In the previous study, we investigated the cation recognition by 12-crown-4 ether with the 3D-RISM theory [Ikuta, Y. et al., Chem. Phys. Lett., 2007, 433, 403], and showed the applicability of the theory to the problem. The results show that a 12-crown-4 ether recognizes Li + and Mg 2+ . 18-crown-6 ether is widely used in comparison with 12-crown-4 ether. In this study, we calculated three-dimensional distributions of water as well as cations (Li + , K + , Mg 2+ , Ca 2+ ) inside and outside 18-crown-6 ether in aqueous solutions of electrolytes. A 18-crown-6 ether recognizes not only Li + and Mg 2+ but also K + and Ca 2+ . K + is dehydrated in the host cavity whereas other cations are hydrated. It is found that a hydration structure, as well as cation size, is important for the molecular recognition by crown ether.
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