8th International Electronic Conference on Synthetic Organic Chemistry. ECSOC-8. 1-30 November 2004. http://www.lugo.usc.es/~qoseijas/ECSOC-8/
Abstract: Microwave irradiation becomes a widely used method to synthesize many useful organic chemicals rapidly, with good
yields and high selectivity. A great many of relevant works suggest only thermal nature of the microwave action,
that means that microwaves are considered to be a method to heat chemical reagents rapidly and without any overheating.
Some other works describe specific non-thermal effects. And the effects are likely to exist. Sometimes the effects are
thought to be only specific forms of heat effects, but not always.
We have recently observed some of the effects in the microwave initiated polymerizations and oxidations when the
microwave power was quite low [1-3]. Here we shall try to discuss briefly possibilities that can be
useful to explain the microwave effects. The discussion suggests selection of theoretical ideas that will serve a basis for future
theories of the microwave-initiated reactions.
Microwave effects result from material-wave interactions.
Thermal effects can result from dipolar polarization as a consequence of dipole-dipole interactions between polar
molecules and the electromagnetic field. The interaction turns into heat evolvement as an outcome of agitation and
molecular friction of molecules when dipoles change their mutual orientation at each alternation of the electric
field at a very high frequency. The heat is considered [4,5] to be the only reason of chemical reaction acceleration
under microwave exposure. The absence of effects being other than thermal could be a general conclusion, when a
careful control of the temperature is carried out, or the rapid increase of the pressure in scaled reactors is taken
in to account for an accurate analysis [4].
However microwave heating is different from conventional heating [4]. Dipolar polarization and rotation of molecules
in an attempt to align them with applied microwave field produces specific effects (superheating, hot spots, volume
uniformity of heating, etc), which cannot be achieved by conventional heating [4].
At present there is no unanimous opinion whether a non-thermal specific activation of chemical reactions by
microwaves exists
or not [4-9].
There exist some phenomena that could not be explained on the basis of thermal effects only. We gathered a collection of them and believed that the collection could prompt components of a theoretical description of the microwave-substance interactions.
Akyel and Bilgen [10] found that amount of energy needed to produce the microwave induce curing of polymers was only 0.3-0.5 Mj per one kilogram of a product. This value is approximately 10-20 times less than that for the heating-induces curing.
T.Cheng et al. [11] observed an increase of the Rydberg atom ionization yields under irradiation by the low and very high power microwaves in the presence of an additional magnetic field.
J. Cheng et al [12-14] obtained additional evidences of that it was no longer possible to ignore the effect of a magnetic component of microwave in the general theory of energy loss in various materials such as metals and metallic composites. They concluded that the induction of eddy currents was clearly a major factor. This set of empirical data might reopen the matter of microwave-material interaction to incorporate more detailed consideration for the effects of magnetic field [14]. At weak fields (when microwave amplitude is equal to 0.01-0.1) the ionization rate changes slightly with magnetic field strength.
There was a great many of publications dedicated to biological effects of microwave, and in particular to the action of irradiation with the lower power microwave. A brief introductory review of the problem can be found on the Internet cite [15]. The effectiveness of microwaves for sterilization has been well established by numerous studies over the previous decades [16-19]. The exact nature of the sterilization effect and whether it is due solely to thermal effects or to the 'microwave effect' has been a matter of controversy for decades.
The energy level of a microwave photon is only 10-5 eV, whereas the energy required to break a covalent bond is 1-4 eV. Based on this fact, it has been stated in the literature that "microwaves are incapable of breaking the covalent bonds of DNA" [19,20], but there is, in fact, plenty of evidence to indicate that there are alternate mechanisms for causing DNA covalent bond breakage without invoking the energy levels of ionizing radiation [21-26]. Still, no theory currently exists to explain the phenomenon of DNA fragmentation by microwaves.
Pologea-Moraru et al [27] found that the low power level microwaves had small but significant effects on photoreceptor cell membranes and changed membrane fluidity of the cells.
X. Zhu et al [28] have demonstrated that the microwave induced polymerization of very low mean power has more significant effect on the polymerization rate enhancement, which verifies a specific non-thermal effects of microwave irradiation on the emulsion polymerization of methyl methacrylate. The molecular weight of polymer was 1.1-2.0 times higher than that under heating. In addition, at the same irradiation energy, the conversion achieved using a lower pulse power was greater than that using a higher pulse power. The irradiation energy of low pulse power had a higher efficiency.
The increase in the conversion of monomers in the microwave-initiated polymerization compared to the thermal one seems to be the general phenomenon [29]. It was observed that the molecular weight of the polystyrene prepared by microwave radiation (Mw=350000) was 1.2 times higher than that of polystyrene prepared by the conventional heating. However it was shown that the dominant effect of the microwave on the emulsion polymerization of the styrene is the thermal effect.
In summary, it would seem there is reason to believe that the microwave effect does indeed exist, even if it cannot yet be adequately explained.
