OPLS-AA force field parameters have been developed and validated for use in the simulation of 68 unique combinations of ionic liquids (ILs) featuring 1-alkyl-3-methylimidazolium [RMIM] (R = Me, Et, Bu, Hex, Oct), N-alkylpyridinium [RPYR], and choline cations, along with Cl-, PF6-, BF4-, NO3-, AlCl4-, Al2Cl7-, TfO-, saccharinate, and acesulfamate anions.  Torsional free-energy profiles and electrostatic charges were obtained at the LMP2/cc-pVTZ(-f)//HF/6-31G(d) level, and the new parameters were fit to reproduce experimental condensed-phase structural and thermodynamic properties.  Monte Carlo (MC) simulations of the ILs gave relative deviations from experimental densities and heats of vaporization of ca. 1-3%.  Transferability of the cations was tested by comparing the new parameters to potentials specific to alkyl chain length.  Both parameter sets gave nearly identical values over a wide temperature range regardless of the anion chosen. As further validation and applications of the newly developed parameters, the Kemp elimination reaction of benzisoxazole via piperidine was computed in 1-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF6] using mixed quantum and molecular mechanics (QM/MM) simulations and was found to give close agreement with the experimental free energy of activation. Also the shift mechanisms for base-induced �-eliminations from 1,1,1-tribromo-2,2-bis(3,4-dimethoxyphenyl)ethane via two bases, piperidine and pyrrolidine, has been computationally investigated using QM/MM/MC simulations in methanol and the ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4] and [BMIM][PF6]. The highly accurate parameters for testing organic reactions in a variety of IL cation/anion combinations should be of considerable interest to the IL modeling community.

Development of OPLS-AA force field parameters for ionic liquid simulations and their applications in elimination reactions

245th National Meeting (2013)

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