Explicit Solvent Modeling of IR and UV–Vis Spectra of 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid

Hydrogen Bonding and Infrared Spectra of Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide/Water Mixtures: A View from Molecular Dynamics Simulations

Simulations of ab initio molecular dynamics have been performed for mixtures of ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) ionic liquid and water. Statistics of donors and acceptors of hydrogen bonds has revealed that with increasing water content, hydrogen bonds between EMIM cations and TFSI anions are replaced by bonds to water molecules. In the mixture of liquids, the total number of bonds (from EMIM cations or water molecules) formed by TFSI acceptors increases. IR spectra obtained from ab initio molecular dynamics trajectories are in good agreement with literature data for ionic liquid/water systems. Analysis of oscillations of individual C-H and O-H bonds has shown correlations between vibrational frequencies and hydrogen bonds formed by an EMIM cation or water molecule and has indicated that the changes in the IR spectrum result from the decreased number of water-water hydrogen bonds in the mixture. The tests of DFTB methodology with tailored parameterizations have yielded reasonably good description of the IR spectrum of bulk water, whereas available parameterizations have failed in satisfactory reproduction of the IR spectrum of EMIM-TFSI/water mixtures in the region above 3000 cm^-1.

Structural, dynamical, and electronic properties of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

We provide a microscopic insight, both structural and electronic, into the multifold interactions occurring in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI] currently targeted for applications in next-generation low-power electronics and optoelectronic devices. To date, practical applications have remained hampered by the lack of fundamental understanding of the interactions occurring both inside the IL and at the interface with the substrate. Our first principles dynamical simulations provide accurate insights into the nature of bonding and non-bonding interactions, dynamical conformational changes and induced dipole moments, along with their statistical distributions, of this ionic liquid, that have so far not been completely unraveled. The mobilities of the two ionic species are obtained by long-lasting dynamical simulations at finite temperature, allowing simultaneous monitoring and quantification of the isomerization occurring in the IL. Moreover, a thorough analysis of the electronic structure and partial charge distributions characterizing the two components, the cation and anion, allow rationalization of the nature of the electrostatic interactions, hydrogen bonding properties of the two ionic counterparts, and the infra-red and dielectric response of the system, especially in the low frequency range, for the full characterization of the IL.

Understanding the Structure of the Hydrogen Bond Network and Its Influence on Vibrational Spectra in a Prototypical Aprotic Ionic Liquid

Analysis of the hydrogen bond network in aprotic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) has been performed based on structures obtained from ab initio or classical molecular dynamics simulations. Statistics of different donor and acceptor atoms and the amount of chelating or bifurcated bonds has been presented. Most of the hydrogen bonds in EMIM-TFSI are formed with oxygen atoms as hydrogen acceptors; and the most probable bifurcated bonds are those with a mixed pair of oxygen and nitrogen acceptors. Spectral graph analysis has shown that the cations may form hydrogen bonds with up to five different anions and the connectivity of the whole hydrogen bond network is supported mainly by H-O bonds. In the structures of the liquid simulated via force field-based dynamics, the number of hydrogen bonds is smaller and fluorine atoms are the most favored hydrogen acceptors. One-dimensional potential energy profiles for hydrogen atom displacements and corresponding vibrational frequencies have been calculated for selected C-H bonds. Individual C-H stretching frequencies vary by 200-300 cm^-1, indicating differences in local environment of hydrogen atoms forming C-H···O hydrogen bonds.

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.