A. Miklavc considers [30] a strong non-thermal microwave acceleration mechanism of chemical reactions as a consequence of rotational effects on collision geometry. The very nature of this mechanism is such that it could arise also due to the large-amplitude oscillatory motions of molecules or reactive groups. In the case of polyatomic and biological molecules, the acceleration may come from an excitation of internal rotation of the group taking part in the reaction.
A. Miklavc tried [30] to answer the main question that all relevant theories had to answer: why the mechanism of the microwave acceleration of chemical reactions has been difficult to elucidate. He noted that the occurrence of the phenomenon he has developed subtly depended on the shape and the steepness of the barrier of a reaction and the ratio of the translational to the rotational velocity.
Fini and Breccia [4] have suggested that possible non-thermal effects include lowering of the activation energy of the reaction either through storage of microwave energy as vibration energy of a molecule (or some of its functional groups) or by an alignment of molecules.
Granichev et al [31] have pointed out that tunneling in high-frequency fields is the most effective mechanism of absorption of radiation in the transparency region of dielectrics between phonon and electron excitations.
There are many works stating that 'all chemical changes of microwave field could be rationalized as a consequences of thermal effects only' [32].
A review of the data shows [15] a common pattern: for the first few minutes of irradiation of biological objects there is no pronounced effect, and then a cascade of microbial destruction occurs. It may simply indicate a threshold temperature has been reached, or it may indicate a two-stage process is at work. Due to [15] the second stage of this process may very well be the accumulation of oxygen radicals. They can be generated by the disruption of a hydrogen bond on a water molecule. These water molecules share a hydrogen bond with component atoms of the DNA backbone, including carbon, nitrogen and other oxygen atoms. At any given point in time one of the hydrogen atoms may be preliminary bonded to either an oxygen atom on the water molecule, or to an oxygen (or other) atom on the DNA backbone. The fluctuating character of these shared and exchanged bonds is enhanced by temperature and by the dynamics induced by microwaves. It must be noted here though, that most of the oxygen radicals produced in this manner would exist only briefly, as they would almost immediately bond to the nearest available site. If this site is an oxygen atom on the DNA backbone, we get a covalent bond break, albeit probably only a brief one. Although DNA tends to repair itself naturally, the simultaneous breakage of a sufficient number of covalent bonds would lead to a catastrophic failure of the entire DNA molecule. Due to the exceedingly large number of bonds involved, the matter boils down to a reproducible function of pure probabilities. In other words, after a set and reproducible amount of time determined by probability functions, you would expect to see DNA disintegration. And so, what we have is a two-stage process of DNA covalent bond breakage resulting from oxygen radicals generated by microwave irradiation.
A great majority of published works deals with microwave sources having a frequency of 2.45 GHz and a power of 500-1000 W. Recently we have described polymerization [1,2] and oxidation [33] processes under irradiation by the low power microwave radiation.
We have observed [34] microradicals appearance when styrene, methyl methacrylate or olygocarbonate methacrylate in the presence of para-nitroaniline were irradiated by microwave with frequency by 9.45 GHz and the incident power of 0.05-0.1 W cm-2. The irradiation time was ca. 30 s, and the constant magnetic field with strength of 0.3-0.7 T was directed perpendicular to the microwave flow. The macroradical chemical natures were studied with e.s.r. experiments utilizing 2-methyl-2-nitrosopropane as a spin trap. We have observed the e.s.r. spectra of the radicals having structures
in the case of styrene, methyl methacrylate or olygocarbonate methacrylate, respectively.
Later we have observed [1] that under the action of the low-intensity microwave radiation (under irradiation conditions described above), the polymerization of methyl methacrylate initiated by the radical products arising from the thermal decomposition of AIBM and 2,2,6,6-tetramethyl-4-oxo-piperidinyl-1-oxyl was carried out at 25°C in absence of oxygen. PMMA was synthesized with a yield of 60±5% and MW = 181500 and Mn = 97500. We have proposed that the reaction involved a microwave sensitive step
Here MW is microwave radiation.
Recently [2] we have obtained some relationships between the yields of polymers and free radicals and the strength of the constant magnetic field (H) and the dose of irradiation for the polymerization of MMA-AIBN-TEMPO, styrene-AIBN-TEMPO, and MMA-para-nitroaniline systems initiated by the low power microwave radiation. The maximum yields of the polymers were achieved at H = 0.25-0.30 and 0.65-0.70 T. It was assumed that this effect enhances the probability of a singlet-triplet transitions in the radical pair involved.
In addition to the microwave-initiated polymerizations we have observed [33] hydrocarbon oxidations by molecular oxygen under irradiation by the low power microwave radiation.
Energy of the microwave quanta in our experiments is about 10-5 eV and the power flux is less than 0.1 W cm-2. We have controlled the increase of temperature in our experiments. It has not been more then 12°C [2]. The last result does not agree with any purely thermal effect. These irradiation parameters have given us an opportunity to relate the observed effects to the non-thermal one. It should be noted that the idea about 'hot spots' under conditions of our experiments could not be excluded, too.
Taking into account that an explanation of a microwave initiated reaction in the non-thermal mode should include "non-activation" (under barrier) processes it is possible to propose some hypothetic models.
Model 1.
