Ultrafast Dynamics near an Anion in the Aqueous Solutions of Ionic Liquids
Aritri Biswas and
Bhabani S. Mallik*
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy-502285, Telangana, India
Ionic liquids (ILs) have drawn interest over the past few years owing to their widespread use in chemical science. Nowadays much of the research focus lies in IL synthesis and its spectral characterization. However, knowledge about structure and dynamics is necessary for tuning IL properties towards specific applications. Spectroscopic techniques like IR, NMR provide information about the structural evolution of the molecular system with time. They can capture structural changes if timescales are slow. 2D IR vibrational echo experiment1 tracks the structural alterations of the probe due to environmental fluctuations in condensed phase IL systems, occurring at ultrafast timescales (ps and fs). However, selectively tagging a particular vibrational mode is difficult in such a complex IL framework due to the lack of desired laser pulse. The experimental results are inconsistent that depend on the accurate and efficient methods of analysis. The problem in the study of IL systems using non-linear 2D IR technique needs to be solved from a computational perspective to bridge the existing gap between experiment and theory. Our main aim is to explore the phenomena of spectral diffusion2 affecting the dynamics of the infrared probe in IL. OD stretch of HOD is the vibrational chromophore of interest here. We observed randomization of the OD vibrational frequency in aqueous solutions of lithium bistriflimide (LiNTf2) and 1-ethyl-3-methylimidazolium bistriflimide (EmimNTf2) because of their differences in interaction caused by environmental variations. We used the flexible model for all ions, adjusting the concentration according to the experimental study3 and employed classical molecular dynamics (MD) using GROMACS 5.0.4 package.4 We performed wavelet transform5 of the trajectories (method of Arevalo and Wiggins), to obtain time-dependent OD frequencies. After that, we calculated the frequency-frequency time correlation function (ffcf), orientation correlation, rotational anisotropy and also the 2D IR spectra to gain deep insight into spectral diffusion dynamics in ILs. To achieve the target water: ion-pair ratio during simulation, the appropriate number of water molecules was added to prevent crystallization of LiNTf2 salt. Our frequency-frequency correlation data agrees well with the experimental findings. Reorientation times are almost similar in the solutions; however, the movement seems to be restricted in viscous EmimNTf2. We observe different structural arrangements in two liquids, and water dynamics depend on the cationic counterpart in the ionic solution.
References:
(1) Zheng, J.; Kwak, K.; Fayer, M. D. Ultrafast 2D IR Vibrational Echo Spectroscopy. Acc. Chem. Res. 2007, 40 (1), 75-83. https://doi.org/10.1021/ar068010d.
(2) Mallik, B. S.; Semparithi, A.; Chandra, A. Vibrational Spectral Diffusion and Hydrogen Bond Dynamics in Heavy Water from First Principles. J. Phys. Chem. A 2008, 112 (23), 5104-5112. https://doi.org/10.1021/jp801405a.
(3) Giammanco, C. H.; Kramer, P. L.; Fayer, M. D. Ionic Liquid versus Li+ Aqueous Solutions: Water Dynamics near Bistriflimide Anions. J. Phys. Chem. B 2016, 120 (37), 9997-10009. https://doi.org/10.1021/acs.jpcb.6b07145.
(4) GROMACS: High-Performance Molecular Simulations through Multi-Level Parallelism from Laptops to Supercomputers. SoftwareX 2015, 1-2, 19-25. https://doi.org/10.1016/j.softx.2015.06.001.
(5) Vela-Arevalo, L. V.; Wiggins, S. TIME-FREQUENCY ANALYSIS OF CLASSICAL TRAJECTORIES OF POLYATOMIC MOLECULES. Int. J. Bifurc. Chaos 2001, 11 (05), 1359-1380. https://doi.org/10.1142/S0218127401002766.