title: Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: I. Ab Initio Calculations.
abstract: This work elucidates the molecular binding mechanism of CO2in [C4C1IM][PF6] ionic liquid (IL) and its interplay with the CO2asymmetric stretch frequency ν3, and establishes computational protocols for the reliable construction of spectroscopic maps for simulating ultrafast 2D-IR data of CO2solvated 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 CO2in [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.
authors.affiliation: Department of Chemistry, University of Pittsburgh , 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States.|Pittsburgh Quantum Institute , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Eric J
authors.identifier: 0000-0001-8186-9522
authors.initials: EJ
authors.lastname: Berquist
authors.affiliation: Department of Chemistry and Biochemistry, University of Notre Dame , Notre Dame, Indiana 46556, United States.
authors.forename: Clyde A
authors.initials: CA
authors.lastname: Daly
authors.affiliation: Department of Chemistry, University of Pittsburgh , 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States.|Pittsburgh Quantum Institute , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Thomas
authors.identifier: 0000-0002-4167-391X
authors.initials: T
authors.lastname: Brinzer
authors.affiliation: Department of Chemistry, University of Pittsburgh , 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States.|Pittsburgh Quantum Institute , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Krista K
authors.initials: KK
authors.lastname: Bullard
authors.affiliation: Department of Chemistry, University of Pittsburgh , 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States.|Pittsburgh Quantum Institute , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Zachary M
authors.initials: ZM
authors.lastname: Campbell
authors.affiliation: Department of Chemistry and Biochemistry, University of Notre Dame , Notre Dame, Indiana 46556, United States.
authors.forename: Steven A
authors.identifier: 0000-0001-6451-4447
authors.initials: SA
authors.lastname: Corcelli
authors.affiliation: Department of Chemistry, University of Pittsburgh , 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States.|Pittsburgh Quantum Institute , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Sean
authors.identifier: 0000-0001-6199-8773
authors.initials: S
authors.lastname: Garrett-Roe
authors.affiliation: Department of Chemistry, University of Pittsburgh , 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States.|Pittsburgh Quantum Institute , Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Daniel S
authors.identifier: 0000-0001-5326-0234
authors.initials: DS
authors.lastname: Lambrecht