Matthews Richard P, Villar-Garcia Ignacio J, Weber Cameron C, Griffith Jeraime, Cameron Fiona, Hallett Jason P, Hunt Patricia A, Welton Tom
Department of Chemistry, Imperial College London, London, SW7 2AZ, UK.
Phys Chem Chem Phys. 2016 Mar 28;18(12):8608-24. doi: 10.1039/c6cp00156d.
The structures of mixtures of ionic liquids (ILs) featuring a common 1-butyl-3-methylimidazolium (C4C1im) cation but different anions have been investigated both experimentally and computationally. (1)H and (13)C NMR of the ILs and their mixtures has been performed both on the undiluted liquids and those diluted by CD2Cl2. These experiments have been complemented by quantum chemical density functional theory calculations and molecular dynamics simulations. These techniques have identified the formation of preferential interactions between H(2) of the imidazolium cation and the most strongly hydrogen bond (H-bond) accepting anion. In addition, a preference for the more weakly H-bond accepting anion to interact above the imidazolium ring through anion-π(+) interactions has been identified. The modelling of these data has identified that the magnitude of these preferences are small, of the order of only a few kJ mol(-1), for all IL mixtures. No clustering of the anions around a specific cation could be observed, indicating that these interactions arise from the reorientation of the cation within a randomly assigned network of anions. π(+)-π(+) stacking of the imidazolium cations was also studied and found to be promoted by ILs with a strong H-bond accepting anion. Stacking interactions are easily disrupted by the introduction of small proportions (<50 mol%) of a weakly coordinating anion due to their propensity to form anion-π(+) interactions. These results suggest that the formation of IL mixtures with different anions leads to subtle structural changes of much lower energy than the Coulombic ordering of ions, accounting for why most IL mixtures exhibit ideal, or nearly ideal, behaviour.
对具有常见的1-丁基-3-甲基咪唑鎓（C4C1im）阳离子但不同阴离子的离子液体（ILs）混合物的结构进行了实验和计算研究。对ILs及其混合物进行了（1）H和（13）C NMR实验，实验对象既有未稀释的液体，也有被CD2Cl2稀释的液体。这些实验得到了量子化学密度泛函理论计算和分子动力学模拟的补充。这些技术确定了咪唑鎓阳离子的H(2)与最强氢键（H键）接受阴离子之间形成了优先相互作用。此外，还确定了较弱H键接受阴离子更倾向于通过阴离子-π(+)相互作用在咪唑鎓环上方相互作用。对这些数据的建模表明，对于所有IL混合物，这些偏好的幅度很小，仅为几kJ mol(-1)左右。未观察到阴离子围绕特定阳离子的聚集，这表明这些相互作用源于阳离子在随机分配的阴离子网络中的重新定向。还研究了咪唑鎓阳离子的π(+)-π(+)堆积，发现具有强H键接受阴离子的ILs会促进这种堆积。由于弱配位阴离子倾向于形成阴离子-π(+)相互作用，因此引入小比例（<50 mol%）的弱配位阴离子会很容易破坏堆积相互作用。这些结果表明，形成具有不同阴离子的IL混合物会导致比离子的库仑有序化能量低得多的细微结构变化，这解释了为什么大多数IL混合物表现出理想或接近理想的行为。
