**Cohesive Properties of Ionic Liquids Calculated from First Principles**

C Cervinka and M Klajmon and V Stejfa, JOURNAL OF CHEMICAL THEORY AND COMPUTATION, 15, 5563-5578 (2019).

DOI: 10.1021/acs.jctc.9b00625

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.

Return to Publications page