The microwave heating mechanism of N-(4-methoxybenzyliden)-4-butylaniline in liquid crystalline and isotropic phases as determined using in situ microwave irradiation NMR spectroscopy

Abstract

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.

Synthesis and Modification of Carbon Nanomaterials utilizing Microwave Heating

“…We only briefly want to name four examples of how research is progressing to investigate potential specific microwave effects. For example, silicon carbide reactor vials were designed to separate thermal and potential non‐thermal microwave effects, a combination of temperature measurement by an internal fiber probe and an external IR sensor was applied to obtain more‐reliable and reproducible heating profiles, and in situ Raman and NMR spectroscopy were used to investigate the molecular mechanisms associated with microwave‐heating effects on chemical reactions.…”

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

“…The local dipolar interaction of individual protons in the liquid crystalline state can be examined by this method via high resolution resonance in the isotropic phase, − and the resulting data can also be used to obtain state-correlated 2D NMR spectra of proteins in both native and denatured states . In situ microwave irradiation NMR spectroscopy was developed later, and the microwave heating process was observed in liquid crystalline samples. , In the liquid crystalline molecules, the different protons were found to indicate significantly different temperature …”

“…30 In situ microwave irradiation NMR spectroscopy was developed later, 31 and the microwave heating process was observed in liquid crystalline samples. 31,32 In the liquid crystalline molecules, the different protons were found to indicate significantly different temperature. 32 Molecular dynamics (MD) simulations have recently been conducted to investigate the atomic-scale properties of molecular systems under an electric field.…”

“…31,32 In the liquid crystalline molecules, the different protons were found to indicate significantly different temperature. 32 Molecular dynamics (MD) simulations have recently been conducted to investigate the atomic-scale properties of molecular systems under an electric field. Tanaka and Sato performed MD simulations of water and ice to investigate their heating processes under microwave irradiation.…”

MRI‐visible liquid crystal thermometer

“…Previous 1 H NMR studies of LCs showed distinct differences in the spectra between the various crystalline and isotropic phases . One group detected the LC phase transition from nematic (crystalline) phase to isotropic phase.…”