure, suggesting specific or nonthermal activation by microwaves. Some of these re-studied reactions occur at... 

The recent literature on microwave-assisted chemistry has reported a multitude of different effects in chemical reactions and processes and attributed them to microwave radiation. Some of these published results cannot be reproduced, however, because the household microwave ovens employed often have serious technical shortcomings. Published experimental procedures are often insufficient and do not enable reproduction of the results obtained. Important factors required for qualification and validation, for example exact records, reproducibility, and transparency of reactions/processes, are commonly not reported, which poses a serious drawback in the industrial development of microwave-assisted reactions and processes for synthesis of fine chemicals, intermediates, and pharmaceuticals. Technical microwave devices for synthetic chemistry have been on the market for a while (cf a.m. explanations) and should enable comparative investigations to be conducted under set conditions. These investigations would enable better assessment of the observed effects. It is, furthermore, possible to obtain a better insight into the often discussed (nonthermal) microwave effects from these experiments (Ref. [138] and Chapter 4 of this book). Technical microwave systems are an important first step toward the use of microwave energy for technical synthesis. The actual scale-up of chemical reactions in the microwave is, however, still to be undertaken. Comparisons between microwave systems with different technical specifications should provide a measure for qualification of the systems employed, which in turn is important for validation of reactions and processes performed in such commercial systems. 

Finally, because the interpretation proposed for specific MW nonthermal effects is rather similar to that for solvent effects (Hughes-Ingold theory), one can expect that aprotic polar solvents can be removed and replaced by MW activation requiring operation with a nonpolar solvent or, much better, under solvent-free conditions. In this situation we add also advantages of green chemistry conditions. In... 

As another consequence of the assumptions above, it might be foreseen that specific nonthermal MW effects could be important in determining the selectivity of some reactions. When competitive reactions are involved, the GS is common for both processes. The mechanism occurring via the more polar TS could, therefore, be favored by use of microwave radiation (Scheme 4.7). 

Microwave-assisted solvent-free reactions have been used by Jenekhe [146] to synthesize quinoline derivatives. An important specific nonthermal microwave effect has been observed compared with conventional heating (Eq. 60). This MW effect is consistent with mechanistic considerations, because the rate-determining step is the internal cyclization depicted in Eq. (60) resulting from nucleophilic attack of the enamine on the carbonyl moiety occurring via a dipolar transition state. 

Nonthermal MW-specific effects were revealed and shown to be more important with the less reactive systems. They can be related to the polarity increases as the reaction progresses involving polar metallacyclobutanes with polar carbon-metal bonds. 

---