Non-thermal microwave effect

Non-thermal microwave effects or specific microwave effects have been posited in order to explain unusual observations in microwave chemistry. As the name suggests, the effects are supposed not to require the transfer of microwave energy into thermal energy. Instead, the microwave energy itself directly couples to energy modes within the molecule or lattice. 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. and Dallinger, D. (2013), Microwave Effects in Organic Synthesis: Myth or Reality? . Angew. Chem. Int. Ed., 52: 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.
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