Conformational Isomerism Influence on the Properties of Piperazinium Bis(trifluoromethylsulfonyl)imide

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• 作者：Xiaoxing Lu ; Qi Cao ; Xi Wu ; Hujun Xie ; Qunfang Lei ; Wenjun Fang
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：July 31, 2014
• 年：2014
• 卷：118
• 期：30
• 页码：9085-9095
• 全文大小：571K
• ISSN：1520-5207

文摘

Investigation of conformational isomerism of ring compounds can help us get a clear comprehension of the ring structure and reveal significant structure鈥揳ctivity relationship. In this study, conformational isomerism of the cationic moiety of ionic liquid 1-ethyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)imide ([C₂C₁C₁⁴pi][NTf₂]) has been investigated by means of ¹H nuclear magnetic resonance spectra. The energy levels for different conformations of the cationic moiety [C₂C₁C₁⁴pi]⁺ are obtained via density functional theory calculations. The predominant cis-conformer in [C₂C₁C₁⁴pi][NTf₂] at its liquid state is observed under ambient conditions, where the ethyl group locates at the equatorial position of quaternary nitrogen atom, consistent with the calculated results. The trans-conformer minorities in the IL convert to the cis-conformers when [C₂C₁C₁⁴pi][NTf₂] is well crystallized. Besides, the addition of polar solvents, such as ethanol, leads to a convenient and complete transformation from the trans-form to the recognizable cis-form. The phase-transition behaviors have been measured by means of differential scanning microcalorimetry (DSC), and the DSC results can be highly affected by the initial state of the IL. Density and viscosity measurements for mixtures of [C₂C₁C₁⁴pi][NTf₂] with ethanol or 1-propanol at different temperatures T = (293.15 to 323.15) K are performed. Conformational isomerism affects the excess molar volumes of [C₂C₁C₁⁴pi][NTf₂] + alcohol systems more significantly than the viscometric property. The behaviors, as comparison, for the mixtures of 1-n-pentyl-1,4-dimethyl-piperazinium bis(trifluoromethylsulfonyl)imide ([C₅C₁C₁⁴pi][NTf₂]) with ethanol are observed with the same phenomena as the common binary systems. On the basis of the experimental and calculated results of the ILs, it can be concluded that conformational isomerism in the cation of [C₂C₁C₁⁴pi][NTf₂] is quite significant, and it should be taken into account when sensitive properties are evaluated.