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

CA Daly and EJ Berquist and T Brinzer and S Garrett-Roe and DS Lambrecht and SA Corcelli, JOURNAL OF PHYSICAL CHEMISTRY B, 120, 12633-12642 (2016).

DOI: 10.1021/acs.jpcb.6b09509

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
(**C(4)C(1)im****PF6**) ionic liquid (IL). DFT is used to compute the
vibrational frequency of 500 statistically independent
CO2-**C(4)C(1)im****PF6** clusters extracted from an MD simulation. When the
map was tested on 500 different CO2-**C(4)C(1)im****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
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.

Return to Publications page