Performance of quantum chemically derived charges and persistence of ion cages in ionic liquids. A molecular dynamics simulations study of 1-n-butyl-3-methylimidazolium bromide.
Performance of quantum chemically derived charges and persistence of ion cages in ionic liquids. A molecular dynamics simulations study of 1-n-butyl-3-methylimidazolium bromide.
复制标题
DOI：
10.1021/jp109612k
复制
发表时间：
2011-02
期刊：
影响因子：
--
通讯作者：
Miriam Kohagen;Martin Brehm;J. Thar;Wei Zhao;F. Müller-Plathe;B. Kirchner
中科院分区：
文献类型：
--
作者：
Miriam Kohagen;Martin Brehm;J. Thar;Wei Zhao;F. Müller-Plathe;B. Kirchner
我们使用标准电荷集和两个基于量子化学的电荷集进行经典分子动力学模拟，以研究离子液体 1-n-丁基-3-甲基溴化咪唑鎓，[C(4)C(1)im][Br]。我们将阳离子分成不同的电荷组，发现咪唑鎓环中的总电荷和电荷分布在三个系统中完全不同，而丁基链的总电荷在这两种方法之间守恒得多。为了进行比较，计算了空间分布函数和径向分布函数以及不同的时间相关函数。对于结构特性，我们在标准和两个基于量子化学的集合之一之间获得了良好的一致性，而第二个基于量子化学的集合的结果导致了完全不同的情况。对于动态特性观察到相反的情况，其在标准组和第二基于量子化学的组之间非常一致，而通过量子化学计算获得的第一电荷组中的动力学进行得太慢，这从总电荷中并不明显。我们观察到，丁基链的结构基本上不受电荷组选择的影响。这是分离成离子部分和非极性区域的间接证明。本文的第二个重点是动态异质性和离子笼的研究。因此，我们分析了三个系统以及具有标准电荷集的系统在五个不同温度下的重新取向动力学。一般来说，我们检测到四个不同的时域。连续氢键和最近邻离子对动力学可以找到最快的运动。在第二时域中，丁基链发生运动。第三时域包括咪唑鎓环的运动增加以及离子笼的连续变形，即几个反离子之一离开中心离子的第一壳层，以及间歇性氢键动力学。剩余的域涉及离子的平移位移。
We carried out classical molecular dynamics simulations with a standard and two quantum chemistry based charge sets to study the ionic liquid 1-n-butyl-3-methylimidazolium bromide, [C(4)C(1)im][Br]. We split the cation up into different charge groups and found that the total charge and the charge distribution in the imidazolium ring are completely different in the three systems while the total charge of the butyl chain is much better conserved between the methods. For comparison, the spatial distribution functions and the radial distribution functions as well as different time correlation functions were calculated. For the structural properties we obtained a good agreement between the standard and one of the two quantum chemistry based sets, while the results from the second quantum chemistry based set led to a completely different picture. The opposite was observed for the dynamic properties, which agree well between the standard set and the second quantum chemistry based set, whereas the dynamics in the first charge set obtained by quantum chemistry calculations proceeded much too slow, which is not obvious from the total charge. We observed, that the structure of the butyl chain is mostly unaffected by the choice of the charge set. This is an indirect proof for separation into ionic parts and nonpolar domains. A second focus of the article is the investigation of dynamical heterogeneity and the ion cages. Therefore, we analyzed the reorientational dynamics in the three systems and at five different temperatures in system with the standard charge set. Generally speaking, we detected four different time domains. The fastest movement can be found for the continuous hydrogen bond and the nearest neighbor ion pair dynamics. In the second time domain the movement of the butyl chain took place. The third time domain consisted in the increasing movement of the imidazolium ring as well as in the continuous distortion of an ion cage, i.e., the departure of one of the several counterions from the central ion's first shell, and the intermittent hydrogen bond dynamics. The remaining domain involves the translational displacement of the ions.