Migliorati Valentina, Serva Alessandra, Aquilanti Giuliana, Pascarelli Sakura, D'Angelo Paola
Dipartimento di Chimica, Università di Roma "La Sapienza", P.le A. Moro 5, 00185 Roma, Italy.
Phys Chem Chem Phys. 2015 Jul 7;17(25):16443-53. doi: 10.1039/c5cp01613d. Epub 2015 Jun 5.
A thorough characterization of the structural properties of alkylimidazolium halide ionic liquids (ILs), namely 1-alkyl-3-methylimidazolium bromide ([Cnmim]Br with n = 5, 6, 8, 10) and iodide ([C6mim]I), has been carried out by combining molecular dynamics simulations and EXAFS spectroscopy. The existence of a local order in [Cnmim]Br ILs has been evidenced, with anions and imidazolium head groups forming a local three-dimensional bonding pattern that is common to all the [Cnmim]Br IL family, regardless of the length of the alkyl chain attached to the cation. On the other hand, upon alkyl chain elongation significant differences have been highlighted in the long-range structure of these ILs. Theoretical X-ray structure factors have been calculated from MD simulations and a low q peak has been found for all [Cnmim]Br ILs, indicating the existence of long-range structural correlations. The low q peak moves to smaller q values corresponding to longer distances, increases in intensity and sharpens with increasing alkyl chain length on the cation. Similarities and differences between the ion three-dimensional arrangements in [C6mim]Br and [C6mim]I were highlighted and the structural arrangement of Br(-) and I(-) was found to be different in the proximity of the most acidic hydrogen atom of the imidazolium ring: the I(-) ion is preferentially located above and below the ring plane, while the Br(-) ion has a high probability also to be coplanar with the imidazolium ring. A quantitative analysis of the Br and I K-edge EXAFS spectra of alkylimidazolium halide ILs has been carried out based on the microscopic description of the systems derived from MD simulations. A very good agreement between theoretical and experimental EXAFS signals has been obtained, allowing us to assess the reliability of the MD structural results for all the alkylimidazolium halide ILs investigated in this work.
通过结合分子动力学模拟和扩展X射线吸收精细结构（EXAFS）光谱，对卤化烷基咪唑鎓离子液体（ILs）的结构性质进行了全面表征，这些离子液体包括1-烷基-3-甲基咪唑溴盐（[Cnmim]Br，n = 5、6、8、10）和碘盐（[C6mim]I）。已证明[Cnmim]Br离子液体中存在局部有序结构，阴离子和咪唑鎓头基形成了局部三维键合模式，这是所有[Cnmim]Br离子液体家族共有的，与连接到阳离子上的烷基链长度无关。另一方面，随着烷基链的延长，这些离子液体的长程结构出现了显著差异。从分子动力学模拟计算出了理论X射线结构因子，并且在所有[Cnmim]Br离子液体中都发现了一个低q峰，表明存在长程结构相关性。该低q峰向对应于更长距离更小的q值移动，强度增加，并随着阳离子上烷基链长度的增加而锐化。突出了[C6mim]Br和[C6mim]I中离子三维排列的异同，并且发现在咪唑环最酸性氢原子附近Br(-)和I(-)的结构排列不同：I(-)离子优先位于环平面上方和下方，而Br(-)离子也有很大概率与咪唑环共面。基于分子动力学模拟得出的系统微观描述，对卤化烷基咪唑鎓离子液体的Br和I K边EXAFS光谱进行了定量分析。理论和实验EXAFS信号之间取得了很好的一致性，这使我们能够评估本工作中研究的所有卤化烷基咪唑鎓离子液体分子动力学结构结果的可靠性。
