Cohesive Properties of Ionic Liquids Calculated from First Principles

Low volatility of ionic liquids (ILs), being one of their most valuable properties, is also the principal factor making reliable measurements of vapor pressures and vaporization (or sublimation) enthalpies of ILs extremely difficult. Alternatively, vaporization enthalpies at the temperature of the triple point can be obtained from the enthalpies of sublimation and fusion. While the latter can be obtained calorimetrically with a fair accuracy, the former is in principle accessible through ab initio computations. This work assesses the performance of the first-principles calculations of sublimation properties of ILs. Namely, 3 compounds, coupling the 1-ethyl-3-methylimidazolium cation [emIm] with either tetrafluoroborate [BF₄], hexafluorophosphate [PF₆], or bis­(trifluoromethylsulfonyl)­imide [NTf₂] anions were selected for a case study. A computational methodology, originally developed for molecular crystals, is adopted for crystals of ILs. It exploits periodic density functional theory (DFT) calculations of the unit-cell geometries and quasi-harmonic phonons and many-body expansion schemes for ab initio refinements of the lattice energies of crystalline ILs. The vapor phase is treated as the ideal gas whose properties are obtained combining the rigid rotor–harmonic oscillator model with corrections from the one-dimensional hindered rotors and molecular-dynamics simulations capturing the contributions from the interionic interaction modes. Although the given computational approach enables one to reach the chemical accuracy (4 kJ mol^–1) of calculated sublimation enthalpies of simple molecular crystals, reaching the same level of accuracy for ionic liquids proves challenging as crystals of ionic liquids are bound appreciably stronger than common molecular crystals, the underlying cohesive energies of solid ionic liquids is up to 1 order of magnitude larger. Still, combination of the mentioned computational and experimental frameworks results in a novel promising scheme that is expected to generate reliable and accurate temperature-dependent data on sublimation (and vaporization) of ILs.