Spectroscopic Evidence for an Enhanced Anion-Cation Interaction from Hydrogen Bonding in Pure Imidazolium Ionic Liquids
Spectroscopic Evidence for an Enhanced Anion-Cation Interaction from Hydrogen Bonding in Pure Imidazolium Ionic Liquids
复制标题
DOI：
10.1002/anie.200905437
复制
发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Ludwig, Ralf
中科院分区：
文献类型：
--
作者：
Wulf, Alexander;Fumino, Koichi;Ludwig, Ralf
离子液体 (IL) 在化学和技术领域具有许多有价值的应用。[1-6] 它们被理解为完全由离子组成且熔点低于 1008C 的液体。离子液体的有趣特性取决于其成分之间相互作用的类型和强度。据认为，氢键对于这些库仑系统的性质和反应动力学起着重要作用。 Seddon等人首先报道了1-烷基-3-甲基咪唑鎓盐结构中氢键的存在。 1986年。[7]此后，人们通过 X 射线衍射、中红外和核磁共振光谱获得了氢键存在的证据。咪唑基离子液体中的局域和定向相互作用（例如氢键）通过较短的 CÀH… 阴离子距离、红移的 CÀH 频率和低场位移的 CÀH 质子化学位移来表明。[8-18] 理论研究也提供了氢键的迹象。[19-22] 然而，最近，一些作者强烈质疑氢键在咪唑基离子液体中的存在。 离子液体，并声称不需要援引氢键来解释离子液体性质。[23-25]为此，我们启动了一个直接光谱观察氢键的计划，通过在一组精心挑选的咪唑基离子液体中连续增加氢键能力。研究和理解这些相互作用是一个真正的挑战，特别是对于 IL 而言。对于基于咪唑的离子液体，红外光谱中的氢键主要与中红外区域 CÀH 伸缩频率的偏移 (Δνs) 有关。然而，在远红外（FIR）光谱中观察氢键本身的拉伸（νσ）和弯曲（νβ）频率更有意义。[26-28]这些模式在方案1中显示和分配。
Ionic liquids (ILs) have many valuable applications in chemistry and technology.[1–6] They are understood as liquids consisting entirely of ions and having melting points below 1008C. The interesting properties of ionic liquids are governed by the type and strength of interaction between its constituents. It is assumed that hydrogen bonding plays an important role for the properties and reaction dynamics of these Coulomb systems. The presence of hydrogen bonding in the structure of 1-alkyl-3-methylimidazolium salt was first reported by Seddon et al. in 1986.[7] Since then, evidence for hydrogen bonding has been obtained from X-ray diffraction and mid-infrared and NMR spectroscopy. Local and directional interactions, such as hydrogen bonds, in imidazoliumbased ILs are indicated by shorter CÀH··· anion distances, redshifted CÀH frequencies, and downfield-shifted CÀH proton chemical shifts.[8–18] Indications of hydrogen bonding is also provided by theoretical studies.[19–22] Recently however, some authors have strongly challenged the presence of hydrogen bonds in ionic liquids, and claimed that hydrogen bonding need not be invoked for explaining IL properties.[23–25] For this reason, we initiated a program of direct spectroscopic observation of hydrogen bonds by successively increasing hydrogen bond abilities in a set of well-chosen imidazolium-based ionic liquids. Studying and understanding these interactions is a real challenge, and in particular for ILs. For imidazolium-based ILs, hydrogen bonding in infrared spectroscopy has been primarily concerned with the shift (Δνs) of the CÀH stretching frequency in the mid-infrared region. However, it is more pertinent to observe the stretching (νσ) and bending (νβ) frequencies of hydrogen bonds themselves in far-infrared (FIR) spectra.[26–28] These modes are shown and assigned in Scheme 1.