Extension of TEAM Force-Field Database to Ionic Liquids

The TEAM force-field database is
extended to cover room-temperature
ionic liquids (IL). The training set for parametrization includes
11 cations in five classes, imidazolium, pyridinium, pyrrolidinium,
piperidinium, and tetraalkylammonium, and 12 anions, chloride, perchlorate,
nitrate, tetrafluoroborate, hexafluorophosphate, thiocyanate, dicyanamide,
tricyanomethanide, tetracyanoborate, bis­(trifluoromethylsulfonyl)­imide,
triflate, and trifluoroacetate. The force field is compatible with
other force fields of the TEAM database. The parameters are primarily
derived from quantum-mechanical data, with a few nonbonded parameters
optimized using experimental data of density and viscosity. The extrapolation
of shear viscosity calculated using the nonequilibrium periodic perturbation
(PP) method is examined by analyzing the Newtonian region in the calculated
shear viscosities. With the identified extrapolation scheme, the PP
method is about five times more efficient than the multiconfiguration-ensemble
Green–Kubo (GK) method to reach similar convergence, and it
is used for iterated calculations of viscosities as a part of the
parametrization. The force field is validated using 46 ILs; the unsigned
deviations from the experimental data are 1.4%, 12.8%, and 3.7% for
density, viscosity, and isobaric heat capacity, respectively. The
force field is used to analyze the interactions in terms of activation
energies of viscosity and liquid structures in terms of distribution
functions and free energy maps for common ILs. The ring–ring
stacking is found to be the most stable local configuration for ILs
of aromatic cation and chloride anion, which has a profound impact
on the high viscosities of the ILs.