Ion pair free energy surface as a probe of ionic liquid structure

Area of Science:

• Computational Chemistry
• Materials Science
• Chemical Physics

Background:

• Ionic liquids (ILs) offer tunable properties through cation-anion combinations.
• Computer simulations are vital for understanding and predicting IL behavior.
• Quantum mechanical (QM) methods are limited in scale, necessitating parameterized force fields for bulk IL studies.

Purpose of the Study:

• To develop a method for comparing QM calculations with polarizable and nonpolarizable force fields.
• To assess the accuracy of force fields in representing ionic liquid structures.
• To provide insights for improving force field parameterization.

Main Methods:

• Calculation of free energy surfaces for anion translational motion around cations in the gas phase.
• Comparison of QM (density functional theory) results with polarizable and nonpolarizable force fields.
• Testing the method with imidazolium-based cations and three different anions: tetrafluoroborate ([BF4]-), dicyanamide ([N(CN)2]-), and bis(trifluoromethylsulfonyl)imide ([NTf2]-).

Main Results:

• The proposed method enables direct comparison between QM and force field models.
• Force fields incorporating polarizability showed better agreement with QM calculations.
• Calculated ion pair free energy surfaces successfully reproduced structural patterns observed in bulk liquid simulations.

Conclusions:

• The developed method is a valuable tool for validating and refining force fields for ionic liquids.
• Polarizable force fields are crucial for accurately capturing the behavior of ionic liquids.
• This approach aids in understanding liquid phase structure and improving force field parameterization for more accurate simulations.