Department of Nanomaterial Science, Chiba University, 1-33 Yayoi, Inage-ku, Chiba 263-8522, Japan.
Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973-5000, USA.
J Chem Phys. 2018 May 21;148(19):193805. doi: 10.1063/1.5010066.
In this study, we investigate the temperature dependence of low-frequency spectra in the frequency range of 0.3-200 cm for ionic liquids (ILs) whose cations possess two systematically different cyclic groups, using femtosecond Raman-induced Kerr effect spectroscopy. The target ILs are bis(trifluoromethylsulfonyl)amide [NTf] salts of 1-cyclohexylmethyl-1-methylpyrrolidinium [CHxmMPyrr], 1-cyclohexylmethyl-3-methylimidazolium [CHxmMIm], -cyclohexylmethylpyridinium [CHxmPy], 1-benzyl-1-methylpyrrolidinium [BzMPyrr], 1-benzyl-3-methylimidazolium [BzMIm], and -benzylpyridinium [BzPy] cations. The aim of this study is to better understand the effects of aromaticity in the cations' constituent groups on the temperature-dependent low-frequency spectral features of the ILs. The low-frequency spectra of these ILs are temperature dependent, but the temperature-dependent spectrum of [CHxmMPyrr][NTf] is different from that of other ILs. While [CHxmMPyrr][NTf] shows spectral changes with temperature in the low-frequency region below 50 cm, the other ILs also show spectral changes in the high-frequency region above 80 cm (above 50 cm in the case of [BzMPyrr][NTf]). We conclude that the spectral change in the low-frequency region is due to both the cation and anion, while the change in the high-frequency region is attributed to the red shift of the aromatic ring librations. On the basis of the plots of the first moment of the spectra vs. temperature, we found that the first moment of the low-frequency spectrum of the IL whose cation does not have an aromatic ring is less temperature dependent than that of the other ILs. However, the intrinsic first moment, the first moment at 0 K, of the low-frequency spectrum is governed by the absence or presence of a charged aromatic group, while a neutral aromatic group does not have much influence on determining the intrinsic first moment.
在这项研究中，我们使用飞秒拉曼诱导克尔效应光谱法研究了具有两个系统不同环状基团的阳离子的离子液体（ILs）在 0.3-200 cm 频率范围内的低频谱随温度的变化。目标 ILs 是双（三氟甲烷磺酰基）酰胺 [NTf]盐，由 1-环己基甲基-1-甲基吡咯烷鎓 [CHxmMPyrr]、1-环己基甲基-3-甲基咪唑鎓 [CHxmMIm]、-环己基甲基吡啶鎓 [CHxmPy]、1-苄基-1-甲基吡咯烷鎓 [BzMPyrr]、1-苄基-3-甲基咪唑鎓 [BzMIm]和 -苄基吡啶鎓 [BzPy]阳离子组成。本研究的目的是更好地了解阳离子组成基团的芳香性对 ILs 随温度变化的低频光谱特征的影响。这些 ILs 的低频光谱随温度变化，但 [CHxmMPyrr][NTf] 的温度依赖性光谱与其他 ILs 不同。虽然 [CHxmMPyrr][NTf] 在低于 50 cm 的低频区显示出随温度变化的光谱变化，但其他 ILs 也在高于 80 cm 的高频区显示出光谱变化（对于 [BzMPyrr][NTf]，高于 50 cm）。我们得出的结论是，低频区的光谱变化是由阳离子和阴离子共同引起的，而高频区的变化则归因于芳香环内振的红移。根据光谱的第一矩随温度的变化图，我们发现没有芳香环的 IL 的低频光谱的第一矩对温度的依赖性小于其他 ILs。然而，低频光谱的固有第一矩，即 0 K 时的第一矩，由带电荷的芳香族基团的存在与否决定，而中性芳香族基团对确定固有第一矩没有太大影响。
