Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
Phys Chem Chem Phys. 2012 Aug 14;14(30):10455-65. doi: 10.1039/c2cp40244k. Epub 2012 Apr 12.
In this paper, we report on our investigation into the vibrational dynamics of the antisymmetric stretching modes of SCN(-) and N(3)(-) in several polar solvents. We used an infrared (IR) pump-probe method to study orientational relaxation processes. In two aprotic solvents (N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO)), the anisotropy decay shows a bimodal feature, whereas in other solvents the anisotropy decay can be fitted well by a single exponential function. We consider that the relative contribution of fast-decaying components is smaller in the other solvents than in DMF and DMSO. We discuss the possible origins of the different anisotropy decay behavior in different solvents. From the three-pulse IR photon echo measurements for SCN(-) and N(3)(-), we found that the time-correlation functions (TCFs) of vibrational frequency fluctuations decay on two different time scales, one of which is less than 100 fs and the other is approximately 3-6 ps. In aprotic solvents, the fast-decaying components of the TCFs on a <100 fs time scale play an important role in the vibrational frequency fluctuation, although the contribution of collective solvent reorganization in aprotic solvents was clearly observed to have small amplitudes. On the other hand, we found that the amplitude of components that decay in a few picoseconds and/or the constant offset of the TCF in protic solvents is relatively large compared with that in aprotic solvents. With the formation and dissociation of hydrogen bonds between ion solute and solvent molecules, the spectra of different solvated species are exchanged with each other and merged into one band. We considered that this exchange may be an origin of slow-decaying components of the TCFs and that the decay of the TCFs corresponds to the time scales of the exchange for protic solvents such as formamide. The mechanism of vibrational frequency fluctuations for the antisymmetric stretching modes of SCN(-) and N(3)(-) is discussed in terms of the difference between protic and aprotic solvents.
在本文中，我们报告了对 SCN(-)和 N(3)(-)的反对称伸缩模式在几种极性溶剂中的振动动力学的研究结果。我们使用红外（IR）泵浦-探针方法研究了取向弛豫过程。在两种非质子溶剂（N，N-二甲基甲酰胺（DMF）和二甲基亚砜（DMSO））中，各向异性衰减呈现双峰特征，而在其他溶剂中，各向异性衰减可以很好地拟合单指数函数。我们认为，在其他溶剂中，快速衰减分量的相对贡献比在 DMF 和 DMSO 中要小。我们讨论了不同溶剂中各向异性衰减行为的可能起源。从 SCN(-)和 N(3)(-)的三脉冲 IR 光子回波测量中，我们发现振动频率波动的时间相关函数（TCF）在两个不同的时间尺度上衰减，其中一个小于 100 fs，另一个约为 3-6 ps。在非质子溶剂中，TCF 在小于 100 fs 的时间尺度上的快速衰减分量在振动频率波动中起着重要作用，尽管非质子溶剂中溶剂重排的集体贡献明显观察到幅度较小。另一方面，我们发现在质子溶剂中，在几皮秒内衰减的分量的幅度和/或 TCF 的常数偏移相对较大，与非质子溶剂相比。随着离子溶质和溶剂分子之间氢键的形成和解离，不同溶剂化物种的光谱相互交换并合并为一个带。我们认为这种交换可能是 TCF 慢衰减分量的起源，并且 TCF 的衰减对应于质子溶剂如甲酰胺的交换时间尺度。根据质子和非质子溶剂之间的差异，讨论了 SCN(-)和 N(3)(-)的反对称伸缩模式的振动频率波动的机制。
