Castiglione Franca, Moreno Margherita, Raos Guido, Famulari Antonino, Mele Andrea, Appetecchi Giovanni Battista, Passerini Stefano
Dipartimento di Chimica, Materiali e Ingegneria Chimica "G. Natta", Politecnico di Milano, Via L. Mancinelli 7, 20131 Milano, Italy.
J Phys Chem B. 2009 Aug 6;113(31):10750-9. doi: 10.1021/jp811434e.
Room-temperature ionic liquids (RTILs) based on the N-butyl-N-methyl pyrrolidinium cation (PYR(14)(+)) combined with three different fluorinated anions have been prepared and characterized by NMR, conductivity, and rheological measurements. The anions are (trifluoromethanesulfonyl)(nonafluorobutanesulfonyl)imide (IM(14)(-)), bis(pentafluoroethanesulfonyl)imide (BETI(-)), and bis(trifluoromethanesulfonyl)imide (TFSI(-)). Intermolecular anion-cation nuclear Overhauser enhancements (NOEs) have been experimentally observed in all titled compounds. These findings indicate the formation of long-lived aggregates in the bulk liquids. The NOE patterns show marked selectivity and can be rationalized assuming that the perfluorinated moieties of the anions tend to adopt a preferential orientation with respect to the cations, with possible formation of mesoscopic fluorous domains. Self-diffusion coefficients D for the anion and the cation have been measured by DOSY NMR. Diffusion data show similar but not identical values for cation and anion, consistent with local ordering at the molecular level. The observed trend in diffusion coefficients, D(cation) > D(anion) for all compounds, is compatible with a higher degree of intermolecular organization of the anions. This nanoscale organization is connected to rather strong deviations of the experimental conductivities from those estimated from the ion diffusion coefficients through the Nernst-Einstein relationship. The measured viscosities and ion diffusion coefficients in PYR(14)IM(14) and in PYR(14)TFSI have similar temperature dependencies, leading to very close values of the activation energies for these processes. Ab initio density functional calculations on models of a PYR(14)TFSI ion pair lead to the identification of several local minima, whose structure and energy can be qualitatively related to the experimental NOE signals and activation energies.
基于N-丁基-N-甲基吡咯烷鎓阳离子（PYR(14)(+)）与三种不同的氟化阴离子组合而成的室温离子液体（RTILs）已被制备出来，并通过核磁共振（NMR）、电导率和流变学测量进行了表征。这些阴离子分别是（三氟甲磺酰基）（九氟丁磺酰基）亚胺（IM(14)(-)）、双（五氟乙磺酰基）亚胺（BETI(-)）和双（三氟甲磺酰基）亚胺（TFSI(-)）。在所有标题化合物中均通过实验观察到了分子间阴离子-阳离子核Overhauser增强效应（NOEs）。这些发现表明在本体液体中形成了长寿命聚集体。NOE模式显示出明显的选择性，并且可以通过假设阴离子的全氟部分倾向于相对于阳离子采取优先取向，可能形成介观氟相域来进行合理解释。已通过扩散有序核磁共振（DOSY NMR）测量了阴离子和阳离子的自扩散系数D。扩散数据显示阳离子和阴离子的值相似但不相同，这与分子水平上的局部有序性一致。对于所有化合物，观察到的扩散系数趋势为D(阳离子) > D(阴离子)，这与阴离子更高程度的分子间组织性相符。这种纳米级组织与实验电导率相对于通过能斯特-爱因斯坦关系从离子扩散系数估算出的值存在相当大的偏差有关。在PYR(14)IM(14)和PYR(14)TFSI中测量的粘度和离子扩散系数具有相似的温度依赖性，导致这些过程的活化能值非常接近。对PYR(14)TFSI离子对模型进行的从头算密度泛函计算导致识别出几个局部极小值，其结构和能量可以在定性上与实验NOE信号和活化能相关联。
