分子动力学模拟研究外电场对咪唑类离子液体振动谱的影响

本文利用分子动力学模拟研究了外电场对咪唑类离子液体1-乙基-3-甲基咪唑六氟磷酸盐(EMIMPF₆)从0到4000 cm⁻¹范围内振动谱的影响。研究结果表明，在没有外电场时利用分子动力学模拟计算得到的从400到4000 cm⁻¹的振动带可以重现实验测得的谱。当外电场从0到9 V·nm⁻¹变化时，在50.0和199.8 cm⁻¹处的振动带强度持续增强然后趋于饱和，而从400到4000 cm⁻¹的振动带强度明显减弱并最终消失。此外，在外电场从0变到2 V·nm⁻¹时，50.0 cm⁻¹的振动带红移了16.7 cm⁻¹，然后当外电场变化到3 V·nm⁻¹及更大时，该振动带红移增大到33.3 cm⁻¹。在外电场从0变到3 V·nm⁻¹时，3396.6 cm⁻¹的振动带红移大约16.7 cm⁻¹,然后当外电场增大到4 V·nm⁻¹甚至更大时，该振动带红移33.3 cm⁻¹,但是从0到4000 cm⁻¹的其他振动带的位置几乎没有变化。基于对模拟结果和先前报道文献的进一步分析，对于50.0 cm⁻¹的振动带，增加的外电场增强了阳离子和阴离子之间的极性使阳离子和阴离子间的偶极矩增大，因此该振动带的强度不断增大然后达到饱和。对于199.8 cm⁻¹的振动带增加的外电场增强了乙基链的扭转，使该振动带的强度增大并达到饱和。对于从400到4000 cm⁻¹的其他振动带，增加的外电场使EMIMPF₆中的阳离子和阴离子的取向更一致，并且可以推测这种更一致的取向可能会削弱振动带的强度甚至使它们消失。50.0 cm⁻¹处振动带的红移可能是由于外电场破坏了EMIMPF₆内部的静电场分布进而减弱了阳离子和阴离子间的相互作用。3396.6 cm⁻¹处振动带的红移可归功于外电场减弱了氮原子与阳离子咪唑环上酸性氢原子间形成的氢键的拉伸振动。对于其他的振动带，由于官能团固有的拉伸、弯曲、转动振动不受外电场的影响，外电场没有改变振动带的位置。

Influence of External Electric Field on Vibrational Spectrum of Imidazolium-Based Ionic Liquids Probed by Molecular Dynamics Simulation

In this study, the influence of an external electric field (EEF) on the vibrational spectra of an imidazolium-based ionic liquid, 1-ethyl-3-methylimidazolium hexfluorophosphate (EMIMPF₆), in the wavenumber range from 0 to 4000 cm⁻¹ was probed by molecular dynamics (MD) simulation at 350 K. The results showed that the experimentally obtained spectrum could be reproduced by the calculated vibrational bands (VBs) in the wavenumber range from 400 to 4000 cm⁻¹ using MD simulation without any EEF. When the EEF applied increased from 0 to 9 V·nm⁻¹, the VB intensities at 50.0 and 199.8 cm⁻¹ increased continuously and then tended to be saturated, while the VB intensities from 400 to 4000 cm⁻¹ decrease and eventually disappear. Moreover, the VB at 50.0 cm⁻¹ was red-shifted to ~16.7 cm⁻¹ and then increased to 33.3 cm⁻¹ as the EEF was increased from 0 to 2 and then to 3 V·nm⁻¹ and higher. The VB at 3396.6 cm⁻¹ was redshifted to ~16.7 cm⁻¹ and then increased to 33.3 cm⁻¹ as the EEF was increased from 0 to 3 and then to 4 V·nm⁻¹ and higher; however, the position of other VBs from 0 to 4000 cm⁻¹ remain almost unchanged. Based on further analysis of the simulation results and previously reported studies, for the VB at 50.0 cm⁻¹, the increasing EEF enhances the polarity between cations and anions; thus, the difference in dipole moment between the cations and the anions increases, which continually increases the VB intensity until saturation is reached. For the VB at 199.8 cm⁻¹, the increasing EEF intensifies the twisting of the ethyl chain, which enhances the VB intensity until saturation. For the other VBs from 400 to 4000 cm⁻¹, the increasing EEF makes the orientation of the cations and anions in EMIMPF₆ more consistent; thus, it can be conjectured that such consistent orientation may weaken the VB intensities and can even make them disappear. The redshift of VB at 50.0 cm⁻¹ may occur because the EEF breaks the distribution of the electrostatic field inside EMIMPF₆ and then weakens the interactions between cations and anions. The redshift of VB at 3396.6 cm⁻¹ may be attributed to the EEF weakening the stretching vibration of the hydrogen bonds formed between the N atoms and the acidic hydrogen atoms on the cationic imidazolium rings. The EEF does not change the positions of the other VBs because the inherent stretching, bending, and rocking vibration of functional groups are not affected by the EEF.