Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources; Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province; Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
J Phys Chem B. 2023 Jun 1;127(21):4858-4869. doi: 10.1021/acs.jpcb.3c01875. Epub 2023 May 19.
The underlying recognition mechanisms of alkali metal ions by crown ethers in aqueous solutions are yet to be fully understood at the molecular level. We report direct experimental and theoretical evidence for the structure and recognition sequence of alkali metal ions (Li, Na, K, Rb, and Cs) by 18-crown-6 in aqueous solutions by wide-angle X-ray scattering combined with an empirical potential structure refinement modeling and molecular dynamics simulation. Li, Na, and K are located in the negative potential cavity of 18-crown-6, with Li and Na deviating from the centroid of 18-crown-6 by 0.95 and 0.35 Å, respectively. Rb and Cs lie outside the 18-crown-6 ring and deviate from the centroid of 18-crown-6 by 0.05 and 1.35 Å, respectively. The formation of the 18-crown-6/alkali metal ion complexes is dominated by electrostatic attraction between the cations and the oxygen atoms (Oc) of 18-crown-6. Li, Na, K, and Rb form the HO···18-crown-6/cation···HO "sandwich" hydrates, while water molecules only hydrate with Cs of the 18-crown-6/Cs complex on the same side of Cs. Based on the local structure, the recognition sequence of 18-crown-6 for alkali metal ions in an aqueous solution follows K > Rb >Na >Li, which is completely different from that (Li > Na > K > Rb > Cs) in the gas phase, confirming that the solvation medium seriously affects the cation recognition of crown ethers. This work provides atomic insights into understanding the host-guest recognition and solvation behavior of crown ether/cation complexes.
冠醚在水溶液中对碱金属离子的潜在识别机制在分子水平上尚未完全被理解。我们通过广角 X 射线散射结合经验势结构精修建模和分子动力学模拟，直接提供了实验和理论证据，证明了 18-冠-6 在水溶液中对碱金属离子（Li、Na、K、Rb 和 Cs）的结构和识别顺序。Li、Na 和 K 位于 18-冠-6 的负电势腔内，Li 和 Na 分别偏离 18-冠-6 的质心 0.95 和 0.35 Å。Rb 和 Cs 位于 18-冠-6 环外，分别偏离 18-冠-6 的质心 0.05 和 1.35 Å。18-冠-6/碱金属离子配合物的形成主要是由阳离子和 18-冠-6 的氧原子（Oc）之间的静电吸引作用驱动的。Li、Na、K 和 Rb 形成了 HO···18-crown-6/阳离子···HO“三明治”水合物，而只有 Cs 与 18-冠-6/Cs 配合物同侧的 Cs 形成水合物。根据局部结构，18-冠-6 在水溶液中对碱金属离子的识别顺序为 K > Rb >Na >Li，与气相中的顺序（Li > Na > K > Rb > Cs）完全不同，这证实了溶剂化介质严重影响冠醚对阳离子的识别。这项工作提供了原子水平的见解，有助于理解冠醚/阳离子配合物的主体-客体识别和溶剂化行为。
