Dynamics and Vibrational Spectroscopy of Alcohols in Ionic Liquids: Methanol and Ethanol

The structure, dynamics, and vibrational spectroscopy of dilute HOD, methanol, and ethanol in the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim][NTf₂], ionic liquid (IL) are investigated with molecular dynamics (MD) simulations. The structure of the ILs around the solutes is qualitatively similar, where the OD bond of the deuterated alcohols donates an interaction to an [NTf₂] anion and the [emim] cations interact with the oxygen atom of the OD group. The slowest time scale for the reorientational dynamics of the OD bond varied considerably for HOD, methanol, and ethanol (27, 71, and 87 ps, respectively). In contrast, the slowest time scales for spectral diffusion of the OD vibrational frequency were 11 ps for each of the three solutes, which indicates that the dynamics of the IL is relatively unchanged by the presence of the alcohols at dilute concentration. The theoretical results for the reorientational and spectral diffusion dynamics compare favorably with prior two-dimensional infrared (2D IR) spectroscopic measurements.