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

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, [C₄C₁im][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.