Atomistic simulations of Imidazolium-based Ionic Liquids: current challenges for theoretical models.

In this communication we report the first X-Ray scattering study of imidazolium-based ionic liquids containing the bromide anion. The system studied was 1-octyl-3-metyl-imidazolium bromide ([C8mim]Br). The study was extended to the analogous salt, containing chloride as anion ([C8mim]Cl) which has been used for comparison. The measured diffraction patterns are compared with the theoretical spectra calculated from model geometries obtained with classical molecular dynamics simulations. The behavior and the performance of the available force fields in the description of bromide ion is discussed. In particular, we shall show how the force employed in our simulations, which is an implementation of the one from Canogia-Lopes et al. (Lopes, J. N. C.; Deschamps, J.; Pádua, A. A. H. J. Phys Chem. B 2004, 108, 2038), is able to reproduce qualitatively all the prominent features of the measured spectra. This is a remarkable result given that the force field used in the present calculations is not specific to the title ILs, but is fully transferable to other systems. Our calculations show, however, that the theoretical simulations employing that field present some serious discrepancy when compared to experiments at least in the case of the ILs considered here which are characterized by long side chains and by relatively less explored anions such as the Br−. In particular we have shown that the predicted density of the [C8mim]Br is 13% off the experimental value and that this effect can be related to features in the radial distribution function that, in fact, are partially corrected when carrying out the simulation at the volume corresponding to the experimental density. Other features that are bound to the linking of the Br− ion to the carbon ring remain very poorly described by the force field. The case of the [C8mim]Cl presents in general a better agreement between experiments and theory, but also in this case we have discrepancies especially for the long range part of the radial distributions. Simple modifications of the force field are currently under study in our laboratories and will be presented along with previous results.