Department of Materials Science and Engineering, 122 S. Central Campus Drive, Rm. 304, University of Utah, Salt Lake City, UT 84112, USA.
Phys Chem Chem Phys. 2010 Jul 14;12(26):7064-76. doi: 10.1039/c001387k. Epub 2010 May 19.
Molecular dynamics simulations of fluoroalkyl-derivatized imidazolium:bis(trifluoromethylsulfonyl)imide (TFSI) room temperature ionic liquids (FADI-RTILs) with cations of the structure 1-F(CF(2))(n)(CH(2))(2)-3-methyl imidazolium have been performed and compared with simulations of alkyl-derivatized 1-H(CH(2))(n+2)-3-methyl imidazolium analogs (ADI-RTILs). Simulations yield RTIL densities, viscosities and ionic conductivities for the FADI-RTILs and ADI-RTILs in reasonably good agreement with experimental data. Partial fluorination results in a larger increase in density than would be anticipated based upon the density difference between perfluoralkane and alkane melts. Similarly, the slowing down in dynamics upon partial fluorination is greater than would be expected based upon the increase in cation volume. Examination of cation-cation, anion-anion and cation-anion centers-of-mass radial distribution functions reveal remarkably little influence of partial fluorination on the spherically averaged intermolecular structure of the RTILs. Similarly, simulations reveal little change in tail conformations and the extent of tail-tail aggregation upon partial fluorination. The interaction of the TFSI anion with the positively charged imidazolium ring hydrogen and nitrogen atoms is also little influenced by partial fluorination. However, the partially fluorinated alkyl tail exhibits increased interaction with the TFSI anion due to the electron withdrawing character of the fluorinated groups. We believe this strong tail-anion electrostatic interaction largely accounts for the higher than expected density and slower than expected dynamics in the FADI-RTILs.
双（三氟甲基磺酰基）亚胺（TFSI）室温离子液体（FADI-RTIL）进行了分子动力学模拟，其阳离子结构为 1-F(CF(2))(n)(CH(2))(2)-3-甲基咪唑鎓，并与烷基取代的 1-H(CH(2))(n+2)-3-甲基咪唑鎓类似物（ADI-RTIL）的模拟进行了比较。模拟结果表明，FADI-RTIL 和 ADI-RTIL 的 RTIL 密度、粘度和离子电导率与实验数据相当吻合。部分氟化导致密度的增加大于基于全氟烷烃和烷烃熔体之间的密度差所预期的增加。类似地，部分氟化导致动力学减慢的程度大于基于阳离子体积增加所预期的程度。考察阳离子-阳离子、阴离子-阴离子和阳离子-阴离子质心径向分布函数，发现部分氟化对 RTIL 的球形平均分子间结构几乎没有影响。类似地，模拟结果表明，部分氟化对尾部构象和尾部聚集程度的变化很小。TFSI 阴离子与带正电荷的咪唑鎓环氢和氮原子的相互作用也几乎不受部分氟化的影响。然而，部分氟化的烷基尾部与 TFSI 阴离子的相互作用增强，这是由于氟化基团的吸电子特性所致。我们认为这种强的尾部-阴离子静电相互作用在很大程度上解释了 FADI-RTIL 中高于预期的密度和低于预期的动力学。
