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Reports: ND652648-ND6: Solvation Dynamics in Ionic Liquids

Steven A. Corcelli, University of Notre Dame

Ionic liquid (IL) research was initiated in my laboratory with generous seed-funding from the New Directions program of the American Chemical Society Petroleum Research Fund (ACS PRF #52648-ND6, "Solvation Dynamics in Ionic Liquids," 9/1/2012 – 8/31/2015). Thus far, the support has resulted in two publications whose results I will briefly summarize.1,2 The first paper sought to determine the molecular mechanism responsible for solvation dynamics in the imidazolium-based IL, [emim][BF4].1 Solvation responses in ILs occur over seven decades in time (from ~10 fs to ~10 ns) and with complex kinetic profiles.3 My laboratory computed the solvation response from a ~5 μs MD simulation of coumarin 153 (C153) in [emim][BF4] over the full range of time scales accessed in the experiments. The solvation response of C153 in [emim][BF4] compared favorably with experiment. An analysis of the structure of the IL in the vicinity of the C153 dye revealed preferential solvation by the [emim] cations. Despite this observation, decomposition of the solvation response into components from translational and rotational motions of the anions and cations revealed that translations of the [BF4] anions were the dominant contributor to solvation dynamics. The kinetics for the translation of the [BF4] anions into and out of the first solvation shell of the dye were found to mimic the kinetic profile of the solvation dynamics response. Following our initial study, we have confirmed that the solvation dynamics mechanism applies to five other imidazolium-based ILs, and we have established that the linear response approximation is valid over the full range of solvation dynamics time scales. Papers on these topics are presently in preparation.

My laboratory has also investigated the dynamics and vibrational spectroscopy of dilute HOD in [bmim][PF6].2 Recently, Fayer and coworkers found that the linear IR absorption spectrum of the OD stretch of HOD is blue-shifted by 168 cm-1 and is just 21 cm-1 wide in [bmim][PF6] compared to 170 cm-1 in H2O.4 In addition, rotational anisotropy and 2D IR measurements revealed that the reorientation and spectral diffusion dynamics of OD stretch of HOD is dramatically slowed in the IL.4 Our approach, which combined MD simulations of dilute HOD in [bmim][PF6] with the spectroscopic maps developed by Lin, Auer, and Skinner for the OD stretch of HOD in electrolyte solutions,5 was able to capture the experimental observations. For example, our calculation of the linear IR absorption spectrum of the OD stretch of HOD in liquid water and in the IL is shown in Figure 6. The calculated spectrum is blue-shifted by 164 cm-1 and is 20 cm-1 wide in the IL, in excellent agreement with experiment. The calculations also captured the dramatic slowing of the timescales for spectral diffusion and reorientation of the OD stretch of HOD in the IL compared to in water. Analysis of the calculated spectral diffusion revealed that the frequency fluctuation dynamics is dominated by the translational motions of the [PF6] anions relative to the OD reporter.

References

1. Z. L. Terranova and S. A. Corcelli, "On the mechanism of solvation dynamics in imidazolium-based ionic liquids" J. Phys. Chem. B 117, 15659 (2013).

2. Z. L. Terranova and S. A. Corcelli, "Molecular dynamics investigation of the vibrational spectroscopy of isolated water in an ionic liquid" J. Phys. Chem. B 118, 8264 (2014).

3. X. X. Zhang, M. Liang, N. P. Ernsting, and M. Maroncelli, "Complete solvation response of coumarin 153 in ionic liquids" J. Phys. Chem. B 117, 4291 (2013).

4. D. B. Wong, C. H. Giammanco, E. E. Fenn, and M. D. Fayer, "Dynamics of isolated water molecules in a sea of ions in a room temperature ionic liquid" J. Phys. Chem. B 117, 623 (2013).

5. Y. S. Lin, B. M. Auer, and J. L. Skinner, "Water structure, dynamics, and vibrational spectroscopy in sodium bromide solutions" J. Chem. Phys. 131, 144511 (2009).