Systematic Refinement of Canongia Lopes–Pádua Force Field for Pyrrolidinium-Based Ionic Liquids

Reliable force field (FF) is a central
issue in successful prediction
of physical chemical properties via computer simulations. While Canongia
LopesPádua (CL&P) FF provides good to excellent
thermodynamics and structure of pure room-temperature ionic liquids
(RTILs), it suffers from drastically and systematically underestimated
ionic motion. This occurs due to neglected partial electron transfer
from the anion to the cation, resulting in unphysically small simulated
self-diffusion and conductivity and high shear viscosities. We report
a systematic refinement of the CL&P FF for six pyrrolidinium-based
RTILs (1-N-butyl-1-methylpyrrolidinium dicyanamide,
triflate, bis­(fluorosulfonyl)­imide, bis­(trifluoromethanesulfonyl)­imide,
tetrafluoroborate, chloride). The elaborated procedure accounts for
specific cation–anion interactions in the liquid phase. Once
these interactions are described effectively, experimentally determined
transport properties can be reproduced with an acceptable accuracy.
Together with the original CL&P parameters, our force field fosters
computational investigation of ionic liquids. In addition, the reported
results shed more light on the chemical nature of cation–anion
binding in various families of RTILs.