Mechanism for microwave heating of 1-(4′-cyanophenyl)-4-propylcyclohexane characterized by in situ microwave irradiation NMR spectroscopy

• Published in: Journal of magnetic resonance (1997) Vol. 254; pp. 27 - 34
• Main Authors: Tasei, Yugo, Yamakami, Takuya, Kawamura, Izuru, Fujito, Teruaki, Ushida, Kiminori, Sato, Motoyasu, Naito, Akira
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.05.2015
• Subjects: Cyclohexanes - radiation effects; High temperature heating; Hot Temperature; Isotropic phase; Liquid crystalline phase; Liquid Crystals; Magnetic Resonance Spectroscopy - methods; Microwave heating; Microwave irradiation NMR spectroscopy; Microwaves; Nitriles - radiation effects; Non-equilibrium local heating; Temperature; Liquid crystalline phase; High temperature heating; Microwave irradiation NMR spectroscopy; Microwave heating; Isotropic phase; Non-equilibrium local heating
• ISSN: 1090-7807; 1096-0856
• DOI: 10.1016/j.jmr.2015.02.002

[Display omitted] •In situ microwave irradiation NMR spectrometer was developed.•1H NMR spectra of liquid crystal PCH3 under microwave irradiation were observed.•Non-equilibrium local heating state of PCH3 was observed as a isotropic phase.•Microwave heating generate very high temperature within a short time.•Temperature of PCH3 under microwave irradiation was estimated by chemical shifts. Microwave heating is widely used to accelerate organic reactions and enhance the activity of enzymes. However, the detailed molecular mechanism for the effect of microwave on chemical reactions is not yet fully understood. To investigate the effects of microwave heating on organic compounds, we have developed an in situ microwave irradiation NMR spectroscopy. 1H NMR spectra of 1-(4′-cyanophenyl)-4-propylcyclohexane (PCH3) in the liquid crystalline and isotropic phases were observed under microwave irradiation. When the temperature was regulated at slightly higher than the phase transition temperature (Tc=45°C) under a gas flow temperature control system, liquid crystalline phase mostly changed to the isotropic phase. Under microwave irradiation and with the gas flow temperature maintained at 20°C, which is 25°C below the Tc, the isotropic phase appeared stationary as an approximately 2% fraction in the liquid crystalline phase. The temperature of the liquid crystalline state was estimated to be 38°C according to the line width, which is at least 7°C lower than the Tc. The temperature of this isotropic phase should be higher than 45°C, which is considered to be a non-equilibrium local heating state induced by microwave irradiation. Microwaves at a power of 195W were irradiated to the isotropic phase of PCH3 at 50°C and after 2min, the temperature reached 220°C. The temperature of PCH3 under microwave irradiation was estimated by measurement of the chemical shift changes of individual protons in the molecule. These results demonstrate that microwave heating generates very high temperature within a short time using an in situ microwave irradiation NMR spectrometer.