Molecular dynamics simulation of the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate

Room-temperature ionic liquids (RTILs) due to their unique properties and solvent capabilities have been motivating an extraordinary growth on experimental and theoretical investigations. Many RTILs have been developed to fulfill specific applications and therefore this class of compounds has been termed “designer solvents”. Therefore, to accomplish this purpose, the understanding of RTILs behavior at the atomistic level is needed. In this work a 5 ns NpT molecular dynamic simulation at T = 300 K and p = 1.0 atm was performed to investigate thermodynamical and structural properties of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate. All force field parameters but charges and geometries were taken from the OPLS-AA. The partial charges and geometry parameters were obtained at the ab initio MP2/6-31g(d) level, with charges computed using the ChelpG procedure. This same level of theory was used to calculate energies for cation-anion interactions at several configurations to validate the force field parameters. The agreement between geometries and energies obtained with ab initio and force field calculations is good. The value obtained for the liquid density, 1.178 g cm−3, is close to the experimental data (1.17 g cm−3). The value estimated for the heat of vaporization, ΔHvap ≈ 413 kJ mol−1, is larger than the ones usually observed for molecular liquids, which is in accordance with the very low vapor pressure observed for RTILs. Structural and dynamics properties, such as radial distribution function and mean square displacement were analyzed and are also in good agreement with data reported in the literature. Comparatively to the anion, the cation self-diffusion constant is larger despite the fact that it is heavier and bigger. Compared to the values observed for molecular liquids the self-diffusion constants obtained for cation and anions are about three orders of magnitude smaller.