Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: I. Ab Initio 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 different 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 within 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.