J. Phys. Chem. B 2008, 112, 4735-4740
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Structures of Ionic Liquids with Different Anions Studied by Infrared Vibration Spectroscopy Yoonnam Jeon,† Jaeho Sung,† Choongwon Seo,† Hyunjin Lim,† Hyeonsik Cheong,† Minhyuck Kang,‡ Bongjin Moon,‡ Yukio Ouchi,§ and Doseok Kim*,† Department of Physics and Interdisciplinary Program of Integrated Biotechnology and Department of Chemistry, Sogang UniVersity, Seoul 121-742, Korea, and Department of Chemistry, Graduate School of Science, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8602, Japan ReceiVed: December 26, 2007
We investigated the structures of ionic liquids (1-butyl-3-methylimidazolium iodide [BMIM][I] and 1-butyl3-methylimidazolium tetrafluoroborate [BMIM][BF4]) and their aqueous mixtures using attenuated total reflection (ATR) infrared absorption and Raman spectroscopy. The ATR spectrum in the CHx (x ) 1, 2, 3) vibration region from 2800 to 3200 cm-1 was very different between [BMIM][BF4] and [BMIM][I] even though all the spectral features in this region were from the butyl chain and the imidazolium ring of the same cation. The spectrum did not change appreciably irrespective of the water concentration for [BMIM][BF4], whereas the spectrum from [BMIM][I] showed significant changes as the water concentration was increased, especially in CH-vibration modes from the imidazolium ring. For very diluted solutions both aqueous mixtures of [BMIM][I] and [BMIM][BF4] showed very similar spectra. Mixing of [BMIM][I] with heavy water (D2O) facilitated the isotopic exchange of the proton attached to the most acidic carbon of the imidazolium ring into deuterium from D2O, whereas even prolonged exposure to D2O did not induce any isotopic exchange for [BMIM][BF4]. Raman spectra around 600 cm-1 indicative of the butyl chain conformation also changed differently as the water concentration was increased between [BMIM][I] and [BMIM][BF4]. These differences are considered to come from the variation in the position of the anion, where I- is expected to be closer to the C(2) hydrogen of the imidazolium cation and interacting more specifically as compared to BF4-.
Introduction Room-temperature ionic liquids (RILs) are receiving much interest owing to their characteristics as environmentally friendly solvents for a range of chemical processes or as possible constituents in electrochemical applications.1-3 Thus molecular design, synthesis, and characterization of RILs have been the focus of many recent scientific investigations.4-6 However, one of the barriers in the application of RILs is the general lack of physical property data for these compounds toward fundamental understanding of the system. For example, the relation between the chemical constituents of ionic liquids and the resulting structure has not been studied much as yet. The structure of RILs became the topic of active research recently, and numerous experimental and theoretical techniques have been utilized to unravel their structures.7-10 From the Raman spectroscopic study of 1-butyl-3-methyl imidazolium chloride ([BMIM][Cl]), Hayashi et al. found that two different local conformations of bulk ionic liquid can coexist in the crystal, which would give a clue to understand why RILs exist as liquid at room temperature.11 This report was supported by X-ray studies which found two local structures can coexist in RILs.12,13 Calculations that followed suggested that the above cation conformation change would alter the position of the anion and change the interaction between the anion and the cation in * Corresponding author. † Department of Physics and Interdisciplinary Program of Integrated Biotechnology, Sogang University. ‡ Department of Chemistry, Sogang University. § Nagoya University.
1-alkyl-3-methylimidazolium halide ionic liquids.14-16 An important issue in the structure of an ionic liquid is the relative position between the anion and cation comprising the bulk liquid. In the study of a mixture of methyl-ethyl imidazolium chloride ([MEI][Cl]) and AlCl3 molten salt, Dieter et al. first proposed that Cl- interacts preferentially with the hydrogen attached to C(2), C(4), and C(5) of the imidazolium ring.17 Following reports that investigated various imidazolium ionic liquids suggested that the structure changes depending on the type of anions such as in BF4- and Br-.4,16,18-23 By contrast, there have been several simulation studies and experiments which proposed that the difference in the relative position is very small for different anion types.9,15,24 Thus, even for prototypical imidazolium ionic liquid, this issue is still unresolved and awaits further investigation, especially as the relative position between the cation and the anion is one of the most important structural information. In this report, we investigated the structures of 1-butyl-3methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1-butyl3-methylimidazolium iodide ([BMIM][I]), and their aqueous mixtures using attenuated total reflection infrared (ATR-IR) and Raman spectroscopy. The infrared spectra from [BMIM]+ were very different between these two ionic liquids and changed differently with the increase in the water concentration. The rate of isotopic substitution of the hydrogen attached to the imidazolium core was also very different. Raman spectra from the butyl chain of the cation were also distinct between these two RILs. The difference in the relative position of the anion with respect to imidazolium cation was proposed to account for the observed differences.
10.1021/jp7120752 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008
4736 J. Phys. Chem. B, Vol. 112, No. 15, 2008
Jeon et al.
Figure 1. Chemical structure of the 1-butyl-3-methyl imidazolium cation [BMIM]+.
Figure 2. ATR-IR spectra from [BMIM][BF4] (solid line) and [BMIM][I] (dashed line).
Experimental Section The tetrafluoroborate salt of 1-butyl-3-methyl imidazolium ([BMIM][BF4], purity better than 99 wt %, water content
