Spectroscopic Tracking of Size Selectivity in Crown Ethers: A Unified Molecular Picture of Encapsulation

Crown ethers exhibit pronounced ion selectivity and have long provided an excellent platform in supramolecular chemistry. This selectivity has been rationalized by a match between the ionic radius and the nominal cavity size of the crown ether, largely driven by the success of the K+18-crown-6 system. However, this concept omits the role of conformational flexibility of the crown ethers and fails to directly connect binding structures with ion selectivity. To gain molecular-level insight into the intrinsic binding structures of crown ethers with alkali metal ions, we present a systematic study of crown ethers with varying ring sizes using cryogenic gas-phase infrared spectroscopy in conjunction with anharmonic analysis of the CH stretch region. The CH stretch region serves as a sensitive probe of ion-binding structures and interactions, enabling definitive structural assignments of Li+-Cs+ complexes across different crown ethers. Our results reveal that the intrinsic binding structures vary systematically with both ionic radius and crown ring size. It appears that successful selectivity for a specific ion-crown ether system depends on two factors: the ability of the crown ether to encapsulate the ion, and having a configuration where the ion is equidistant from the oxygen atoms within a plane formed by the etheric oxygens. While encapsulation of Li+, Na+ and K+ is achieved by 18-crown-6, only K+18-crown-6 has the oxygen distances uniformly positioned about this plane, resulting in truly selective encapsulation. For 12-crown-4 and 15-crown-5, the most favorable structures, involving Li+ and Na+, respectively, are unable to achieve selective encapsulation, as the ions are positioned at varying distances with respect to the plane of the etheric oxygens. These distinctions can be traced to the differing conformational flexibility of the crown ethers.