Virtual Site OPLS Force Field for Imidazolium-Based Ionic Liquids

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.