Abstract
The terahertz and infrared frequency vibration modes of room-temperature ionic liquids with imidazolium cations and halogen anions were extensively investigated. There is an intermolecular vibrational mode between the imidazolium ring of an imidazolium cation, a halogen atomic anion with a large absorption coefficient and a broad bandwidth in the low THz frequency region (13–130 cm−1), the intramolecular vibrational modes of the alkyl-chain part of an imidazolium cation with a relatively small absorption coefficient in the mid THz frequency region (130–500 cm−1), the intramolecular skeletal vibrational modes of an imidazolium ring affected by the interaction between the imidazolium ring, and a halogen anion with a relatively large absorption coefficient in a high THz frequency region (500–670 cm−1). Interesting spectroscopic features on the interaction between imidazolium cations and halogen anions was also obtained from spectroscopic studies at IR frequencies (550–3300 cm−1). As far as the frequency of the intermolecular vibrational mode is concerned, we found the significance of the reduced mass in determining the intermolecular vibration frequency.
Highlights
Ionic liquids, which are ambient-temperature molten salts, have been extensively studied because of their unique material properties such as a wide liquid range, extremely low vapor pressure, high electrical conductivity, excellent thermal stability, low combustibility, and excellence as a solvent in catalyzed reactions and the dissociation of biopolymers [1,2,3,4,5]
As far as the intermolecular vibrational mode found in the low THz frequency region is concerned, we found that the reduced mass (μ) calculated according to the mass of the methyl-imidazolium ring and the mass of the halogen anion, and the force constant (k) play an important role in determining the central frequency of the inter-molecular vibration
Since the absorption frequency does not depend on the alkyl-chain length of the imidazolium cation as shown in Figures 2–4 as long as the halogen anion is the same, we considered the effective mass of the cation for the intermolecular vibration to be the mass (m = 83) of the methyl-imidazolium ring part [mim+]
Summary
Ionic liquids, which are ambient-temperature molten salts, have been extensively studied because of their unique material properties such as a wide liquid range, extremely low vapor pressure, high electrical conductivity, excellent thermal stability, low combustibility, and excellence as a solvent in catalyzed reactions and the dissociation of biopolymers [1,2,3,4,5]. These properties are expected to have a wide range of applications [6,7,8]. We believe that a detailed spectroscopic characterization in the basic system will be useful benchmark with which to explore the interactions between an imidazolium cation and anionic molecule as more complex system as well as to consider the difference between these systems
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