Cryogenic Ion Vibrational Spectroscopy of Cucurbit[5]Uril Complexes With One and Two Alkali Metal Endcaps

Ion receptors have been used in chemical contexts such as drug delivery, separation science, and catalysis. Designing new receptors targeting specific ions is challenging and requires insight into ion-receptor interactions and well as the competition of solvation with ion binding. Ion-receptor interactions and the intermolecular forces governing them are encoded in the structure, symmetry, and vibrational modes of such complexes and can be revealed using infrared (IR) spectroscopy. Gas-phase studies eliminate solvation effects and isolate the ion-receptor interactions.

Cucurbit[5]uril (CB[5]) complexes with one and two alkali metal endcaps have been studied in the gas phase using cryogenic ion vibrational spectroscopy (CIVS) and density functional theory (DFT) calculations. The C=O stretching modes encode structural information of how guest cations A⁺ (A = Li, Na, K, Cs) fit into the binding site of CB[5], and the number of bound ions (i.e., one vs. two) determines the coupling of the vibrational modes of the two binding sites.