|Name||Mr. Brian Doherty|
|Organization or Institution||University of Miami|
Development of a virtual site OPLS force field for ionic liquid simulations
Brian Doherty, Xiang Zhong, and Orlando Acevedo
Department of Chemistry, University of Miami, Coral Gables, Florida 33146
Ionic liquids (IL) are often utilized as alternatives to volatile organic solvents. These “designer” solvents are generally composed of a low symmetry organic cation and a weakly coordinating inorganic or organic anion. In this work, the cation of interest is 1-alkyl-3-methylimidazolium [RMIM] (R = E for ethyl, B for butyl, etc.), which is popular for its simultaneous hydrogen bonding and pi-pi stacking abilities. In many cases, [RMIM] can enhance both the rate of reaction and product specificity for multiple organic reaction classes. Molecular simulations can provide further insight into the origin of these reaction enhancements. Problematically, existing non-polarizable force fields have several limitations that include poor solvent dynamics, the underestimation of hydrogen bond strength, and improper solvent organization. A new non-polarizable force field called OPLS-VSIL has been developed that features a virtual site bisecting the nitrogen atoms in the [RMIM] cation that offloads negative charge to inside the ring. Empirically derived partial charges and Lennard-Jones terms were guided by free energy of hydration calculations in order to mimic polarization and charge transfer effects. The predicted bulk-phase properties were significantly improved compared to prior force fields for 20 different ion pair combinations and often gave quantitative agreement with experimental measurements. Solvent ordering and intermolecular interactions monitored by radial and combined distribution functions were also nearly identical to ab initio molecular dynamics simulations.