Roget Sean A, Carter-Fenk Kimberly A, Fayer Michael D
Department of Chemistry, Stanford University, Stanford, California 94305, United States.
J Am Chem Soc. 2022 Mar 9;144(9):4233-4243. doi: 10.1021/jacs.2c00616. Epub 2022 Feb 28.
In highly concentrated salt solutions, the water hydrogen bond (H-bond) network is completely disrupted by the presence of ions. Water is forced to restructure as dictated by the water-ion and ion-ion interactions. Using ultrafast polarization-selective pump-probe (PSPP) spectroscopy measurements of the OD stretch of dilute HOD, we demonstrate that the limited water-water H-bonding present in concentrated lithium chloride solutions (up to four waters per ion pair) is, on average, stronger than that occurring in bulk water. Furthermore, information on the orientational dynamics and the angular restriction of water H-bonded to both water oxygens and chloride anions was obtained through analysis of the frequency-dependent anisotropy decays. It was found that, when the salt concentration increased, the water showed increasing restriction and slowing at frequencies correlated with strong H-bonding. The angular restriction of the water molecules and strengthening of water-water H-bonds are due to the formation of a water-ion network not present in bulk water and dilute salt solutions. The structural evolution of the ionic medium was also observed through spectral diffusion of the OD stretch using 2D IR spectroscopy. Compared to bulk water, there is significant slowing of the biexponential spectral diffusion dynamics. The slowest component of the spectral diffusion (13 ps) is virtually identical to the time for complete reorientation of HOD measured with the PSPP experiments. This result suggests that the slowest component of the spectral diffusion reflects rearrangement of water molecules in the water-ion network.
在高浓度盐溶液中，离子的存在会完全破坏水的氢键（H键）网络。水被迫根据水-离子和离子-离子相互作用进行重构。通过对稀HOD的OD伸缩进行超快偏振选择泵浦-探测（PSPP）光谱测量，我们证明了浓氯化锂溶液中存在的有限的水-水H键（每个离子对最多四个水分子）平均比本体水中的H键更强。此外，通过分析频率相关的各向异性衰减，获得了与水氧原子和氯离子都形成H键的水的取向动力学和角度限制的信息。研究发现，当盐浓度增加时，水在与强H键相关的频率处表现出越来越大的限制和变慢。水分子的角度限制和水-水H键的增强是由于形成了本体水和稀盐溶液中不存在的水-离子网络。还通过二维红外光谱对OD伸缩的光谱扩散观察了离子介质的结构演变。与本体水相比，双指数光谱扩散动力学有明显减慢。光谱扩散最慢的成分（13皮秒）实际上与用PSPP实验测量的HOD完全重新取向的时间相同。这一结果表明，光谱扩散最慢的成分反映了水-离子网络中水分子的重排。
