Atom Substitution Effects in Ionic Liquids: A Microscopic View by Femtosecond Raman- Induced Kerr Effect Spectroscopy

It is well known that the physical and chemical properties of ionic liquids (ILs) vary when the combination of cations and anions changes. It is thus possible to tune the properties of ILs on demand by choosing a suitable cation and anion combination. Consequently, ILs are commonly referred to as designer fluids. Many studies have demonstrated the usefulness of this strategy for controlling the properties of ILs (Wasserscheid & Welton, 2008; Ohno, 2005). It is possible to systematically examine the physical and chemical properties of ILs by changing an ionic species, while leaving a counter ion unchanged, in order to understand the relationships between an IL’s chemical and physical properties and its constituent ion species. Extensive efforts along this line of study have clarified the relationships between the physical and chemical properties and the ionic species. On the other hand, the effects of replacing an atomic element in an ionic species on the physical and chemical properties of ILs have not been very well understood. It is obvious that one of the main themes in chemistry is to develop an understanding of how chemical and physical properties depend on the atomic elements in molecular liquids as well as in ILs. In 2005, C and Si in a side group of an imidazolium cation (1-methyl-3-neopentylimidazolium: [C-MIm]+ and 1-methyl-3-trimethylsilylmethylimidazolium: [Si-MIm]+) were compared (Shirota & Castner, 2005). The shear viscosities of the silicon substituted ILs are substantially reduced in comparison with those of the respective carbon ILs, when ILs with the same anions are compared. Actually, this feature of ILs is the opposite of that observed in conventional neutral molecular liquids. For example, the shear viscosity of bromobenzene at a standard ambient condition is approximately 1.4 times larger than that of chlorobenzene, and the ambient shear viscosity of diethyl sulfide is approximately 1.9 times larger than that of diethyl ether (Lide, 2008). Heavier atom substitution in the hexafluoropnictogenate anion ([XF6]-; X: P, As, and Sb) for 1-butyl-3-methylimidazolium ILs also gives a lower shear viscosity (Shirota et al., 2009). In addition to aromatic cation-based ILs, reductions in the shear viscosities of ILs with heavier atoms in the same position were also confirmed in nonaromatic cation-based ILs (Tsunashima & Sugiya, 2007; Seki et al., 2009; Shirota et al., 2010). Interionic interaction is a key parameter for determining the physical and chemical properties of ILs. Because the interionic interactions in ILs are delicately balanced between