Kakinuma Shohei, Shirota Hideaki
J Phys Chem B. 2019 Feb 14;123(6):1307-1323. doi: 10.1021/acs.jpcb.8b10269. Epub 2019 Feb 5.
We investigated the temperature dependence of the intermolecular vibrational dynamics of pyrrolidinium-based ionic liquids (ILs) with 10 different anion species using femtosecond Raman-induced Kerr effect spectroscopy. The features of the temperature-dependent vibrational spectra vary with the different anions. In the case of the ILs with spherical top anions, such as tetrafluoroborate and hexafluorophosphate, and trifluoromethanesulfonate, the spectral intensity in the low-frequency region below 50 cm increases with rising temperature, while that in the high-frequency region above 50 cm remains almost unchanged. Similar temperature-dependent features were also found in the bis(fluorosulfonyl)amide and bis(perfluoroalkylsulfonyl)amide salts. However, the difference spectra at respective temperature relative to 293 K indicate that the spectra of the bis(fluorosulfonyl)amide and bis(perfluoroalkylsulfonyl)amide salts are more temperature-sensitive in the low-frequency region below 50 cm compared to those of the tetrafluoroborate, hexafluorophosphate, and trifluoromethanesulfonate salts. The spectra of 1-butyl-1-methylpyrrolidinium-based ILs with dicyanamide and tricyanomethide anions show a characteristic temperature dependence; in addition to an increase of the spectral intensity in the low-frequency region below 50 cm, a red shift of the spectra in the high-frequency side above 50 cm was observed with increasing temperature. This implies that the librational motions of planar dicyanamide and tricyanomethide anions contribute substantially to the low-frequency spectra. We also compared the temperature-dependent low-frequency spectra of 1-butyl-1-methylpyrrolidinium- and 1-(2-methoxyethyl)-1-methylpyrrolidinium-based ILs with some anions. Although the spectral shapes are slightly different in the range of 70-150 cm, which can be attributed to the intramolecular vibrational modes of the cations, the temperature dependence of the spectral shapes is quite similar, indicating that the ether substitution in the cation side groups has little effects on the temperature dependence of the low-frequency spectra. The fragilities of the ILs were also estimated from the temperature-dependent viscosities and the glass-transition temperatures. The fragility parameter seems to be correlated with the temperature dependence of the first moment of the low-frequency spectral bands mainly arising from the intermolecular vibrations of the ILs.
我们使用飞秒拉曼诱导克尔效应光谱研究了具有10种不同阴离子种类的吡咯烷鎓基离子液体（ILs）分子间振动动力学的温度依赖性。随温度变化的振动光谱特征因不同阴离子而异。对于具有球形顶部阴离子的离子液体，如四氟硼酸盐、六氟磷酸盐和三氟甲磺酸盐，低于50 cm的低频区域的光谱强度随温度升高而增加，而高于50 cm的高频区域的光谱强度几乎保持不变。在双（氟磺酰）酰胺盐和双（全氟烷基磺酰）酰胺盐中也发现了类似的温度依赖性特征。然而，相对于293 K的各温度下的差谱表明，与四氟硼酸盐、六氟磷酸盐和三氟甲磺酸盐相比，双（氟磺酰）酰胺盐和双（全氟烷基磺酰）酰胺盐在低于50 cm的低频区域的光谱对温度更敏感。具有双氰胺和三氰甲基化物阴离子的1-丁基-1-甲基吡咯烷鎓基离子液体的光谱呈现出特征性的温度依赖性；除了低于50 cm的低频区域的光谱强度增加外，还观察到高于50 cm的高频侧光谱随温度升高发生红移。这意味着平面双氰胺和三氰甲基化物阴离子的摆动运动对低频光谱有很大贡献。我们还比较了具有某些阴离子的1-丁基-1-甲基吡咯烷鎓基和1-（2-甲氧基乙基）-1-甲基吡咯烷鎓基离子液体的温度依赖性低频光谱。尽管在70 - 150 cm范围内光谱形状略有不同，这可归因于阳离子的分子内振动模式，但光谱形状的温度依赖性非常相似，表明阳离子侧基中的醚取代对低频光谱的温度依赖性影响很小。还从温度依赖性粘度和玻璃化转变温度估计了离子液体的脆性。脆性参数似乎与主要源于离子液体分子间振动的低频光谱带第一矩的温度依赖性相关。
