Terahertz time-domain spectroscopy of imidazolium ionic liquids.
Terahertz time-domain spectroscopy of imidazolium ionic liquids.
复制标题
DOI：
10.1021/jp067171w
复制
发表时间：
2007-04
期刊：
影响因子：
--
通讯作者：
Kohji Yamamoto;M. Tani;M. Hangyo
中科院分区：
文献类型：
--
作者：
Kohji Yamamoto;M. Tani;M. Hangyo
我们采用太赫兹时域光谱（THz-TDS）技术，在5（0.15 THz）至140 cm（-1）（4.2 THz）频率范围内测量了咪唑离子液体的太赫兹（THz）复介电谱。所研究的离子液体为1-乙基-3-甲基咪唑（EMIm+）/三氟甲磺酸盐（TfO-）、EMIm+/四氟硼酸盐（BF（4）-）、1-丁基-3-甲基咪唑（BMIm+）/TfO-和BMIm+/BF（4）-。离子液体在太赫兹波段的介电值与短链醇类的介电值相似。太赫兹介电值与亚皮秒到皮秒动力学有关。在经验极性ET（30）中也观察到了同样的趋势，尽管它与极性和氢键能力的静态特性有关。两种类型的液体之间的差异，观察在太赫兹介电光谱形状：离子液体显示结构化的线形，但短链醇显示少得多的结构。离子液体的结构化线形反映了分子内振动和/或内振动的低频运动。当不同咪唑阳离子组成的离子液体含有相同的阴离子作为抗衡离子时，它们在20 cm（-1）以上的密度归一化THz介电谱在形状和大小上具有很强的相似性。这清楚地表明，太赫兹光谱不是起源于咪唑阳离子的分子内振动。除TfO-的CF 3-SO 3挠率外，所有阴离子的分子内振动都位于140 cm（-1）以上，不能单独解释离子液体的宽THz介电谱.因此，我们可以得出结论，而不是分子内振动的固有振动的太赫兹介电谱占主导地位。结果表明，即使在液相中，离子液体也具有类似于其固相结构的局部结构。
We have measured the terahertz (THz) complex dielectric spectra of imidazolium ionic liquids by THz time-domain spectroscopy (THz-TDS) in the frequency range from 5 (0.15 THz) to 140 cm(-1) (4.2 THz). The ionic liquids investigated are 1-ethyl-3-methylimidazolium (EMIm+)/trifluoromethanesulfonate (TfO-), EMIm+/tetrafluoroborate (BF(4)-), 1-butyl-3-methylimidazolium (BMIm+)/TfO-, and BMIm+/BF(4)-. The dielectric values of the ionic liquids in the THz region are similar to those of short-chain alcohols. The THz dielectric values are related to subpicosecond-to-picosecond dynamics. The same trend has been observed in the empirical polarity ET(30) although it is related to the static characteristics of polarity and hydrogen bonding ability. A difference between the two types of liquids is observed in the THz dielectric spectral shapes: the ionic liquids show structured lineshapes but short-chain alcohols show much less structured ones. The structured lineshapes of the ionic liquids reflect the low-frequency motions of interion and/or intramolecular vibrations. When the ionic liquids composed of the different imidazolium cations contain the same anions as counterions, their density-normalized THz dielectric spectra above 20 cm(-1) bear strong resemblance to each other in shape and magnitude. It shows clearly that the THz spectra do not originate from the intramolecular vibrations of the imodazolium cations. All of the intramolecular vibrations of the anions are located above 140 cm(-1) except the CF3-SO3 torsion of TfO-, the band of which alone cannot explain the broad THz dielectric spectra of the ionic liquids. Therefore, we conclude that the interion vibrations rather than the intramolecular vibrations dominantly contribute to the THz dielectric spectra. The results strongly indicate that even in the liquid phase the ionic liquids have local structures similar to their solid-phase structures.