Non-thermal microwave effects or specific microwave effects have been posited in order to explain unusual observations in microwave chemistry. The main effect of the absorption of microwaves by dielectric materials is a brief displacement in the permanent dipoles which causes rotational entropy. Since the frequency of the microwave energy is much faster than the electrons can absorb, the resultant energy can cause frictional heating of nearby atoms or molecules. If the material is rigid there will be no release of rotational energy, and therefore no heating. There are no "Non-thermal effects". If the material is not a dielectric material with dipoles or an ionic distribution, there is no interaction with microwaves and no heating. Non-thermal effects in liquids are almost certainly non-existent,[1][2] as the time for energy redistribution between molecules in a liquid is much less than the period of a microwave oscillation. A 2005 review has illustrated this in application to organic chemistry, though clearly supports the existence of non-thermal effects.[3] It has been shown that such non-thermal effects exist in the reaction of O + HCl(DCl) -> OH(OD) + Cl in the gas phase and the authors suggest that some mechanisms may also be present in the condensed phase.[4] Non-thermal effects in solids are still part of an ongoing debate. It is likely that through focusing of electric fields at particle interfaces, microwaves cause plasma formation and enhance diffusion in solids[5] via second-order effects.[6][7][8] As a result, they may enhance solid-state sintering processes. Debates continued in 2006 about non-thermal effects of microwaves that have been reported in solid-state phase transitions.[9] A 2013 essay concluded the effect did not exist in organic synthesis involving liquid phases.[10] A 2015 perspective[11] discusses the non-thermal microwave effect (a resonance process) in relation to selective heating by Debye relaxation processes.

References

  1. ↑ Stuerga, D.; Gaillard, P. Journal of Microwave Power and Electromagnetic Energy, 1996, 31, 101-113. http://jmpee.org/JMPEE_temp/31-2_bl/JMPEEA-31-2-Pg101.htm
  2. ↑ Stuerga, D.; Gaillard, P. Journal of Microwave Power and Electromagnetic Energy, 1996, 31, 87-99. http://jmpee.org/JMPEE_temp/31-2_bl/JMPEEA-31-2-Pg87.htm
  3. ↑ Microwaves in organic synthesis. Thermal and non-thermal microwave effects, Antonio de la Hoz, Angel Diaz-Ortiz, Andres Moreno, Chem. Soc. Rev., 2005, 164-178. doi:10.1039/B411438H
  4. ↑ Strong Acceleration of Chemical Reactions Occurring Through the Effects of Rotational Excitation on Collisional Geometry, Adolf Miklavc, ChemPhysChem, 2001, 552-555.doi:10.1002/1439-7641(20010917)2:8/9<552::AID-CPHC552>3.0.CO;2-5
  5. ↑ Whittaker, A.G., Chem. Mater., 17 (13), 3426 -3432, 2005.
  6. ↑ Booske, J. H.; Cooper, R. F.; Dobson, I. Journal of Materials Research 1992, 7, 495-501.
  7. ↑ Booske, J. H.; Cooper, R. F.; Freeman, S. A. Materials Research Innovations 1997, 1, 77-84.
  8. ↑ Freeman, S. A.; Booske, J. H.; Cooper, R. F. J. Appl. Phys., 1998, 83, 5761.
  9. ↑ Robb, G.; Harrison, A.; Whittaker, A. G. Phys. Chem. Comm., 2002, 135-137
  10. ↑ Kappe, C. O.; Pieber, B.; Dallinger, D. (2013). "Microwave Effects in Organic Synthesis: Myth or Reality?". Angew. Chem. Int. Ed. 52 (4): 1088–1094. doi:10.1002/anie.201204103.
  11. ↑ Dudley, G. B.; Richert, R.; Stiegman, A. E. (2015). "On the existence of and mechanism for microwave-specific reaction rate enhancement". Chem. Sci. 6 (4): 2144. doi:10.1039/c4sc03372h. PMC 5639434. PMID 29308138.
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