Dynamics of Isolated Water Molecules in a Sea of Ions in a Room Temperature Ionic Liquid

The vibrational dynamics of the antisymmetric and symmetric stretching modes of very low concentration spatially isolated D₂O molecules in the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate (BmImPF₆) were examined using two-dimensional infrared (2D IR) vibrational echo spectroscopy and infrared pump–probe experiments. In BmImPF₆, D₂O’s antisymmetric and symmetric stretching modes are well resolved in the IR absorption spectrum in spite of the fact that the D₂O is surrounded by a sea of ions, making it is possible to study inter- and intramolecular dynamics. Both population exchange between the modes and excited-state relaxation to the ground state contribute to the population dynamics. The kinetics for the incoherent population exchange (scattering) between the two modes was determined by the time dependence of the exchange peaks in the 2D IR spectrum. In addition, coherent quantum beats were observed at short time in both the amplitudes and 2D IR band shapes of the modes. The quantum beat decay is caused by dephasing due to both inhomogeneous and homogeneous broadening of the spectral lines. Analysis of the oscillations of the 2D line shapes demonstrates that there is some degree of anticorrelation in the inhomogeneous broadening of the two modes. It is proposed that a distribution in the coupling strength between the local modes that give rise to symmetric and antisymmetric eigenstates is responsible for the anticorrelation. Spectral diffusion, caused by structural evolution of the medium, occurs on multiple time scales and is identical for the two modes within experimental error. The spectral diffusion is fast compared to the time scale for complete orientational randomization of the RTIL. Spectral diffusion of the OD stretch of HOD in BmImPF₆ was also measured, and is essentially the same as that of the D₂O modes. Orientational anisotropy measurements of HOD in BmImPF₆ determined the orientational relaxation dynamics of the isolated HOD molecules