McDaniel Jesse G, Son Chang Yun, Yethiraj Arun
Department of Chemistry , University of Wisconsin , 1101 University Avenue , Madison , Wisconsin 53706 , United States.
J Phys Chem B. 2018 Apr 12;122(14):4101-4114. doi: 10.1021/acs.jpcb.8b01221. Epub 2018 Mar 23.
Room-temperature ionic liquids (ILs) composed of organic anions bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), and trifluoromethanesulfonate (OTf) exhibit interesting physical properties and are important for many electrochemical applications. TFSI and FSI form "hydrophobic" ILs, immiscible with water but miscible with many organic solvents and polymers; for computer simulation studies, it is thus essential to develop force fields for these anions that are transferable among this wide variety of chemical environments. In this work, we develop entirely ab initio force fields for the TFSI, FSI, and OTf anions and predict the properties of corresponding 1-butyl-3-methylimidazolium ILs. We discuss important subtleties in the force field development related to accurately modeling conformational flexibility, that is, relaxed torsional profiles and intramolecular electrostatic interactions. The TFSI anions have notable conformational flexibility in the IL, and we predict approximately 70% cisoid and 20% transoid conformations, which is largely driven by cation/anion ion-pair interactions and is opposite to the trend expected from the anion ab initio potential energy surface. The favorable interactions between the cation and cisoid TFSI conformations result in a shoulder in the cation/anion radial distribution function at short distances, whereas interconversion between cisoid and transoid conformations occurs on a commensurate time scale as ion diffusion processes. In addition to this physical insight on anion effects, we expect that these force fields will have important applications for studying a variety of complex electrolyte systems.
由有机阴离子双（三氟甲磺酰）亚胺（TFSI）、双（氟磺酰）亚胺（FSI）和三氟甲磺酸盐（OTf）组成的室温离子液体（ILs）具有有趣的物理性质，并且对许多电化学应用都很重要。TFSI和FSI形成“疏水”离子液体，与水不混溶，但可与许多有机溶剂和聚合物混溶；因此，对于计算机模拟研究来说，开发适用于这些阴离子的、可在多种化学环境之间转移的力场至关重要。在这项工作中，我们为TFSI、FSI和OTf阴离子开发了完全从头算的力场，并预测了相应的1-丁基-3-甲基咪唑鎓离子液体的性质。我们讨论了力场开发中与准确模拟构象灵活性相关的重要细微之处，即松弛的扭转轮廓和分子内静电相互作用。TFSI阴离子在离子液体中具有显著的构象灵活性，我们预测其顺式构象约为70%，反式构象约为20%，这在很大程度上是由阳离子/阴离子离子对相互作用驱动的，并且与阴离子从头算势能面预期的趋势相反。阳离子与顺式TFSI构象之间的有利相互作用导致阳离子/阴离子径向分布函数在短距离处出现一个峰，而顺式和反式构象之间的相互转化发生在与离子扩散过程相当的时间尺度上。除了对阴离子效应的这种物理见解之外，我们预计这些力场将在研究各种复杂电解质系统方面有重要应用。
