Insights into the structure and dynamics of a room-temperature ionic liquid: ab initio molecular dynamics simulation studies of 1-n-Butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) and the [bmim][PF₆]−CO₂ mixture

Abstract

Ab initio molecular dynamics (AIMD) studies have been carried out on liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) and its mixture with CO₂ using the Car−Parrinello molecular dynamics (CPMD) method. Results from AIMD and empirical potential molecular dynamics (MD) have been compared and were found to differ in some respects. With a strong resemblance to the crystal, the AIMD simulated neat liquid exhibits many cation−anion hydrogen bonds, a feature that is almost absent in the MD results. The anions were observed to be strongly polarized in the condensed phase. The addition of CO₂ increased the probability of this hydrogen bond formation. CO₂ molecules in the vicinity of the ions of [bmim][PF₆] exhibit larger deviations from linearity in their instantaneous configurations. The polar environment of the liquid induces a dipole moment in CO₂, lifting the degeneracy of its bending mode. The calculated splitting in the vibrational mode compares well with infrared spectroscopic data. The solvation of CO₂ in [bmim][PF₆] is primarily facilitated by the anion, as seen from the radial and spatial distribution functions. CO₂ molecules were found to be aligned tangential to the PF₆ spheres with their most probable location being the octahedral voids of the anion. The structural data obtained from AIMD simulations can serve as a benchmark to refine interaction potentials for this important room-temperature ionic liquid.