| Ionic liquids?ILs?,as a new environmentally friendly organic solvent,play an important role in the development of green chemistry.There are many excellent properties for IL materials,for example,non-flammability,high conductivity,less volatility,structural tenability and recyclability and so on,and they are often used in various industrial processes,such as organic synthesis,catalytic extraction and electrochemistry et al.They can react not only under normal environmental conditions but also at extreme environments such as external electric or magnetic field.The mechanism of changing for external electric or magnetic field regulating the microstructure and macroscopic properties of ILs,electric field drive of their polymer,and electrowetting phenomenon of ILs have become a concerned research area.In addition,vibrational spectrum is an effective tool for studying the microstructure of liquid molecules,and the study of the response law of ILs to external electric field from molecular/atomic level is beneficial to the analysis of the change law of ILs microstructure under external electric field by the method of spectral characterization.Therefore,it is proposed that vibrational spectrum can be performed by molecular dynamics?MD?method to study how external electric field regulates the change of ILs microstructure,so that the deeper study on the IL materials structure and properties under electric or magnetic field critical conditions can be made.Herein,MD method is employed to investigate the variation of the vibrational spectrum for nano-scale imidazole IL system with the change of external electric field strength for the first time,and the mechanism of the changes of IL vibrational spectrum induced by external electric field are comprehensively analyzed.At 350 K,the vibrational spectra of IL and anion,cation are obtained according to the analysis and calculation of simulation results of the imidazole IL 1-ethyl-3-methylimidazolium hexafluorophosphate?[EMIM][PF6]?cube study model,and the vibrational peaks are assigned in detail on the basis of the work of the predecessors.The results indicate that the main characteristic peaks of the measured spectrum obtained from the experiment can be reproduced by the MD method.Furthermore,in this paper the change of the vibrational spectra for[EMIM][PF6]IL under external electric field varying from 0 V/?to 1.0 V/?are firstly investigated,and the intrinsic reasons of the changes for vibrational bands?VBs?at 50.0,183.2,3196.8 and 3396.6 cm-1 are systematically analyzed,respectively.As the external electric field increases from 0V/?to 1.0 V/?,the intensities of VBs at 50.0 cm-11 and 183.2 cm-1 are continuously enhanced,which can be attributed to the increase of the average values of the total dipole moment for per anion and cation in the IL system?from 4.34 D to 5.46 D?,the orientations for cations are more consistent,respectively.While the intensity of VBs at 3196.8 cm-1 and 3396.6 cm-1 are significantly weakened,due to that the numbers of hydrogen atoms around the carbon atoms on the cationic methyl and ethyl side chains increase and the stretching vibrations of the carbon-hydrogen bonds on alkyl chains are subject to certain limit,the number of intermolecular+C-H???F-hydrogen bonds in the IL system is obviously reduced and the stretching vibration intensity of the carbon-hydrogen bonds on imidazole ring is weakened.Moreover,as the external electric field increases,the VBs at 50.0 and 3396.6 cm-1 both produce a redshift phenomenon,which can be separately attributed to the decrease of average interaction energy for per IL in the system(from-378.7 to-298.0 kJ?mol-1)and the existence of the“relaxation effect”that contributes to the formation of+C-H???F-hydrogen bonds under the external electric field.The research results of this paper will be of great guiding significance for the future exploration on the mechanism of changes of microstructure and macroscopic properties for new IL materials induced by external electric or magnetic field,so that the IL materials would be better applied in various industrial processes. |
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