物理化学学报 >> 2021, Vol. 37 >> Issue (10): 2001004.doi: 10.3866/PKU.WHXB202001004

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外电场作用下离子液体振动光谱变化的分子动力学模拟研究

陈文琼1, 关永吉1, 张姣1, 裴俊捷1, 张晓萍1,*(), 邓友全2,*()   

  1. 1 兰州大学信息科学与工程学院光电子与电磁信息研究所,兰州 730000
    2 中国科学院兰州化学物理研究所绿色化学与催化中心,兰州 730000
  • 收稿日期:2020-01-02 录用日期:2020-03-19 发布日期:2020-03-23
  • 通讯作者: 张晓萍,邓友全 E-mail:zxp@lzu.edu.cn;ydeng@licp.cas.cn
  • 基金资助:
    国家重点研发项目(2017YFA0403101);兰州大学国际师资博士后项目和兰州大学“中央高校基本科研业务费”优秀研究生创新项目(lzujbky-2018-it62)

Atomistic Insight into Changes in the Vibrational Spectrum of Ionic Liquids under External Electric Field

Wenqiong Chen1, Yongji Guan1, Jiao Zhang1, Junjie Pei1, Xiaoping Zhang1,*(), Youquan Deng2,*()   

  1. 1 Institute of Optoelectronics and Electromagnetic Information, School of Information Science and Engineering, Lanzhou University, Lanzhou 730000, China
    2 Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
  • Received:2020-01-02 Accepted:2020-03-19 Published:2020-03-23
  • Contact: Xiaoping Zhang,Youquan Deng E-mail:zxp@lzu.edu.cn;ydeng@licp.cas.cn
  • About author:Email: ydeng@licp.cas.cn; Tel.: +86-931-4968116 (Y.D.)
    Email: zxp@lzu.edu.cn (X.Z.)
  • Supported by:
    the National Key Research and Development Program of China(2017YFA0403101);the Lanzhou University International Teacher Postdoctoral Scholarship Fund and the Fundamental Research Funds for the Central Universities, China(lzujbky-2018-it62)

摘要:

振动光谱学是研究液体分子微观结构的强有力工具,利用振动光谱学探究外界条件作用下液体结构性质的变化是可行的。本文利用分子动力学模拟方法研究了从0到10 V·nm-1变化的外加电场对1-乙基-3-甲基咪唑六氟磷酸盐离子液体的振动光谱的影响,并且详细分析了位于50,183,3196,3396 cm-1的振动峰的强度及位置随外加电场变化而变化的内在原因。随着外加电场从0增大到10 V·nm-1,由于模拟体系中平均每对阴阳离子的总偶极矩增大(从4.34到5.46 Debye)和阳离子的取向更加一致,使得位于50、183 cm-1的振动峰的强度相应地逐渐增强。位于3196 cm-1的振动峰的强度明显减弱,因为外加电场的作用使甲基和乙基侧链上碳原子周围的氢原子不断增多,从而使烷基链上碳氢键的伸缩振动受限。外加电场作用下分子间+C-H···F-氢键的减少使位于3396 cm-1的振动峰的强度逐渐减弱。而位于50和3396 cm-1的振动峰的红移现象分别归因于每对离子平均相互作用能的减小(从-378.7到-298.0 kJ·mol-1)和有利于氢键结合的弛豫效应。

关键词: 外加电场, 振动光谱, 离子液体, 分子动力学模拟

Abstract:

Vibrational spectroscopy is a powerful tool for studying the microstructure of liquids, and anatomizing the nature of the vibrational spectrum (VS) is promising for investigating changes in the properties of liquid structures under external conditions. In this study, molecular dynamics (MD) simulations have been performed to explore changes in the VS of 1-ethyl-3-methylimidazolium hexafluorophosphate ([Emim][PF6]) ionic liquid (IL) under an external electric field (EEF) ranging from 0 to 10 V·nm-1 at 350 K. First, the vibrational spectra for [Emim][PF6] IL as well as its cation and anion are separately obtained, and the peaks are strictly assigned. The results demonstrate that the VS calculated by MD simulation can well reproduce the main characteristic peaks in the experimentally measured spectrum. Then, the vibrational spectra of the IL under various EEFs from 0 to 10 V·nm-1 are investigated, and the intrinsic origin of the changes in the vibrational bands (VBs) at 50, 183, 3196, and 3396 cm-1 is analyzed. Our simulation results indicate that the intensities of the VBs at 50 and 183 cm-1 are enhanced. In addition, the VB at 50 cm-1 is redshifted by about 16 cm-1 as the EEF is varied from 0 to 2 V·nm-1, and the redshift wavenumber increases to 33 cm-1 as the EEF is increased to 3 V·nm-1 and beyond. However, the intensities of the VBs at 3196 and 3396 cm-1 show an obvious decrease. Meanwhile, the VB at 3396 cm-1 is redshifted by about 16 cm-1 when the EEF increases to 3 V·nm-1, and the redshift increases to 33 cm-1 with an increase in the EEF beyond 4 V·nm-1. The intensity of the VB at 50 cm-1 increases because of the increase in the total dipole moment of each anion and cation (from 4.34 to 5.46 D), and the redshift is attributed to the decrease in the average interaction energy per ion pair (from -378.7 to -298.0 kJ·mol-1) with increasing EEF. The intensity of the VB at 183 cm-1 increases on account of the more consistent orientations for cations in the system with increasing EEF. The VB at 3196 cm-1 weakens visibly because a greater number of hydrogen atoms appear around the carbon atoms on the methyl/ethyl side chains and the vibrations of the corresponding carbon-hydrogen bonds are suppressed under the action of the EEF. Furthermore, the intensity of the VB at 3396 cm-1 decreases due to the decrease in the intermolecular +C-H···F- hydrogen bonds (HBs), while the relaxation effect that is beneficial for the formation of HBs simultaneously exists in the system under the varying EEF, thus causing a redshift of the VB at 3396 cm-1.

Key words: External electric field, Vibrational spectrum, Ionic liquids, Molecular dynamics simulation

MSC2000: 

  • O641