Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, Japan.
J Phys Chem A. 2012 Mar 8;116(9):2177-83. doi: 10.1021/jp212460v. Epub 2012 Feb 27.
There have been a growing number of articles that report dramatic improvements in the experimental performance of chemical reactions by microwave irradiation compared to that under conventional heating conditions. We theoretically examined whether nonthermal microwave effects on intramolecular reactions exist or not, in particular, on Newman-Kwart rearrangements and intramolecular Diels-Alder reactions. The reaction rates of the former calculated by the transition state theory, which consider only the thermal effects of microwaves, agree quantitatively with experimental data, and thus, the increases in reaction rates can be ascribed to dielectric heating of the solvent by microwaves. In contrast, for the latter, the temperature dependence of reaction rates can be explained qualitatively by thermal effects but the possibility of nonthermal effects still remains regardless of whether competitive processes are present or not. The effective intramolecular potential energy surface in the presence of a microwave field suggests that nonthermal effects arising from potential distortion are vanishingly small in intramolecular reactions. It is useful in the elucidation of the reaction mechanisms of microwave synthesis to apply the present theoretical approach with reference to the experiments where thermal and nonthermal effects are separated by screening microwave fields.
已有越来越多的文章报道，与传统加热条件相比，微波辐射可显著提高化学反应的实验性能。我们从理论上研究了微波对分子内反应是否存在非热效应，特别是对 Newman-Kwart 重排和分子内 Diels-Alder 反应。通过仅考虑微波热效应的过渡态理论计算的前者的反应速率与实验数据定量一致，因此，反应速率的增加可以归因于微波对溶剂的介电加热。相比之下，对于后者，反应速率的温度依赖性可以通过热效应定性解释，但无论是否存在竞争过程，非热效应的可能性仍然存在。在微波场存在下的有效分子内势能表面表明，分子内反应中由势能变形引起的非热效应非常小。在通过屏蔽微波场将热效应和非热效应分开的实验中，应用本理论方法有助于阐明微波合成的反应机制。
