A density functional theory based approach for predicting melting points of ionic liquids

Area of Science:

• Physical Chemistry
• Computational Chemistry

Background:

• Accurate prediction of ionic liquid (IL) melting points is crucial for fundamental understanding and practical applications, such as screening ILs for low melting points and expanding their operational temperature range.
• ILs are salts that are liquid below 100°C, with unique properties making them suitable for various applications.

Purpose of the Study:

• To present an ab initio computational approach for calculating the melting points of ILs with known crystal structures.
• To apply this method to a series of 11 ILs featuring imidazolium/pyrrolidinium cations and halide/polyatomic fluoro-containing anions.

Main Methods:

• Melting point determination based on the temperature where Gibbs free energy of fusion is zero, utilizing the Born-Fajans-Haber cycle.
• Estimation of lattice enthalpy, entropy, and free energy using dispersion-corrected density functional theory (DFT) with PBE-D3 and HSE06-D3 functionals.
• Calculation of ion solvation free energies using the SMD-generic-IL model at the M06-2X/6-31+G(d) level.

Main Results:

• Melting points computed with the HSE06-D3 functional showed good agreement with experimental data (mean absolute error of 30.5 K, mean relative error of 8.5%).
• The model accurately reproduced trends in melting points with variations in alkyl substituents and anion types.
• Lattice energies for ILs with polyatomic fluoro-containing anions were well-approximated by a volume-based thermodynamic approach, unlike those with halide anions.

Conclusions:

• The developed ab initio approach provides accurate predictions of IL melting points and trends.
• Crystal structure prediction for organic salts with halide anions is essential for improving melting point predictions without prior structural knowledge.
• This work advances the computational screening of ILs for tailored applications.