PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, USA.
J Chem Phys. 2012 Jan 7;136(1):014501. doi: 10.1063/1.3665140.
We have investigated the rotational dynamics of lithium thiocyanate (LiNCS) dissolved in various polar solvents with time and polarization resolved vibrational spectroscopy. LiNCS forms multiple distinct ionic structures in solution that can be distinguished with the CN stretch vibrational frequency of the different ionic assemblies. By varying the solvent and the LiNCS concentration, the number and type of ionic structures present in solution can be controlled. Control of the ionic structure provides control over the volume, shape, and dipole moment of the solute, critical parameters for hydrodynamic and dielectric continuum models of friction. The use of solutes with sizes comparable to or smaller than the solvent molecules also helps amplify the sensitivity of the measurement to the short-ranged solute-solvent interaction. The measured orientational relaxation dynamics show many clear and distinct deviations from simple hydrodynamic behavior. All ionic structures in all solvents exhibit multi-exponential relaxation dynamics that do not scale with the solute volume. For Lewis base solvents such as benzonitrile, dimethyl carbonate, and ethyl acetate, the observed dynamics strongly show the effect of solute-solvent complex formation. For the weak Lewis base solvent nitromethane, we see no evidence for solute-solvent complex formation, but still see strong deviation from the predictions of simple hydrodynamic theory.
我们利用时间分辨和极化分辨振动光谱研究了不同极性溶剂中溶解的硫氰酸锂（LiNCS）的旋转动力学。LiNCS 在溶液中形成多种不同的离子结构，可通过不同离子组装体的 CN 伸缩振动频率来区分。通过改变溶剂和 LiNCS 浓度，可以控制溶液中存在的离子结构的数量和类型。离子结构的控制可以提供对溶质的体积、形状和偶极矩的控制，这是流体动力学和介电连续体摩擦模型的关键参数。使用与溶剂分子大小相当或更小的溶质也有助于提高测量对短程溶质-溶剂相互作用的灵敏度。测量的取向弛豫动力学表现出许多明显且明显偏离简单流体动力学行为的差异。所有溶剂中的所有离子结构都表现出多指数弛豫动力学，与溶质体积不成比例。对于路易斯碱溶剂，如苯腈、碳酸二甲酯和乙酸乙酯，观察到的动力学强烈显示出溶质-溶剂络合形成的影响。对于弱路易斯碱溶剂硝基甲烷，我们没有看到溶质-溶剂络合形成的证据，但仍然看到强烈偏离简单流体动力学理论的预测。
