**14. Conclusion**

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

along the x-axis as a function of time. The black and green lines show the oscillation of the dipole moments along the x-axis under the applied electric field at 293 and 315 K, respectively. Due to the applied electric field, the dipole moment (black and green) at 293 and 315 K were delayed by around 32.5 and 18 ps, respectively. The values are close to the experimentally measured dielectric relaxation time of MBBA at 28 and 23.7 ps at 20 and 50°C, respectively [60]. In contrast, there is no oscillation of the dipole moment without the electric field at 293 K or 315 K, which indicates that the applied electric field affected the polar groups of the MBBA molecules. The net direction of C-O and C=N groups of MBBA during these simulations were also analyzed, and are shown in **Figure 8(g)** and **(h)**, respectively. The α' and 7′ protons respectively bind to the C-O and C=N groups in the MBBA molecule, as shown in **Figure 6A**. **Figure 8(g)** and **(h)** show the net directions of the C-O and C=N groups as a function of time. The black and green lines show the net direction of C-O or C=N groups under the electric field at 293 and 315 K, respectively. The red and blue lines display the net directions in the simulations at 293 and 315 K without an electric field. The oscillation phase of the C-O groups is consistent with the phase of the dipole moments, as shown in **Figure 8(f )**, and their amplitudes are higher than those of the C=N groups shown in **Figure 8(h)**. This indicates that microwave heating has a more significant influence on the C-O groups of MBBA

These simulations showed the ordering of the polar molecules in the presence of an electric field as a non-thermal microwave effect. This polarization can only appear under microwave irradiation. Furthermore, this type of ordered state is also considered a low entropy state and is a distinct state occurring under microwave

According to the MD simulations under microwave irradiation, the interactions between coherently ordered polar groups increased the number of hydrogen bonds in the ethanol-hexane mixed solution due to their coherent polarization (**Figures 7** and **8**). The formation of hydrogen bonds is due to the interaction between two OH groups, thus causing the energy of the work term to be supplied as a non-thermal microwave effect. This non-thermal microwave effect was experimentally verified

This coherently ordered state of OH groups only appears under microwave irradiation and is different from the molecular order achieved by conventional thermal heating, even at the same bulk temperature of the system. In this microwaveinduced ordered state, polar molecules are coherently aligned along with the alternately oscillated electric field. These coherently ordered molecules enable interaction between polar groups. Furthermore, the coherently ordered low entropy state may accelerate the chemical reaction rate between molecules with polar

Similar to under microwave irradiation, polarizable molecules, or those with a dipole moment, will also gradually align with the direction of an oriented external electric field (OEEF). A sufficiently strong OEEF can completely orient a molecule or a molecular complex in space through interacting with its dipole and polarizability, thereby removing, in principle, the difficulty in orienting the molecules and the OEEF. Therefore, it is possible to enhance or control the chemical reactivity in

H chemical shift of the OH protons in ethanol.

**13. Microwave effects on chemical reaction processes**

groups, as in the formation of hydrogen bonding in ethanol.

catalysis by a decrease in the activation energy of the reaction [61, 62].

by the observation of a lower field 1

**182**

molecules.

irradiation.

The CSC-temperatures of an ethanol-hexane mixed solution and MBBA in the isotropic state under microwave irradiation were accurately evaluated using the linear relationship of temperature with respect to the 1 H chemical shift changes (Δδ) of individual protons. A CSC-temperature increase was observed as a function of the microwave irradiation time for CH2 and CH3 non-polar protons. The CSCtemperature for non-polar protons reflects the bulk temperature of the solution. A lowered CSC-temperature with lower field <sup>1</sup> H chemical shift was observed for OH polar protons than that with CH2 and CH3 non-polar protons in ethanol, and higher CSC-temperature was observed for H-C=N (7′) and CH3-O (α') protons in MBBA. The lowered CSC-temperature of OH protons in ethanol under microwave irradiation which was lower than the bulk temperature is concluded to be the experimental evidence of a non-thermal microwave effect. In the microwave heating process, microwave energy is absorbed into the polar molecular system by the formation of an ordered state with lower entropy. Ordered dipolar molecules cannot completely follow the oscillating electric field; therefore, the ordered state becomes partly disordered, increasing the entropy. Microwave energy is simultaneously dissipated to the system as thermal and non-thermal microwave effects. These coherently ordered molecules interact strongly with each other to form hydrogen bonds between the OH groups of ethanol, and these interactions are considered to be due to a non-thermal microwave effect. MD simulation was carried out to confirm the theoretical validity of the experimentally observed increased lower field <sup>1</sup> H chemical shift, and the results were found to agree well. These non-thermal microwave effects play an important role in the intrinsic acceleration of chemical reaction rates between polar molecules under microwave irradiation. It is considered that the coherently ordered state reduces the activation energy for the reaction, which increases the reaction rate as catalysis.
