Schmollngruber Michael, Schröder Christian, Steinhauser Othmar
Department of Computational Biological Chemistry, University of Vienna, Währingerstrasse 17, A-1090 Vienna, Austria.
Phys Chem Chem Phys. 2014 Jun 14;16(22):10999-1009. doi: 10.1039/c4cp01236d.
For the three molecular ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate and 1-butyl-3-methylimidazolium tetrafluoroborate, dielectric spectra were calculated from molecular dynamics simulations based on polarizable force fields. Using the reaction field continuum model the dielectric spectra were converted to the solvation dynamics of coumarin 153. It is shown in detail that the inclusion of the static conductivity in this model is essential. When simplifying the dielectric spectrum to the static conductivity hyperbola, the solvation response function becomes mono-exponential. Taking into account the frequency dependence of the conductivity, the typical two time-regimes of the solvation response function in ionic liquids are already obtained. However, the mean relaxation time remains the same. When converting the complete dielectric spectrum, i.e. also including frequency-dependent dielectric permittivity, quantitative changes are observed, but the qualitative shape is conserved. In accordance with previous experimental studies, solvation dynamics in ionic liquids predicted by the reaction field continuum model is too fast for longer times. This correlates with the suppression of the fine structure of the dielectric spectrum at low frequencies by the static conductivity hyperbola. By scaling down the static conductivity this effect can be partially amended. In addition to the impact of the solvent dielectric spectrum on solvation dynamics, solute-specific effects, i.e. anisotropy in shape and charge distribution as well as polarizability, were also studied.
对于三种分子离子液体1-乙基-3-甲基咪唑四氟硼酸盐、1-乙基-3-甲基咪唑三氟甲磺酸盐和1-丁基-3-甲基咪唑四氟硼酸盐，基于可极化力场，通过分子动力学模拟计算了介电谱。使用反应场连续介质模型，将介电谱转换为香豆素153的溶剂化动力学。详细表明，在该模型中包含静态电导率至关重要。当将介电谱简化为静态电导率双曲线时，溶剂化响应函数变为单指数形式。考虑到电导率的频率依赖性，已经得到了离子液体中溶剂化响应函数典型的两个时间区域。然而，平均弛豫时间保持不变。当转换完整的介电谱，即也包括频率相关的介电常数时，会观察到定量变化，但定性形状得以保留。与先前的实验研究一致，反应场连续介质模型预测的离子液体中的溶剂化动力学在较长时间内太快。这与静态电导率双曲线对低频介电谱精细结构的抑制相关。通过降低静态电导率，这种效应可以得到部分修正。除了溶剂介电谱对溶剂化动力学的影响外，还研究了溶质特异性效应，即形状和电荷分布的各向异性以及极化率。
