Umebayashi Yasuhiro, Yamaguchi Taishi, Fukuda Shuhei, Mitsugi Takushi, Takeuchi Munetaka, Fujii Kenta, Ishiguro Shin-ichi
Department of Chemistry, Faculty of Science, Kyushu University, Higashi, Fukuoka 812-8581, Japan.
Anal Sci. 2008 Oct;24(10):1297-304. doi: 10.2116/analsci.24.1297.
The Raman spectra for 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [BMI][TFSA] containing alkaline metal salts of TFSA(-), MTFSA (M = Li, Na, K and Cs), were recorded in the frequency range of 200-1800 cm(-1), with varying salt concentrations at 298 K. With Li(+) and Na(+) ions, at the frequency range of 730-760 cm(-1), new Raman bands ascribable to the anion bound to the ions appeared at higher frequency relative to that found in the neat ionic liquid. On the other hand, with K(+) and Cs(+) ions, single Raman bands were solely observed. According to the difference Raman spectra for the ionic liquids containing K(+) and Cs(+), evaluated by subtracting Raman spectra for the neat ionic liquid, it turned out that two-state approximation, i.e., bulk TFSA(-) and TFSA(-) bound to K(+) and Cs(+) ions, could hold, as Li(+) and Na(+) ions. By careful analyses of Raman band intensity arising from bulk TFSA(-) as a function of the salt concentration, the solvation numbers for the respective ions were successfully evaluated to be 1.95 for Li(+), 2.88 for Na(+), 3.2 for K(+) and 3.9 for Cs(+), respectively. By taking into account that TFSA(-) acts as a bidentate ligand, the atomic coordination numbers are proposed to be 4, 6, 6 and 8 for Li(+), Na(+), K(+) and Cs(+), respectively. Raman shifts for the TFSA(-) bound to the metal ions relative to that of the bulk TFSA(-) were plotted against the ionic radii for the solvated alkaline metal ions estimated via Shannon's ionic radii, to yield a straight line with a slope of almost unity, suggesting that the electrostatic interaction predominantly operates in the ion-ion interaction between the alkaline metal ions and TFSA(-), as expected. Moreover, the Raman spectra in the frequency range of 370-450 cm(-1) strongly depend on the alkaline metal ions, indicating that cis TFSA(-) is favored in the first solvation sphere of the Li(+) ion of a relatively small ionic radius, and that such a preferred conformational isomerism of TFSA(-) diminishes with an increase of the ionic radii of the central metal ions.
记录了含有TFSA(-)的碱金属盐MTFSA（M = Li、Na、K和Cs）的1-丁基-3-甲基咪唑双（三氟甲磺酰）亚胺[BMI][TFSA]在298K下、盐浓度不同时200 - 1800cm(-1)频率范围内的拉曼光谱。对于Li(+)和Na(+)离子，在730 - 760cm(-1)频率范围内，相对于纯离子液体中发现的频率，出现了可归因于与离子结合的阴离子的新拉曼带。另一方面，对于K(+)和Cs(+)离子，仅观察到单一拉曼带。通过减去纯离子液体的拉曼光谱评估含K(+)和Cs(+)离子的离子液体的差分拉曼光谱，结果表明，与Li(+)和Na(+)离子一样，双态近似，即本体TFSA(-)以及与K(+)和Cs(+)离子结合的TFSA(-)成立。通过仔细分析本体TFSA(-)产生的拉曼带强度随盐浓度的变化，成功评估出各离子的溶剂化数分别为Li(+)为1.95、Na(+)为2.88、K(+)为3.2、Cs(+)为3.9。考虑到TFSA(-)作为双齿配体，提出Li(+)、Na(+)、K(+)和Cs(+)的原子配位数分别为4、6、6和8。将与金属离子结合的TFSA(-)相对于本体TFSA(-)的拉曼位移与通过香农离子半径估算的溶剂化碱金属离子的离子半径作图，得到一条斜率几乎为1的直线，这表明如预期的那样，静电相互作用在碱金属离子与TFSA(-)之间的离子-离子相互作用中起主要作用。此外，370 - 450cm(-1)频率范围内的拉曼光谱强烈依赖于碱金属离子，表明在离子半径相对较小的Li(+)离子的第一溶剂化层中，顺式TFSA(-)更受青睐，并且随着中心金属离子离子半径的增加，TFSA(-)这种优选的构象异构体减少。
