低温下离子液体[BMIM][TFSI]的拉曼光谱

Raman Spectroscopy of Ionic Liquid [BMIM][TFSI] at Low Temperature

结合差示扫描量热技术和变温拉曼光谱,研究了咪唑类离子液体1-丁基-3-甲基咪唑双三氟甲磺酰亚胺盐([BMIM][TFSI])的相变和分子构象随温度的变化。研究结果表明:样品从常温降至190 K过程中,一直保持液态,继续降温至184 K左右出现玻璃化转变。升温过程中,在240 K左右出现放热峰,同时有新的拉曼峰出现,表明[BMIM][TFSI]由无序结构转变为有序的结晶相。当温度升高到270 K左右时,[BMIM][TFSI]熔化回到初始液态,说明[BMIM][TFSI]在温度变化过程中的相变是可逆的。通过对[BMIM][TFSI]分子构象的拉曼特征峰进行定量分析可知:在常温液态时反式和顺式两种构象共存,但随着温度降低,反式构象的比例增加。升温时,伴随240 K时结晶态的转变,反式构象消失,表明通过冷结晶得到的晶体仅有顺式构象。

The temperature-induced phase transition and molecular conformation of the imidazole ionic liquid 1-butyl-3-methylimidazolium bis{( trifluoromethyl) sulfonyl} imide( [BMIM][TFSI]) was studied by differential scanning calorimetry( DSC) and raman spectroscopy. The results show that the sample remains liquid state when cooling down from room tempenture to 190 K. Fuether cooling to 184 K,the super-cooling liquid sample transforms into the glassy state. During the warming process,the exothermic peak appears at about 240 K,and new Raman peaks appear,indicating that [BMIM][TFSI] changes from a disordered structure to an ordered crystalline phase. When the temperature rises to about 270 K,[BMIM][TFSI]melts back to the initial liquid state,indicating that the process is reversible. The quantitative analysis of the Raman peaks of the molecular conformation shows that both the trans and cis conformations coexist at room temperature,but the proportion of the trans conformation increases as the temperature decreases. With increasing temperature,the trans conformation disappears with the transformation of the crystalline state at 240 K,indicating that the crystal obtained by cold crystallization has only cis conformation.