State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, P. R. China.
J Phys Chem B. 2012 Jan 26;116(3):1007-17. doi: 10.1021/jp205435u. Epub 2012 Jan 11.
Due to their unique properties, ionic liquids (ILs) have attracted the academic and industrial attentions. However, recent controversies have focused on what are the main forces to determine the behaviors of ILs. In this work, a detailed DFT calculation was carried out to investigate the intermolecular interactions in two typical ILs, [Emim][BF(4)] and [Bmim][PF(6)]. The results indicate that hydrogen bonds (H-bonds) are the major intermolecular structural feature between cations and anions. Although the electrostatic force remains the major noncovalent force (70% of the total energy by energy decomposition calculation), the interaction energies calculated at different theoretical levels indicate that H-bond and van der Waals interactions cannot be ignored. However, the H-bonded capacities from natural bond orbital (NBO) delocalization energies do not show the consistent changes in the total interaction energies and number of H-bonds. Based on the canonical orbitals analysis, it is found that the σ-type orbital overlap and the partial charges transfer between anion and cation, finally, result in the significant energy reduction and rationalize the preferable location of anion, which is an essential understanding for the interaction and structure in the ion pair. Additionally, the strong agreement between the experimental IR spectra and the calculated vibrations implies that the structures of the larger ion clusters provide a reasonable depiction for bulk ILs at room temperature condition.
由于其独特的性质，离子液体（ILs）引起了学术界和工业界的关注。然而，最近的争议集中在是什么主要力量决定了 ILs 的行为。在这项工作中，进行了详细的 DFT 计算，以研究两种典型的 ILs，[Emim][BF4]和[Bmim][PF6]中的分子间相互作用。结果表明，氢键（H 键）是阳离子和阴离子之间的主要分子间结构特征。尽管静电作用力仍然是主要的非共价力（通过能量分解计算占总能量的 70%），但在不同理论水平上计算的相互作用能表明，H 键和范德华相互作用不能被忽略。然而，自然键轨道（NBO）离域能的 H 键合能力并没有在总相互作用能和 H 键的数量上显示出一致的变化。基于正则轨道分析，发现阴离子和阳离子之间的σ 型轨道重叠和部分电荷转移，最终导致能量显著降低，并合理说明阴离子的优选位置，这对于离子对中的相互作用和结构是一个重要的理解。此外，实验 IR 光谱与计算振动之间的强烈一致性表明，较大离子簇的结构为室温条件下的块状 ILs 提供了合理的描述。
