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We investigated the temperature dependence of the intermolecular dynamics, including intermolecular vibrations and collective orientational relaxation, of one of the most typical deep eutectic solvents, reline, using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), subpicosecond optical Kerr effect spectroscopy (ps-OKES), and molecular dynamics (MD) simulations. According to fs-RIKES results, the temperature-dependent intermolecular vibrational band peak at ∼90 cm-1 exhibited a redshift with increasing temperature. The density-of-state (DOS) spectrum of reline by MD simulations reproduced this fs-RIKES spectral feature. The decomposition analysis of the DOS spectra showed that all constituent components, including urea, cholinium cation, and chloride anion, also exhibited similar magnitudes of redshifts upon heating, indicating that the three species intermolecularly interact one another. The temperature sensitivity of the intermolecular vibrational frequency was high compared to that of ionic liquids. According to ps-OKES results, the slow orientational relaxation rate increased with increasing temperature; however, this phenomenon was not well explained by the Stokes-Einstein-Debye hydrodynamic model. Analysis of the orientational relaxation time based on the Stokes-Einstein-Debye model indicates that the decoupling between the orientational relaxation time and viscosity occurs at temperatures below ∼330 K. Quantum chemistry calculations of urea and the cholinium cation based on the MP2/6-311++G(d,p) level of theory confirmed that the contribution of intramolecular vibrational bands to low-frequency bands below 200 cm-1 was minimal. The densities, viscosities, electrical conductivities, and surface tensions of reline at various temperatures were also estimated and compared with the dynamics data.
Shirota et al. (Mon,) studied this question.