Department of Chemistry, Stanford University, Stanford, California 94305, USA.
J Phys Chem B. 2013 Jan 17;117(2):623-35. doi: 10.1021/jp310086s. Epub 2013 Jan 7.
The vibrational dynamics of the antisymmetric and symmetric stretching modes of very low concentration spatially isolated D(2)O molecules in the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate (BmImPF(6)) were examined using two-dimensional infrared (2D IR) vibrational echo spectroscopy and infrared pump-probe experiments. In BmImPF(6), D(2)O's antisymmetric and symmetric stretching modes are well resolved in the IR absorption spectrum in spite of the fact that the D(2)O 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 BmImPF(6) was also measured, and is essentially the same as that of the D(2)O modes. Orientational anisotropy measurements of HOD in BmImPF(6) determined the orientational relaxation dynamics of the isolated HOD molecules.
室温离子液体 1-丁基-3-甲基咪唑六氟磷酸盐（BmImPF(6)）中非常低浓度空间分离的 D(2)O 分子的反对称和对称伸缩模式的振动动力学，通过二维红外（2D IR）振动回声光谱和红外泵浦探测实验进行了研究。在 BmImPF(6)中，尽管 D(2)O 被离子海洋包围，但 D(2)O 的反对称和对称伸缩模式在红外吸收光谱中得到了很好的分辨，使得研究分子间和分子内动力学成为可能。两种模式之间的群体交换和激发态弛豫到基态都会导致群体动力学。通过 2D IR 光谱中交换峰的时间依赖性来确定两种模式之间的非相干群体交换（散射）的动力学。此外，在两种模式的振幅和 2D IR 带形中，在短时间内都观察到了相干量子拍频。量子拍频的衰减是由于谱线的非均匀和均匀展宽导致的退相。对 2D 线形状的振荡分析表明，两种模式的非均匀展宽存在一定程度的反相关。有人提出，导致对称和反对称本征态的局部模式之间的耦合强度分布是导致反相关的原因。由介质结构演化引起的光谱扩散发生在多个时间尺度上，在实验误差范围内，两种模式的光谱扩散是相同的。与 RTIL 完全取向随机化的时间尺度相比，光谱扩散是快速的。在 BmImPF(6)中，HOD 的 OD 伸缩的光谱扩散也进行了测量，与 D(2)O 模式的光谱扩散基本相同。BmImPF(6)中 HOD 的各向异性测量确定了孤立 HOD 分子的取向弛豫动力学。
