title: Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: II. Spectroscopic Map.
abstract: 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 CO2in 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 R = 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 CO2angle, the electrostatics of the surrounding solvent, and the Lennard-Jones interaction between the CO2and the IL. The contribution of each term in the map was investigated using symmetry-adapted perturbation theory calculations.
authors.affiliation: Department of Chemistry and Biochemistry, University of Notre Dame , 251 Nieuwland Science Hall, Notre Dame, Indiana 46656, 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, University of Pittsburgh , 3943 O'Hara Street, 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, University of Pittsburgh , 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States.|Pittsburgh Quantum Institute, University of Pittsburgh , 3943 O'Hara Street, 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, University of Pittsburgh , 3943 O'Hara Street, 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, University of Pittsburgh , 3943 O'Hara Street, Pittsburgh, Pennsylvania 15260, United States.
authors.forename: Daniel S
authors.identifier: 0000-0001-5326-0234
authors.initials: DS
authors.lastname: Lambrecht
authors.affiliation: Department of Chemistry and Biochemistry, University of Notre Dame , 251 Nieuwland Science Hall, Notre Dame, Indiana 46656, United States.
authors.forename: Steven A
authors.identifier: 0000-0001-6451-4447
authors.initials: SA
authors.lastname: Corcelli