Tasei Yugo, Tanigawa Fumikazu, Kawamura Izuru, Fujito Teruaki, Sato Motoyasu, Naito Akira
Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan.
Phys Chem Chem Phys. 2015 Apr 14;17(14):9082-9. doi: 10.1039/c5cp00476d. Epub 2015 Mar 10.
Microwave heating effects are widely used in the acceleration of organic, polymerization and enzymatic reactions. These effects are primarily caused by the local heating induced by microwave irradiation. However, the detailed molecular mechanisms associated with microwave heating effects on the chemical reactions are not yet well understood. This study investigated the microwave heating effect of N-(4-methoxybenzylidene)-4-butylaniline (MBBA) in liquid crystalline and isotropic phases using in situ microwave irradiation nuclear magnetic resonance (NMR) spectroscopy, by obtaining (1)H NMR spectra of MBBA under microwave irradiation. When heated simply using the temperature control unit of the NMR instrument, the liquid crystalline MBBA was converted to the isotropic phase exactly at its phase transition temperature (Tc) of 41 °C. The application of microwave irradiation at 130 W for 90 s while maintaining the instrument temperature at 20 °C generated a small amount of isotropic phase within the bulk liquid crystal. The sample temperature of the liquid crystalline state obtained during microwave irradiation was estimated to be 35 °C by assessing the linewidths of the (1)H NMR spectrum. This partial transition to the isotropic phase can be attributed to a non-equilibrium local heating state induced by the microwave irradiation. The application of microwave at 195 W for 5 min to isotropic MBBA while maintaining an instrument temperature of 50 °C raised the sample temperature to 160 °C. In this study, the MBBA temperature during microwave irradiation was estimated by measuring the temperature dependent chemical shifts of individual protons in the sample, and the different protons were found to indicate significantly different temperatures in the molecule. These results suggest that microwave heating polarizes bonds in polar functional groups, and this effect may partly explain the attendant acceleration of organic reactions.
微波加热效应广泛应用于有机反应、聚合反应和酶促反应的加速过程中。这些效应主要是由微波辐射引起的局部加热所致。然而，与微波加热效应影响化学反应相关的详细分子机制尚未得到充分理解。本研究使用原位微波辐射核磁共振（NMR）光谱，通过获取微波辐射下N-（4-甲氧基亚苄基）-4-丁基苯胺（MBBA）的（1）H NMR谱，研究了MBBA在液晶相和各向同性相中的微波加热效应。当仅使用NMR仪器的温度控制单元进行加热时，液晶态的MBBA恰好在其41℃的相变温度（Tc）下转变为各向同性相。在将仪器温度保持在20℃的同时，以130W的功率施加微波辐射90s，在大量液晶中产生了少量的各向同性相。通过评估（1）H NMR谱的线宽，估计微波辐射期间获得的液晶态样品温度为35℃。这种向各向同性相的部分转变可归因于微波辐射引起的非平衡局部加热状态。在将仪器温度保持在50℃的同时，对各向同性的MBBA施加195W的微波5分钟，使样品温度升至160℃。在本研究中，通过测量样品中各个质子的温度依赖性化学位移来估计微波辐射期间的MBBA温度，发现不同的质子在分子中显示出显著不同的温度。这些结果表明，微波加热使极性官能团中的键极化，这种效应可能部分解释了随之而来的有机反应加速现象。
