Molecular force field for ionic liquids III: imidazolium, pyridinium, and phosphonium cations; chloride, bromide, and...

This is the third set of parameters of a force field for the molecular simulation of ionic liquids, developed within the spirit of the OPLS-AA model and thus oriented toward the calculation of equilibrium thermodynamic and structural properties. The parameter sets reported here concern the cations alkylimidazolium, tetra-alkylphosphonium, and N-alkylpyridinium, and the anions chloride, bromide, and dicyanamide. The force field is built in a stepwise manner that allows the construction of models for an entire family of cations, with alkyl side chains of different length, for example. Due to the transferability of the present force field, the ions studied here can be combined with those reported in our two previous publications to create a large variety of ionic liquids that can be studied by molecular simulation. The parameters reported were obtained through different series of ab initio calculations concerning the geometry, force constants, torsion energy profiles, and electrostatic charge distributions of the ions under study. Validation of the force field consisted of comparison with experimental crystal structure and liquid density data.