Ab Initio Force Fields for Organic Anions: Properties of [BMIM][TFSI], [BMIM][FSI], and [BMIM][OTf] Ionic Liquids

Room-temperature
ionic liquids (ILs) composed of organic anions
bis­(trifluoromethanesulfonyl)­imide (TFSI), bis­(fluorosulfonyl)­imide
(FSI), and trifluoromethanesulfonate (OTf) exhibit interesting physical
properties and are important for many electrochemical applications.
TFSI and FSI form “hydrophobic” ILs, immiscible with
water but miscible with many organic solvents and polymers; for computer
simulation studies, it is thus essential to develop force fields for
these anions that are transferable among this wide variety of chemical
environments. In this work, we develop entirely ab initio force fields
for the TFSI, FSI, and OTf anions and predict the properties of corresponding
1-butyl-3-methylimidazolium ILs. We discuss important subtleties in
the force field development related to accurately modeling conformational
flexibility, that is, relaxed torsional profiles and intramolecular
electrostatic interactions. The TFSI anions have notable conformational
flexibility in the IL, and we predict approximately 70% cisoid and
20% transoid conformations, which is largely driven by cation/anion
ion-pair interactions and is opposite to the trend expected from the
anion ab initio potential energy surface. The favorable interactions
between the cation and cisoid TFSI conformations result in a shoulder
in the cation/anion radial distribution function at short distances,
whereas interconversion between cisoid and transoid conformations
occurs on a commensurate time scale as ion diffusion processes. In
addition to this physical insight on anion effects, we expect that
these force fields will have important applications for studying a
variety of complex electrolyte systems.