Due to this model microwaves lead to an increase of on electron tunneling probability. The tunneling followed by a proton transfer produces a radical pair in the singlet state.
or
Here B· is deprotonated form of D·+.
Magnetic fields both external and being a component of a microwave increase the intersystem crossing probability for transitions between the singlet and triplet states of the radical pair. The radicals composing the triplet radical pair diffuse out into a reaction volume.
This model was proposed in our works [1-3].
Of course, the tunneling has low probability but the electric field component of microwaves can enhance it tremendously [31]. In addition, even at room temperatures there are non-zero probabilities for a proton and a hydrogen atom tunneling [35].
Model 2.
The microwave action to molecules can be characterized by very fast (within 10-14 s) shift of electron density in a reaction system (electronic polarization) and the electronic distribution fixation under an influence of the microwave fields for a period of time of about 10-9 s. Positions of atoms in molecules are changed only slightly.
During the said period of the time the activated molecule may take part in 10-1000 collisions.
The electronic density shift does not depend on temperature and may be called as 'non-thermal effect'.
Recently this model was described by Kubrakova [36], and its elements were published earlier [37].
It is possible to go further and to propose that during the fixation time atoms in a molecule begins their moving from their stationary positions (characteristic for the initial molecule) towards 'false stationary positions' suitable for fixed electronic density distribution. If the distances between the two minima on the potential energy surface are little then the molecule can move to the new position under the influence of microwave.
The variant of the model suggests that a microwave forces the wave function of the initial molecule to take a contribution from the wave function of the anti-bonding state corresponding to the bond under dissociation.
Model 3.
The most noticeable feature of microwave action to molecules is their polarization. The molecules having induced dipole moments attract each other and form a cluster. The cluster can grow up in the course of irradiation. When the irradiation ceased the molecules remain packed in the clusters for some time (time of a cluster destruction). That time might be enough for the reaction act to occur.
The model could be of a special use in the case of polymerization in the liquid state. The molecules packed in the cluster contain pairs of molecules having nearest contacts suitable for the polymerization (for example, double bonds having parallel orientation). Vibrations of molecules and their components of the critical value might be the starting point of the polymerization process.
Why are non-thermal reactions relatively rare?5. A borderline between thermal and non-thermal microwave reactions.
One of the most important questions, which have to be answered by any relevant theory of the specific non-thermal effect, is why non-thermal effects are relatively rare. The next one is whether thermal and non-thermal effects are alternative (excluding each other) and whether complementary.
There is no doubt now-a-day that there exist thermal effects of microwaves, and when the power is large enough their contribution is proportional to the microwave power. They are Arrenius-like by their nature and develop themselves when the microwave power is large enough.
However if the microwave power is lower or an activation barrier of a reaction is large then the non-thermal effects come to the scene. Some years ago Lewis has stated that "slower reacting systems tend to show a greater effect under microwave radiation than faster reacting ones" [38]. A microwave effect can be important when steric effects are involved in the reaction.
Usually the non-thermal effects operate concurrently with thermal effects. In addition heat evolved as a result of the thermal action of microwaves disturb the conditions necessary for a successful action of the non-thermal effects. The main conditions are
- non-activation mechanism of the reaction initiating;
- polarization induced ordering or staking of the reagent molecules;
- existence of the chemical amplification step (namely, a chain propagating step);
- little distance between a minimum of starting reagent and a minimum of the final product.
Thus we can state that the borderline between thermal and non-thermal modes of the microwave-initiated reaction is determined by a level of the microwave power from one hand and by a high of the activation barrier of a reaction under investigation from the other hand. The more the former and the less the latter the more should be contribution of the thermal mode. The less the microwave power and the more the activation barrier the more proper conditions for the non-thermal processes involvement.
6. Conclusions.
We have presented some literature and our results saying that the non-thermal effects of microwave are likely to exist. But they usually operate simultaneously with the thermal effects that are considerably much. In addition heat that is the main result of their operating violents the requirements for an efficient involvement of the non-thermal effects. But when the microwave power drops significantly the thermal effects diminish greatly and the non-thermal effects appear themselves. If we want to watch them we shall choose a process that adopts so called 'a chemical amplification'. That means that the microwave-initiated processes have to contain a step where the microwave initiated particles take part in the growing of the reaction chains. The examples of the steps are chain reactions such as polymerization, induced radical oxidation, and biological multiplication of the alterations in DNA.
The exact mechanisms of the microwave initiated non-thermal effects are usually not known. We tried to present a tiny collection of them. They must be non-activation processes that is the processes that need only very little barrier to overcome. Tunneling of electrons, protons, or hydrogen atoms, spin chemistry effects under magnetic field influence, electric field induced shifts of electronic density followed by instantaneous rearrangement of atoms according to new minimum of energy achieved etc. They could be acting separately or collectively and at present we cannot identify them up to the end.
Acknowledgments. This work was supported by the Russian Foundation for Basic Research (project no. 04-03-33030), the Scientific Program "Universities of Russia" (project no. UR.05.01.019), and the Competitive Center of Basic Natural Science, St. Petersburg State University (grant E02-5.0-369).
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