Towards molecular dynamics simulations of chiral room-temperature ionic liquids

Abstract

We developed an all-atom non-polarizable force field for simulations of two chiral room-temperature ionic liquids (RTILs) derived from 1-n-butyl-3-methylimidazolium bromide ([bmim][Br]); namely, (R)-1-butyl-3-(3-hydroxy-2-methylpropyl)imidazolium bromide (hydroxypropyl) and 1-butyl-3-[(1R)-nopyl]imidazolium bromide (nopyl). The force field adopts the CHARMM parameters for intramolecular and repulsion–dispersion interactions, and it employs reduced partial atomic charges of the ions which we derived by quantum-mechanical calculations. The atomic charges of a cation are first assigned by the ChelpG method to yield the integer net charge +e. The ChelpG charges of the cation are then uniformly scaled down by a factor equal to the absolute value of a charge on the bromine anion obtained by the QTAIM method for the cation–anion pair. The net charges of the ions are around ±0.8e, which mimics the anion to cation charge transfer and many-body effects. Molecular dynamics (MD) simulations are used to estimate the bulk density and heat of vaporization of the RTILs at 300K and 1bar. MD simulations in the slab geometry combined with the intrinsic interface analysis are then employed to compare the vapor–liquid interface of the chiral RTILs with the interface of [bmim][Br] at 300K. We focus on the structural properties of the interfaces by computing the density profiles and surface roughness. The chiral RTILs are currently tested for separation of racemic mixtures of α-pinenes and the simulations complement experimental characterization of the chiral RTILs.