Self-diffusion behaviors of ionic liquids by MD simulation based on united-atom force field introduced charge scaling by ab initio MO simulation

We performed ab initio molecule orbital (MO) calculations to obtain the stable states of the ions in ionic liquids (ILs), partial atomic charges, and scaling factors for a charge scaling. Then, MD simulations based on united-atom (UA) force field introduced a charge scaling were carried out in ILs. The partial atomic charges of ions in ILs were determined by using charges from electrostatic potentials using a grid based method (CHelpG) from the B3LYP/6-311G(d,p) level wave function for cations B3LYP/6-311+G(d) one for anions. The scaling factors for [C2C1Im], [TFSA] and [FSA] were considered from the dipole interactions. The calculated densities by MD simulation were reproduced the experimental values at 298 K very well and there were errors within 5.0% deviations, because the partial atomic charges of the ions in ILs were scaling down and the coulomb interactions became weaker. Moreover, the self-diffusion coefficients of [C2C1Im][TFSA] by MD simulation based on UA force field introduced a charge scaling showed the same order for the experimental values at 423 K. Furthermore, the self-diffusion coefficients and the center-of-mass radial distribution functions in [P2225][TFSA] at 298, 323, 348, 373, 398, and 423 K were simulated. The calculated self-diffusion coefficients had a similar tendency with experimental ones. The position of first solvation shell withdrew about 0.25 Å and its height became lower due to increase the temperatures. Both the first minimum position around 9-10 Å and the second maximum position around 13-14 Å also went away. This simulation result indicated that the intermolecular interaction strength between cations and anions became weaker with increasing temperature.