Thiocyanate as a Local Probe of Ultrafast Structure and Dynamics in Imidazolium-Based Ionic Liquids: Water-Induced Heterogeneity and Cation-Induced Ion Pairing

Ultrafast two-dimensional infrared spectroscopy (2D-IR) of thiocyanate ([SCN]⁻) in 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([C₄C₁im]­[NTf₂]) and 1-butyl-2,3-dimethylimidazolium bis­(trifluoromethylsulfonyl)­imide ([C₄C₁C₁²im]­[NTf₂]) ionic liquids probes local structure and dynamics as a function of the water content, solute counterion, and solute concentration. The 2D-IR spectra of the water-saturated ionic liquids resolve two distinct kinds of dynamics. This dynamical heterogeneity is explained as two subensembles, one with and one without a water molecule in the first solvation shell. When the countercation is K⁺, ion pairs between K⁺ and [SCN]⁻ that persist for >100 ps are detected by long-lasting vibrational frequency correlations. The observed dynamics are invariant to [SCN]⁻concentration, which indicates that the [SCN]⁻ does not cluster in ionic liquid solution. Taken together, these results are consistent with a picture of thiocyanate as a local probe that can interrogate ultrafast structure and dynamics at a small spatial scale in ionic liquids

Enthalpic Driving Force for the Selective Absorption of CO₂ by an Ionic Liquid

Molecular dynamics (MD) simulations validated against two-dimensional infrared (2D-IR) measurements of CO₂ in an imidazolium-based ionic liquid have revealed new insights into the mechanism of CO₂ solvation. The first solvation shell around CO₂ has a distinctly quadrupolar structure, with strong negative charge density around the CO₂ carbon atom and positive charge density near the CO₂ oxygen atoms. When CO₂ is modeled without atomic charges (thus removing its strong quadrupole moment), its solvation shell weakens and changes significantly into a structure that is similar to that of N₂ in the same liquid. The solvation shell of CO₂ evolves more quickly when its quadrupole is removed, and we find evidence that solvent cage dynamics is measured by 2D-IR spectroscopy. We also find that the solvent cage evolution of N₂ is similar to that of CO₂ with no atomic charges, implying that the weaker quadrupole of N₂ is responsible for its higher diffusion and lower absorption in ionic liquids

Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: II. Spectroscopic Map

The primary challenge for connecting molecular dynamics (MD) simulations to linear and two-dimensional infrared measurements is the calculation of the vibrational frequency for the chromophore of interest. Computing the vibrational frequency at each time step of the simulation with a quantum mechanical method like density functional theory (DFT) is generally prohibitively expensive. One approach to circumnavigate this problem is the use of spectroscopic maps. Spectroscopic maps are empirical relationships that correlate the frequency of interest to properties of the surrounding solvent that are readily accessible in the MD simulation. Here, we develop a spectroscopic map for the asymmetric stretch of CO₂ in the 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄C₁im]­[PF₆]) ionic liquid (IL). DFT is used to compute the vibrational frequency of 500 statistically independent CO₂-[C₄C₁im]­[PF₆] clusters extracted from an MD simulation. When the map was tested on 500 different CO₂-[C₄C₁im]­[PF₆] clusters, the correlation coefficient between the benchmark frequencies and the predicted frequencies was <i>R</i> = 0.94, and the root-mean-square error was 2.7 cm^–1. The calculated distribution of frequencies also agrees well with experiment. The spectroscopic map required information about the CO₂ angle, the electrostatics of the surrounding solvent, and the Lennard-Jones interaction between the CO₂ and the IL. The contribution of each term in the map was investigated using symmetry-adapted perturbation theory calculations

Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: I. <i>Ab Initio</i> Calculations

This work elucidates the molecular binding mechanism of CO₂ in [C₄C₁IM]­[PF₆] ionic liquid (IL) and its interplay with the CO₂ asymmetric stretch frequency ν₃, and establishes computational protocols for the reliable construction of spectroscopic maps for simulating ultrafast 2D-IR data of CO₂ solvated in ILs. While charge transfer drives the static frequency shift between <i>different</i> ionic liquids (J. Chem. Phys. 2015, 142, 212425), we find here that electrostatic and Pauli repulsion effects dominate the dynamical frequency shift between different geometries sampled from the finite-temperature dynamics <i>within</i> a single ionic liquid. This finding is also surprising because dispersion interactions dominate the CO₂–IL interaction energies, but are comparably constant across different geometries. An important practical consequence of this finding is that density functional theory is expected to be sufficiently accurate for constructing potential energy surfaces for CO₂ in [C₄C₁IM]­[PF₆], as needed for accurate anharmonic calculations to construct a reliable spectroscopic map. Similarly, we established appropriate computational and chemical models for treating the extended solvent environment. We found that a QM/MM treatment including at least 2 cation-ion pairs at the QM level and at least 32 pairs at the MM level is necessary to converge vibrational frequencies to within 1 cm^–1. Using these insights, this work identifies a computational protocol as well as a chemical model necessary to construct accurate spectroscopic maps from first principles

Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: I. <i>Ab Initio</i> Calculations

This work elucidates the molecular binding mechanism of CO₂ in [C₄C₁IM]­[PF₆] ionic liquid (IL) and its interplay with the CO₂ asymmetric stretch frequency ν₃, and establishes computational protocols for the reliable construction of spectroscopic maps for simulating ultrafast 2D-IR data of CO₂ solvated in ILs. While charge transfer drives the static frequency shift between <i>different</i> ionic liquids (J. Chem. Phys. 2015, 142, 212425), we find here that electrostatic and Pauli repulsion effects dominate the dynamical frequency shift between different geometries sampled from the finite-temperature dynamics <i>within</i> a single ionic liquid. This finding is also surprising because dispersion interactions dominate the CO₂–IL interaction energies, but are comparably constant across different geometries. An important practical consequence of this finding is that density functional theory is expected to be sufficiently accurate for constructing potential energy surfaces for CO₂ in [C₄C₁IM]­[PF₆], as needed for accurate anharmonic calculations to construct a reliable spectroscopic map. Similarly, we established appropriate computational and chemical models for treating the extended solvent environment. We found that a QM/MM treatment including at least 2 cation-ion pairs at the QM level and at least 32 pairs at the MM level is necessary to converge vibrational frequencies to within 1 cm^–1. Using these insights, this work identifies a computational protocol as well as a chemical model necessary to construct accurate spectroscopic maps from first principles