Multiscale simulations on the structure and dynamics of ionic liquids
Final Report Abstract
We systematically investigated the chemical and physical mechanisms dominating the nature of room temperature ionic liquids. Starting with very accurate quantum chemical calculations on the single ion pair level we were able to determine the ground states of dimethylimidazolium chloride and the corresponding transition states. This enabled us to benchmark our computations on the DFT level in terms of vibrational spectra and structures. In addition to bridge the gap between “ab initio” calculations and classical molecular dynamics simulations we employed the DFT approach on larger system up to 8 ion pairs. In our studies we especially concentrated on the electrostatics as they are expected to govern the properties of this highly charged systems. Analysis of the electric dipole and higher electric multipole moments by a Wannier analysis of the charge distribution showed a well-defined behaviour when increasing the size of the investigated system. However we already observed a convergence of the absolute value of the cation dipole moment at a system size of 4 ion pairs. For this reason we concluded that we are dealing with very localized effects. This point is supported by our current studies and has also been found by other groups. Simultaneously we explored the capability of available classical force fields to describe the statics and dynamics of ionic liquids. The studied systems contained ethylmethylimidazolium as cations and anions of different size and shape were chosen for comparison. In terms of spatial ordering the preferred position of the anion has been observed at the hydrogen above the imidazolium ring while with increasing size of the anions the preference changes to the hydrogens below the ring. Studying of the hydrogen bonding reveals a moderate network similar to the one of water while the distribution of the partial charges plays a major role for the lifetime of the hydrogens bond which were found to have to longest lifetime for hydrogens at the imidazolium ring. The calculated lifetimes are consistent with values reported from the dielectric relaxation method. Besides the comparison of different anions we also compared the results of different force fields. The results generally agree with each other apart from the H2 hydrogen bond which has a significantly longer lifetime for LHW, an important fact that has been discussed by other groups. However both force fields fail in reproducing the dynamics of an ionic liquid system. We analyzed the rotational reorientation, diffusion, and the electric zero-field conductivity and found always a too slow behaviour of the system. Finally we cannot recommend any of the investigated force fields. Our main conclusion is that methods for force field refinement or parametrization have to be developed for this kind of systems. Especially the parametrization of the partial charges of the force field should include the effects of polarization occurring in bulk-like phases.
Publications
-
A comparative study of two classical force fields on statics and dynamics of [EMIM][BF4] investigated via molecular dynamics simulations. J. Chem. Phys., 129(22):224501, 2008
F. Dommert, J. Schmidt, B. Qiao, Y. Zhao, C. Krekeler, L. Delle Site, R. Berger, and C. Holm
-
Effect of anions on static orientational correlations, hydrogen bonds, and dynamics in ionic liquids: A simulational study. J. Phys. Chem. B, 112(6):1743-1751, 2008
B. Qiao, C. Krekeler, R. Berger, L. Delle Site, and C. Holm
-
Study of 1,3-dimethylimidazolium chloride with electronic structure methods and force field approaches. J. Chem. Phys., 129(17):174503, 2008
C. Krekeler, J. Schmidt, Y.Y. Zhao, B. Qiao, R. Berger, C. Holm, and L. Delle Site
