Thermal and Nonthermal Microwave Effects of Ethanol and Hexane-Mixed Solution as Revealed by In Situ Microwave Irradiation Nuclear Magnetic Resonance Spectroscopy and Molecular Dynamics Simulation

Abstract

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 1H chemical shifts (chemical shift calibrated (CSC)-temperature). The CSC-temperature increased to 58 °C for CH2 and CH3 protons, while it increased to 42 °C for OH protons during microwave irradiation. The CSC-temperature of CH2 and CH3 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.

Microwave Heating of Liquid Crystals and Ethanol-Hexane Mixed Solution and Its Features (Review)

“…It is important to note that the orientation behavior of OH bonds along the electric field direction can be observed in these figures. In contrast, the directions of the OH groups were mostly oriented in random directions in the absence of an electric field [34]. Similar results were observed in other simulations at 313, 323, and 333 K (data are not shown).…”

“…It is stressed that the in situ temperature of the bulk solution was accurately determined with the CSC-temperature for individual non-polar proton groups under microwave irradiation. [34]. Copyright (2020) American Chemical Society.…”

“…It is important to note that polar molecules follow the oscillating electric field in a coherent manner. In this case, coherently aligned polar molecules are able to [34]. Copyright (2020) American Chemical Society.…”

“…In situ microwave irradiation NMR spectroscopy was developed later [32], and the microwave heating process was observed in liquid crystalline samples [32,33] and ethanol-hexane mixed solution [34]. The in situ temperature of the bulk solution was determined using the relation between 1 H chemical shift and temperature.…”

“…In N-(4-methoxybenzylidene)-4-butylaniline (MBBA) molecules, H-C=N (7′), and CH 3 -O (α') protons showed significantly higher CSC-temperatures than the bulk temperature in the isotropic state [33]. In the ethanol-hexane mixed solution, OH proton showed lower CSC-temperature than that of the bulk solution [34].…”