Concentration Dependence of Dynamics and Structure among Hydrated Magnesium Ions: An Ultrafast Infrared Study
Concentration Dependence of Dynamics and Structure among Hydrated Magnesium Ions: An Ultrafast Infrared Study
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DOI：
10.1021/acs.jpcb.3c00300
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发表时间：
2023-03-30
影响因子：
3.3
通讯作者：
Fayer，Michael D.
中科院分区：
文献类型：
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作者：
Hung，Samantha T.;Roget，Sean A.;Fayer，Michael D.
利用超快二维红外和偏振选择性泵浦-探测光谱研究了氯化镁水溶液从相对稀（0.5 μ m）到近饱和（4.2 μ m）浓度的动力学。实验进行了两个光谱上不同的腈伸缩频率的selenocyanate振动探针，对应于CN氮孤对与水和Mg 2+。在实验时间尺度（100 ps）上没有观察到两种物质的化学交换，从而可以直接分析它们的动力学。由Mg 2+相关峰报告的动力学比水相关峰慢，表明水合Mg 2+的直接环境不同于溶液的其余部分。值得注意的是，Mg 2+相关的峰显示三个光谱扩散时间尺度，最慢的是30 ps，而水相关的峰衰减为更快的双指数。从完整的取向弛豫时间和流体力学理论，镁的水化数为6，这是在很好的协议与NMR和X-射线衍射研究。这个水合数保持所有浓度，直到接近饱和，当线宽和动力学偏离线性趋势，指示Mg 2+溶剂化结构的变化，导致充分溶剂化所需的水分子的短缺。
The dynamics of aqueous magnesium chloride solutions, from relatively dilute (0.5 m) to near saturated (4.2 m) concentrations, were investigated using ultrafast two dimensional infrared and polarization selective pump-probe spectroscopies. The experiments were performed on two spectrally distinct nitrile stretch frequencies of the selenocyanate vibrational probe, corresponding to the CN nitrogen lone pair being associated with water and with Mg2+. No chemical exchange of the two species was observed over the experimental time scale (∼100 ps), enabling straightforward analysis of their dynamics. The dynamics reported by the Mg2+-associated peak are slower than those of the water-associated peak, suggesting that the immediate environment of the hydrated Mg2+is different from the rest of the solution. Notably, the Mg2+-associated peak displays three spectral diffusion time scales, the slowest being ∼30 ps, while the water-associated peak decays as a faster biexponential. From the complete orientational relaxation time and hydrodynamic theory, a magnesium hydration number of six was obtained, which is in good agreement with NMR and X-ray diffraction studies. This hydration number holds for all concentrations until near saturation, when the linewidths and the dynamics deviate from linear trends, indicative of Mg2+solvation structure changes resulting from a shortage of water molecules needed for full solvation.