Virtual Site OPLS Force Field for Imidazolium-Based Ionic Liquids

Abstract

Molecular simulations of ionic liquids can provide deeper insight into the relationship between intermolecular interactions and macroscopic measurements for the solvents. However, many existing force fields have multiple shortcomings, including poor solvent dynamics, the underestimation of hydrogen-bonding strength, and errors in solvent interactions/organization. A new force field, called optimized potentials for liquid simulation-ionic-liquid virtual site (OPLS-VSIL), has been developed for imidazolium-based ionic liquids featuring a novel topology incorporating a virtual site bisecting the nitrogen atoms that offloads negative charge to inside the plane of the ring. Guided by free energy of hydration calculations, an empirically derived set of partial charges and nonbonded Lennard-Jones terms for both 1-alkyl-3-methylimidazolium and 11 different anions provided accurate bulk-phase ionic-liquid properties and produced radial distribution functions nearly indistinguishable from ab initio molecular dynamics simulations. For example, overall mean absolute errors (MAEs) of 3.1-3.4% were computed for the density, heat of vaporization, and viscosity of approximately 20 different ion pair combinations. Additional physical properties, such as, self-diffusion coefficients, heat capacity, and surface tension also gave significant MAE improvements using OPLS-VSIL compared to the existing fixed-charge ionic-liquid force fields. Local interactions, including cation-anion hydrogen bonding and π-π stacking between the imidazolium rings, were also accurately reproduced.

Thermodynamic, structural and dynamic properties of ionic liquids [C4mim][CF3COO], [C4mim][Br] in the condensed phase, using molecular simulations

“…10,11 Recent publications have stressed the existence of multiple shortcomings in some of the available classical FF, including inadequate solvent dynamics, errors in the prediction of hydrogen-bonding strength, inadequate mixture phase behavior, and errors in solvent structure and interactions. [12][13][14][15] Recent contributions have modied the atomic partial charges and Lennard-Jones parameters, in computer simulations by considering the dielectric constant as a target property in order to mimic the miscibility observed experimentally between two different liquids. 16,17 In the literature, there is a constant need for reliable and accurate IL FF 14,18,19 able to capture, among others, the local electrostatic properties, 20 microscopic dynamics, 21 and vibrational spectra; 22 as indicated by Maginn, 18 many FFs already published have not been validated yet against reliable and sensitive properties such as enthalpy of vaporization, and dynamic properties.…”

Accurate Diels-Alder Energies and Endo Selectivity in Ionic Liquids Using the OPLS-VSIL Force Field

“…The use of density functional theory (DFT) instead of SQM significantly increased the cost of the simulations, for example, approximately 3-4 days for M06-2X/6-31G(d) compared to 10-15 min for PDDG/PM3 per 1.5 million MC configurations. The ionic liquid solvent was treated using the OPLS-VSIL or 0.8*OPLS-2009IL force fields [65,66]. Free-energy profiles were computed by perturbing a reaction coordinate (RC) defined by the distance between two dummy atoms located at the midpoint of the diene terminal carbon atoms and the midpoint of the diene C=C bond (Figure 1), as done in our previous computational investigation of the reaction in chloroaluminate ionic liquids [42].…”

“…Mixed quantum and molecular mechanical calculations were carried out using a BOSS-Gaussian interface [69] with solutes treated using the M06-2X/6-31G(d) theory level, which has been shown to yield highly accurate transition structure geometries and energies for Diels-Alder reactions [70,71]. The OPLS-VSIL and 0.8*OPLS-2009IL force fields [41,65,66] anions) for OPLS-VSIL and 376 ion pairs for 0.8*OPLS-2009IL were constructed that were periodic and tetragonal with long-range electrostatics treated using Ewald sums. Potentials of mean force (PMF) calculations coupled to Metropolis Monte Carlo statistical mechanics were utilized to compute a free-energy profile for the Diels-Alder reaction at 25 • C and 1 atm.…”

“…However, in order to dramatically increase the accuracy of the computed ionic liquid dynamics, thermochemical properties and local cation-anion interactions, a new force field parameterization effort was needed. Novel imidazolium-based cation parameters were created that utilized a virtual site bisecting the ring nitrogen atoms [66]. This optimized potentials for liquid simulations-virtual site ionic liquid (OPLS-VSIL) force field featured empirical charges guided by free energy of hydration calculations and new Lennard-Jones terms that were fine-tuned for the 1-alkyl-3-methylimidazolium cations and 11 different anions.…”

Computational Spectroscopy of Ionic Liquids for Bulk Structure Elucidation

“…Depending on the atomic resolution, it is possible to have fully atomistic (FA) or coarse‐grained (CG) versions. A popular FF of the first type for ILs has been developed by Canongia‐Lopes and Padua; nonetheless, there is still an active research in this area that cannot be thoroughly covered here. Coarse‐graining reduces the number of explicit sites, thus reducing the computer power needed, so it is an interesting option for ILs where strong electrostatic interactions require long simulation times (because of the slow dynamics) and large simulations boxes (because of the large spatial correlations related to microphase separation).…”