Graduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan.
School of Engineering and Applied Sciences, National University of Mongolia, Ulaanbaatar 14201, Mongolia.
J Phys Chem B. 2020 Oct 29;124(43):9615-9624. doi: 10.1021/acs.jpcb.0c06383. Epub 2020 Oct 20.
Microwave heating is widely used to accelerate the organic synthesis reaction. However, the role of the nonthermal microwave effect in the chemical reaction has not yet been well characterized. The microwave heating processes of an ethanol-hexane mixed solution were investigated using in situ microwave irradiation nuclear magnetic resonance spectroscopy and molecular dynamics (MD) simulation. The temperature of the solution under microwave irradiation was estimated from the temperature dependence of the H chemical shifts (chemical shift calibrated (CSC)-temperature). The CSC-temperature increased to 58 °C for CH and CH protons, while it increased to 42 °C for OH protons during microwave irradiation. The CSC-temperature of CH and CH protons reflects the bulk temperature of solution by the thermal microwave effect. The lower CSC-temperature of the OH proton can be attributed to a nonthermal microwave effect. MD simulation revealed that electron dipole moments of OH groups ordered along the oscillated electric field decreased the entropy by absorbing microwave energy and simultaneously increased the entropy by dissipating energy to the solution as the thermal and nonthermal microwave effect. Ordered polar molecules interact to increase hydrogen bonds between OH groups as the nonthermal microwave effect, which explains the lower CSC-temperature of the OH protons. The nonthermal microwave effects contribute to the intrinsic acceleration of the organic reaction.
微波加热广泛用于加速有机合成反应。然而，非热微波效应在化学反应中的作用尚未得到很好的描述。本研究采用原位微波辐照核磁共振波谱和分子动力学（MD）模拟研究了乙醇-己烷混合溶液的微波加热过程。通过 H 化学位移（化学位移标定（CSC）-温度）的温度依赖性来估计溶液在微波辐照下的温度。在微波辐照下，CH 和 CH 质子的 CSC-温度升高到 58°C，而 OH 质子的 CSC-温度升高到 42°C。CH 和 CH 质子的 CSC-温度通过热微波效应反映了溶液的体相温度。OH 质子较低的 CSC-温度可归因于非热微波效应。MD 模拟表明，OH 基团的电子偶极矩沿振荡电场有序排列，通过吸收微波能量降低了熵，同时通过将能量耗散到溶液中作为热和非热微波效应增加了熵。有序偶极分子相互作用增加了 OH 基团之间的氢键，这是非热微波效应，解释了 OH 质子较低的 CSC-温度。非热微波效应有助于有机反应的内在加速。
