Intermolecular forces in an ionic liquid ([Mmim][Cl]) versus those in a typical salt (NaCl).
Intermolecular forces in an ionic liquid ([Mmim][Cl]) versus those in a typical salt (NaCl).
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DOI：
10.1002/anie.200705526
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发表时间：
2008-04
影响因子：
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通讯作者：
Stefan Zahn;F. Uhlig;J. Thar;C. Spickermann;B. Kirchner
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文献类型：
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作者：
Stefan Zahn;F. Uhlig;J. Thar;C. Spickermann;B. Kirchner
理解化学键和分子间作用力是化学领域的主要课题之一。一般来说，化合物是根据它们的性质来分类的。离子液体（ILs）这类物质自上个世纪初就为人所知。由于其具有可调谐和低蒸汽压的特性，近年来成为一个具有广泛应用前景的研究热点。更好地理解离子液体的一个有希望的途径是确定主要的分子间力，并将它们与NaCl等例子中的分子间力进行比较，NaCl在室温下是固体，换句话说，是同类化合物的代表。人们在探索离子液体的一般性质时必须小心，因为对一般性质的探索导致了许多关于离子液体的神话。离子液体的性质通常是由特定物质的化学性质来解释的，而不是一般的特征。以咪唑基离子液体为例，我们选择了1,3-二甲基咪唑氯（[Mmim][Cl]）。如果考虑从从头算中理论预测的典型的正离子和阴离子在典型IL中平衡距离处的总相互作用能，这些能量范围在300到400千兆摩尔之间，与参考文献[13]一致（有关方法的详细信息，请参阅支持信息）。对NaCl计算同样的能量，得到545.0 kmol。这有力地指向了将这些能量与相应体系的熔点联系起来。从图1可以明显看出，预测能量与熔点之间没有相关性。一个简单的熔点估计模型表明，ILs的液体行为很可能归因于具有高构象柔韧性的大而不对称的离子。近年来的研究揭示了具有扩展侧链的咪唑基体系具有微异相极性和非极性结构域的复杂结构；当侧链较短时，不会观察到这种现象。据推测，除了纯库仑相互作用外，离子液体中还必须有其他作用力。因此，我们采用对称自适应摄动理论（SAPT）方法，将[Mmim][Cl]和NaCl的一个离子对的总相互作用能分解为类似于多极展开的不同贡献（图2和3）。注意，为了提供可比性，平衡距离被设置为零。对于NaCl对（图2中的红色菱形），色散项可以忽略不计，而对于离子液体对[Mmim][Cl]的两个构象（图2中的蓝色和绿色菱形），色散项的贡献在量级上与诱导项相当。总能量的主要贡献来自于所有物种的静电相互作用（图2中的圆圈），这与参考文献[13]一致。对于NaCl，总能量仅由静电、交换和感应贡献组成（见图3；带有红色方块的曲线与带有菱形的曲线几乎完全匹配）。在图3中，我们可以对最小值进行另一个有趣的观察：尽管NaCl对在平衡距离处（在零处，见黑色虚线）具有所有曲线的最小值，但令人惊讶的是，[Mmim][Cl]对并非如此（见黑色图1）。熔点是根据几种不同离子的一个阳离子和一个阴离子之间的相互作用能绘制的。每个IL的球棒模型也在图中给出。1:[以][AlCl4), 2: [Mmim] (Cl), 3:[以]BF4, 4:以(Cl), 5:以(DCA), 6:以(SCN)。Emim=1-乙基-3-甲基咪唑离子，DCA=二氰酰胺。
Understanding chemical bonding and intermolecular forces is one of the major topics in chemistry. In general, chemical compounds are divided into classes based on their properties. The class of ionic liquids (ILs) has been known since the beginning of the last century. Owing to their tuneable properties and low vapor pressure, ILs have become a hot research area with a wide range of applications in recent years. A promising route towards better understanding ionic liquids is to determine the dominating intermolecular forces and compare them to those in an example such as NaCl, which is solid at room temperature and, in other words, a compound representative of its class. One must be careful in probing for general properties of ILs, because the search for generality has led to many myths about ionic liquids. Often properties of ionic liquids are explained by the chemical nature of the particular substance and are not a general feature. As an example for imidazolium-based ionic liquids, we chose 1,3-dimethylimidazolium chloride ([Mmim][Cl]). If one considers the theoretically predicted total interaction energies from ab initio calculations between a typical cation and an anion at the equilibrium distance in a typical IL, these energies range from 300 to 400 kJmol , in agreement with Ref. [13] (for details on the methods see the Supporting Information). Calculating the same energy for NaCl gives a value of 545.0 kJmol . This strongly points toward correlating these energies with melting points of the corresponding bulk system. It is obivous from Figure 1 that there is no correlation between the predicted energies and the melting points. A simple model for estimating the melting points of ILs suggests that most likely the liquid behavior of ILs can be attributed to large, unsymmetrical ions with high conformational flexibility. Recent studies reveal complex structures having microheterogenous polar and nonpolar domains for imidazolium-based systems with extended side chains; this phenomenon is not observed when the side chains are shorter. It has been inferred that other forces besides pure Coulombic interactions must play a role in ionic liquids. Thus, we decompose the total interaction energy of one ion pair of [Mmim][Cl] and one ion pair of NaCl by the symmetry-adapted perturbation theory (SAPT) method into different contributions in analogy to a multipole expansion (Figures 2 and 3). Note that the equilibrium distance is set to zero in order to provide comparability. For the NaCl pair (red diamonds in Figure 2) the dispersion term is negligible, whereas this contribution is comparable in magnitude to the induction term for the two conformers of the ionic liquid pair [Mmim][Cl] (blue and green diamonds in Figure 2). The main contribution to the total energy stems from the electrostatic interaction for all species, (circles in Figure 2) in agreement with Ref. [13]. For NaCl the total energy consists of only electrostatic, exchange, and induction contributions (see Figure 3; the curve with red squares almost exactly matches the curve with diamonds). In Figure 3 we can make another interesting observation concerning the minima: Whereas the NaCl pair features the minima for all curves exactly at the equilibrium distance (at zero, see black dotted vertical line), this is, surprisingly, not the case for the [Mmim][Cl] pairs (see black Figure 1. Melting points plotted against the interaction energies between one cation and one anion for several different ILs. A ball-andstick model of each IL is also given in the figure. 1: [Emim][AlCl4] , 2 : [Mmim][Cl], 3 : [Emim][BF4], 4 : [Emim][Cl], 5 : [Emim][DCA], 6 : [Emim][SCN]. Emim=1-ethyl-3-methylimidazolium ion, DCA=dicyanamide.