Unusual Li+ Ion Solvation Structure in Bis(fluorosulfonyl)amide Based Ionic Liquid
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- 作者：Kenta Fujii ; Hiroshi Hamano ; Hiroyuki Doi ; Xuedan Song ; Seiji Tsuzuki ; Kikuko Hayamizu ; Shiro Seki ; Yasuo Kameda ; Kaoru Dokko ; Masayoshi Watanabe ; Yasuhiro Umebayashi
- 刊名：Journal of Physical Chemistry C
- 出版年：2013
- 出版时间：September 26, 2013
- 年：2013
- 卷：117
- 期：38
- 页码：19314-19324
- 全文大小：433K
- 年卷期：v.117,no.38(September 26, 2013)
- ISSN：1932-7455
文摘
Raman spectra of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide [C2mIm+][FSA鈥?/sup>] ionic liquid solutions dissolving LiFSA salt of various concentrations were measured at 298 K. FSA鈥?/sup> ((FSO2)2N鈥?/sup>) is an analogue anion of bis(trifluoromethanesulfonyl)amide ((CF3SO2)2N鈥?/sup>; TFSA鈥?/sup>). We found that a solvation number of the Li+ ion in [C2mIm+][FSA鈥?/sup>] is 3, though it has been well established that Li+ ion is solvated by two TFSA鈥?/sup> anions in the corresponding ionic liquids below the Li+ ion mole fraction of xLi+ < 0.2. To yield further insight into larger solvation numbers, Raman spectra were measured at higher temperatures up to 364 K. The Li+ ion solvation number in [C2mIm+][FSA鈥?/sup>] evidently decreased when the temperature was elevated. Temperature dependence of the Li+ ion solvation number was analyzed assuming an equilibrium between [Li(FSA)2]鈭?/sup> and [Li(FSA)3]2鈥?/sup>, and the enthalpy 螖H掳 and the temperature multiplied entropy T螖S掳 for one FSA鈥?/sup> liberation toward a bulk ionic liquid were successfully evaluated to be 35(2) kJ mol鈥? and 29(2) kJ mol鈥?, respectively. The 螖H掳 and 螖S掳 suggest that the Li+ ion is coordinated by one of bidentate and two of monodentate FSA鈥?/sup> at 298 K, and that the more weakly solvated monodentate FSA鈥?/sup> is liberated at higher temperatures. The high-energy X-ray diffraction (HEXRD) experiments of these systems were carried out and were analyzed with the aid of molecular dynamics (MD) simulations. In radial distribution functions evaluated with HEXRD, a peak at about 1.94 脜 appeared and was attributable to the Li+鈥揙(FSA鈥?/sup>) correlations. The longer Li+鈥揙(FSA鈥?/sup>) distance than that for the Li+鈥揙(TFSA鈥?/sup>) of 1.86 脜 strongly supports the larger solvation number of the Li+ ions in the FSA鈥?/sup> based ionic liquids. MD simulations at least qualitatively reproduced the Raman and HEXRD experiments.