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

Abstract

Numerous combinations of cations and anions are possible for the production of ionic liquids with fine-tuned properties once the correlation with the molecular structure is known. In this sense, computer simulations are useful tools to explain and even predict the properties of ionic liquids. However, quantum mechanical methods are usually restricted to either small clusters or short time scales so that parameterized force fields are required to study the bulk liquids. In this work, a method is proposed to enable a comparison between the quantum mechanical system and both polarizable and nonpolarizable force fields by means of the calculation of free energy surfaces for the translational motion of the anion around the cation in gas phase. This method was tested for imidazolium-based cations with 3 different anions, [BF4]−, [N(CN)2]−, and [NTf2]−. Better agreement was found with the density functional theory calculations when polarizability is introduced in the force field. In addition, the ion pair free energy surfaces reproduced the main structural patterns observed in the first coordination shell in molecular dynamics simulations of the bulk liquid, proving to be useful probes for the liquid phase structure that can be computed with higher level methods and the comparison with forcefields can indicate further improvements in their parameterization.

Role of density and electrostatic interactions in the viscosity and non-newtonian behavior of ionic liquids – a molecular dynamics study

“…It is known that the introduction of polarizability weakens the long-range structure in relation to the short-range structure of ILs, and that produces significant effects over the dynamics of the IL. 52,54,55 Here, our model reproduces the rheology as well as the polarizable model (Fig. 4), however, an overall reduction of electrostatic interactions instead of the introduction of explicit polarizability may lead to an overstructation in long-range and understructuration at the first coordination shell.…”

Themis: A Software to Assess Association Free Energies via Direct Estimative of Partition Functions

“…Hamiltonian 23 as implemented in the MOPAC software. 24 More recently, it was shown that free energy surfaces of first coordination shell of ion pairs of ionic liquids obtained at DFT level produces results in excellent agreement with the ones obtained from liquid phase MD simulations, 25,26 demonstrating that even in some complex cases, Themis is able to probe the structure of condensed phases. This enables the comparison of forcefields with higher theory methods such as DFT, which are too costly to be applied even for small clusters.…”

Anti-electrostatic hydrogen bonding between anions of ionic liquids: a density functional theory study

“…4 Ionic liquids (ILs), comprised of anions and cations, show many advantages such as low vapor pressure and wide electrochemical window. 5,6,[18][19][20] The HB networks in ILs play a very important role in their physicochemical properties. 5,6,[21][22][23] Gamrad et al.…”